All good things must come to an end. It's been an honor and a pleasure to guest-blog on BoingBoing for the past two weeks, and to engage with so many of you on topics near and dear to my heart. We talked about why the existence of extraterrestrial life is the most thrilling question humans can now answer. We noticed thousands of missing terrestrial planets, and found out how to find them. We discussed science communication, and astrobiology's asymptotic frontier. We weighed the worth of our world, and then crunched the numbers on some new exoplanets. We visited the Earth-like future of Saturn's moon, Titan, and learned the real science behind the forest moon of Endor. We explored a Keplerian orrery, and found new ways to visualize exoplanet data. We saw the birthing pangs of planets. We debated the most promising nearby stars, and wondered how we might one day reach them.

Special thanks goes out to those who directly or indirectly helped with these entries: Maggie Koerth-Baker, Rob Beschizza, Jer Thorp, Debra Fischer, Greg Laughlin, David Kipping, Dan Fabrycky, Sara Seager, and Lucianne Walkowicz and the rest of the Kepler science team. Thank you. And thanks to all of you reading this, particularly those of you who joined the discussion and passed story links around. I appreciate your attention and your interest.

There was much more I wished to write about and say, and indeed there will be one more multi-part post coming soon, but for now it's time to step aside. I hope all of you will keep reading, keep looking up, and start following and talking with me on Twitter. For media-related inquiries on commissions, reprints, and the like, I can be reached via FirstInitialFollowedByLastName at Google's e-mail service.

A final thought: In these past two weeks, our world has significantly changed—not only in Egypt, with the crowdsourced revolution that led to the fall of an autocrat, Hosni Mubarak, but also with the news from NASA's Kepler mission, which hinted that our universe is likely brimming with other living planets. The writing isn't on the wall so much as it's in the stars: We are not alone.

The juxtaposition of these two events inescapably brings to mind Voyager 1's famed "Pale Blue Dot" image of Earth from the depths of space, and Carl Sagan's timeless meditation on its meaning. It has been oft-quoted, but bears repeating:

Look again at that dot. That's here. That's home. That's us. On it everyone you love, everyone you know, everyone you ever heard of, every human being who ever was, lived out their lives. The aggregate of our joy and suffering, thousands of confident religions, ideologies, and economic doctrines, every hunter and forager, every hero and coward, every creator and destroyer of civilization, every king and peasant, every young couple in love, every mother and father, hopeful child, inventor and explorer, every teacher of morals, every corrupt politician, every "superstar," every "supreme leader," every saint and sinner in the history of our species lived there--on a mote of dust suspended in a sunbeam.

The Earth is a very small stage in a vast cosmic arena. Think of the rivers of blood spilled by all those generals and emperors so that, in glory and triumph, they could become the momentary masters of a fraction of a dot. Think of the endless cruelties visited by the inhabitants of one corner of this pixel on the scarcely distinguishable inhabitants of some other corner, how frequent their misunderstandings, how eager they are to kill one another, how fervent their hatreds.

Our posturings, our imagined self-importance, the delusion that we have some privileged position in the Universe, are challenged by this point of pale light. Our planet is a lonely speck in the great enveloping cosmic dark. In our obscurity, in all this vastness, there is no hint that help will come from elsewhere to save us from ourselves.

The Earth is the only world known so far to harbor life. There is nowhere else, at least in the near future, to which our species could migrate. Visit, yes. Settle, not yet. Like it or not, for the moment the Earth is where we make our stand.

It has been said that astronomy is a humbling and character-building experience. There is perhaps no better demonstration of the folly of human conceits than this distant image of our tiny world. To me, it underscores our responsibility to deal more kindly with one another, and to preserve and cherish the pale blue dot, the only home we've ever known.

Sagan's words are a summation of the value of a cosmic perspective, and I doubt they will ever be surpassed in their eloquence. I'll leave you with them.

Yesterday, we talked about which stars might be the most important ones for the near future of the search for habitable and inhabited planets. All the stars I mentioned are relatively close by and pretty bright, and some of them are already known to have planets. If and when potentially Earth-like worlds are found around these or other nearby stars, astronomers will begin lavishing them with attention in a process of discovery that will span generations. In all likelihood, entire careers and even subdisciplines of astronomy and planetary science will emerge from studying all the data we can remotely gather from a handful of promising worlds scattered among the nearest stars. If we are extremely lucky, and find signs of not only extraterrestrial life but also extraterrestrial intelligence, the consequences will spread beyond our sciences to shape and change our religion, philosophy, literature, and art.

And, if we did locate another pale blue dot circling a nearby star, for many people the next logical step would be to attempt to send people or machines there for direct investigation. It sounds simple enough, to send a spacecraft from point A through mostly empty space to point B. The Moon hangs shining in the sky along with the stars, and we've already sent explorers there, as well as robotic emissaries to all the solar system's planets. Reaching the stars shouldn't be that much harder—but it is.

Consider the problem from the simple viewpoint of velocity. It's easy to forget that until very recently, the fastest anyone had ever traveled on planet Earth was almost certainly about 200 kilometers per hour (kph), the terminal velocity of a plummeting human form past which air resistance impedes further acceleration. But then our species learned to build machines that use the fossilized sunlight in coal, gas, and oil to go even faster.

In 1906, a Bostonian named Fred Marriott finally surpassed the millennia-old record—and lived to tell about it—traveling over 200 kph in a steam-powered car across the sands of Daytona Beach, Florida. Scarcely forty years later, a West Virginian test pilot named Chuck Yeager flew a rocket-propelled plane at more than 1,000 kph, faster than the speed of sound. A decade after that, gargantuan rockets were accelerating men and machines to nearly 28,000 kph, fast enough to orbit the Earth and gain a god's-eye view of the planet. That's how we sent astronauts to the Moon, and robotic probes to other planets. Surely we can go even faster, and undertake interstellar voyages.

But space is vast, and even the distance to the nearest star is mind-boggling. Let's say the Sun is the size of a large orange, 10 centimeters in diameter. Place the orange on the ground, walk a bit more than 10 meters away, and you're in Earth's orbit. Finding our planet might prove challenging—it would be the size of a millimeter grain of sand. The walk out to Pluto, a speck of dust ten times smaller than our sand-grain Earth, would be nearly a half-kilometer, and along the way you'd be lucky to encounter any of the planets: Even the largest, Jupiter, would be no bigger than a small marble.

From Pluto in this scale model, to reach the nearest star system, Alpha Centauri, you'd have to travel some 2900 kilometers: roughly the distance between Memphis and San Francisco, or about how far you'd have to dig straight down into the Earth before reaching its outer core. At this scale, light, the fastest thing in the universe, would travel through space at just over 2 centimeters per second. In actuality, light travels at 300,000 kilometers per second, and requires nearly four and a half years to reach Alpha Centauri from our solar system.

Today, the fastest humans on Earth and in history are three elderly Americans, all of whom Usain Bolt could demolish in a footrace. They're the astronauts of Apollo 10, who in 1969 re-entered the Earth's atmosphere at a velocity of 39,897 kph upon their return from the Moon. At that speed you could get from New York to Los Angeles in less than six minutes. Seven years after Apollo 10, we hurled a probe called Helios II into an orbit that sends it swinging blisteringly deep into the Sun's gravity well. At its point of closest approach, the probe travels at almost 253,000 kph—the fastest speed yet attained by a manmade object. The fastest outgoing object, Voyager I, launched the year after Helios II. It's now almost 17 billion kilometers away, and travels another 17 kilometers further away each and every second. If it were headed toward Alpha Centauri (it's not), it wouldn't arrive for more than 70,000 years. Even then, it wouldn't be able to slow down. Of the nearest 500 stars scattered like sand around our own, most would require hundreds of thousands of years (or more) to reach with current technology.

Part of the problem is rocketry—an inescapable fact of accelerating by venting material out of a nozzle is that it's not terribly efficient. Not even accounting for food, water, and other consumables, you must carry all your fuel along with you, and the faster you wish to go, the more fuel you'll need—fuel that itself requires additional amounts of fuel to accelerate the additional mass. We've already almost maxed-out the velocities attainable through Apollo-style chemical rockets. But even so, there are no insurmountable physical barriers preventing people and machinery from going much, much faster than the pioneers of forty and fifty years ago. A small, scattered vanguard of idealistic scientists and engineers around the world still obsessively concoct new ways of harnessing more energy, of achieving more velocity, of going faster and farther than anyone has ever gone before. Maybe the stars are within reach.

We already know how to build speedier and more efficient rockets powered by electricity instead of chemicals, but they won't do much to get us to nearby stars. For that, only a handful of schemes could suffice. Some researchers suggest building rockets fueled by antimatter, an energy source so potent that the amount required to send you on a month-long crossing to Mars would be measured in grams. Others call for constructing gossamer-thin thousand-kilometer-wide "sails" in space, which would ride on powerful laser or particle beams out of solar system. These options and their more exotic variations theoretically offer velocities that are a significant fraction of the speed of light.

Sadly, while the physics may be on our side, the economics aren't: Based on present production rates and costs, producing and storing enough antimatter to fuel an interstellar mission would quite possibly bankrupt the planet. As for an interstellar sail, such an endeavor would dwarf the largest single piece of space-based infrastructure yet built, the International Space Station, a construction project that has so far cost an estimated $150 billion. Constructing an interstellar sail would probably cost far more—and that's not including the truly astronomical electric bill associated with powering the multimillion-gigawatt laser that would need to shine on the outbound sail for years on end.

At present, the most economically viable fast boat out of the solar system would probably be a spacecraft propelled by regular pulses of detonating atomic explosives. We do, after all, already have plenty of nuclear bombs lying around for no other real purpose than destroying civilization. Perhaps it's not unreasonable to co-opt them for a more productive endeavor. The US government actually funded a study of the concept in 1958, an ambitious program called Project Orion that seriously proposed, among other things, building a nuclear-pulse spacecraft that could send humans to the moons of Saturn as early as the 1970s. But legitimate concerns over radioactive fallout and the dual-use possibilities of miniaturized thermonuclear explosives forced Project Orion's eventual cancellation.

A more recent effort to design a nuclear-pulse spacecraft began in September 2009, and is called Project Icarus. Though, to be fair, Icarus itself is based on another highly regarded study, the 1970s-era Project Daedalus, named after the mythological craftsman who flew free from imprisonment on wings he constructed from feathers and wax. Both projects plan spacecraft that would voyage to the stars propelled by thermonuclear fusion.

Fusion occurs when nuclei of light elements like hydrogen or helium are slammed together with such force that they merge, releasing a flood of energy. It's a process that creates and destroys: It's what gives hydrogen bombs their fearsome power, but it also is how stars shine, glomming together light nuclei in their cores to form heavier elements. Stellar fusion is what made the calcium in your bones, the carbon in your DNA, and the oxygen that you breathe.

If fusion reactions could somehow be used in a propulsion system, they could accelerate a spacecraft to perhaps 10 percent the speed of light. Daedalus envisioned replicating that pressure and heat via arrays of high-powered lasers that would focus on small fuel pellets, compressing them past the fusion threshold and channeling the resulting plasma through a magnetic nozzle to produce thrust. The Icarus team is considering that approach, but has yet to decide on its thermonuclear propulsion method of choice.

Like Daedalus before it, Icarus is a project run entirely by volunteers, scientists and engineers who spend their idle time dreaming of starflight and performing laborious calculations to learn how it might be practically achieved. But unlike the Daedalus volunteers, who relied on the liberal use of slide-rules, brand-new HP-35 calculators, and an occasional sketch on the back of a bar napkin, the Icarus team is leveraging the power of more than 30 additional years of technological progress. Our advances in velocity may have petered out over the past few decades, but our prowess in information processing and communication has steadily accelerated.

Each individual volunteer on the Icarus team today could marshal more computing power than was available to most nation-states in the 1970s, and can electronically access the bulk of the world's accrued scientific and technical knowledge within seconds. Rather than gathering in pubs, they are formulating their starship design via internet telephony, private messaging forums, and the occasional post on the official Icarus blog. Still, while anyone can crunch numbers, actually building a starship would consume a large chunk of the Earth's entire economy, and likely would require creating massive economies off-world.

In the Daedalus plan, for instance, constructing a nearly 200-meter-long, 4,000-metric-ton spacecraft in Earth orbit was actually the easy part, nevermind that such a ship would be roughly the same size as one of the UK's Queen Elizabeth-class aircraft carriers. The harder task was acquiring 50,000 metric tons of the necessary thermonuclear fuel, an isotope of helium that is vanishingly rare on Earth. The Daedalus solution was to harvest the fuel from gas-giant planets like Jupiter, by building and operating a fleet of balloon-borne robotic extraction factories in their atmospheres. In other words, the easiest way the Daedalus volunteers found to fuel their starship was, in effect, the industrialization of the outer solar system.

Additional obstacles abound. Traveling at a significant fraction of light-speed can be compared to staring down the barrel of a gun: Running into a small piece of dust, or, heavens forbid, a sand grain, could cause catastrophic damage. The preferred Daedalus countermeasure was a 50-ton beryllium shield placed at the ship's prow. Even if no damaging impacts occur, a starship on a mission of decades or centuries would still require maintenance as parts and components wore out or broke down. For Daedalus, the solution was to pack a number of autonomous robotic wardens onboard the spacecraft to repair damage as it occurred. Creating such artificially intelligent robots capable of tending a starship for decades on end might be a bit more difficult than designing a Roomba to autonomously vacuum your living room.

And all that effort would only send a 500-ton payload, sans humans, strictly on a one-way flyby of a star. There would be no slowing down, stopping, or returning home. The Daedalus probe would fly through the alien star system in only a matter of hours, gradually trickling data homeward via a parabolic radio antenna. After absorbing untold treasure, time, and talent to reach another star, the Daedalus starship would have sent back scarcely more than the cosmic equivalent of a postcard. Icarus has upped the ante: The team intends to design a starship that can enter orbit around its target star, perhaps to monitor any potentially habitable planets there, and then, somehow, send large amounts of data back to Earth.

Suffice to say, engineering at these scales makes "rocket science" look like child's play.

Even further, consider the disruptive, unanticipated effects of the technologies a project like Icarus currently uses—not even including the ones it hopes to eventually employ for its starship: The ubiquitous computing and information networking that now allows Icarus to break out of local pubs and stretch across the world also seems to be turning many people's focuses inward, simultaneously connecting and unweaving the world. The velocity of our technology may ultimately be too fast, rather than too slow, and like the Daedalus probe of yore, could accelerate past its target in a flash, never to return. In other words, we could all too easily become lost in the virtual worlds we make for ourselves, and lose interest in the stars. Or, more probably, we could squander our resources and experience profound and irreversible technological regression. Sometimes, I pessimistically hold with some combination of these two extremes.

Given the magnitude and number of extreme technological and economic challenges that must be overcome to achieve starflight, it's difficult to imagine what, in fact, a civilization capable of interstellar travel would look like. Probably not much like us--more than anything else, projects like Icarus and Daedalus seem to tell us that we are presently as distant from interstellar travel as the stars are from Earth. And, at least until our culture's prioritization of short-term profit once again aligns with pushing the limits of the ultimately possible, that's likely to remain the case.

Perhaps someday one of these starship designs will take us out of the solar system on voyages to other living planets, other cosmic oases, strewn among the stars. Or maybe all the methods conceived today will in the fullness of time bear no more resemblance to actual starships than airplanes have to birds. Either way, it's worth remembering that the 100,000-year duration of interstellar voyages we can undertake right now is but the blink of an eye in cosmic terms. It may actually be more effective to adapt our expectations to those timescales, and to attempt to master such long-term planning rather than trying to brute-force our way to Alpha Centauri.

In expanding outward into space, patience, not velocity, may be the greatest virtue. After all, we're already on an interstellar spacecraft called the Earth, sailing with the Sun and its retinue of other planets around the Milky Way in circuits lasting 250 million years. Only by carefully preserving and cultivating the relatively bountiful and accessible resources of our planet and the solar system will we ever escape their confines. For now, it's wise to reflect that in our headlong rush to go ever faster and farther, we may only be fooling ourselves.

One reason I like writing about space science is because it offers so many gorgeous, mind-blowing images. Each and every day, they pulse from observatories that dot the Earth, and trickle down from our probes in the sky. The flow of visual data is already too much for our planet's limited number of professional astronomers, and is only set to ramp up further in the immediate future as multiple new deep-looking telescopes and all-sky surveys come online. This means there will be more and more opportunities for amateurs, average folks with just a bit of time and interest, to make real discoveries by sifting through images that the pros didn't have time to closely examine.

A good example of the visual depths waiting to be plumbed is this new image of the North America Nebula from NASA's infrared Spitzer Space Telescope. The North America Nebula is an emission nebula, essentially just a huge cloud of dust and molecular hydrogen that has been partially ionized and lit up by massive stars somewhere deep inside it. It doesn't seem that interesting until you actually look at it, preferably in multiple wavelengths. The thicker clumps of gas and dust occlude light at optical wavelengths, masking processes taking place inside, but infrared light can pass straight through these regions, revealing their mysterious inner workings. And we really want to know what's taking place there, because in all likelihood our Sun and its planets formed in a nebular cloud very much like this one.

Each little pinpoint speck of light in Spitzer's image is a young star at some particular point in its development. Some are still undergoing their initial gravitational collapse, and haven't even become true stars yet—that only occurs when thermonuclear fusion kicks off in their cores. Others have begun their stardom, but are still sheathed in spherical cocoons of gas and dust, shells of material that will gradually grow puffy and vaporous from the inner star's light and heat, until they whisper away on stellar winds. Many of these points of light are ringed by thick accretion disks of material that formed from the angular momentum of their initial gravitational collapse. Sometimes parts of the disk get sucked too close to the star, and are shocked into plasma and spun away and out from the star's poles in powerful collimated jets that can sculpt and shape the surrounding gas and dust into abstract whorls and tendrils. And, in the background, almost unnoticed against all the stellar fireworks, in all probability planets are slowly and surely forming. Perhaps, on a few them, the seeds of life are already being sown by comets and meteorites, the infalling detritus of star formation delivering water and complex chemical compounds brewed in the stellar clouds.

You could while away an entire afternoon just exploring one small patch of this single image. You won't, unfortunately, be able to resolve individual details as small as planets, but you will be able to get a sense of the scale and grandeur that lurk in the origins of every lowly rock and square inch of our own planet. To actually see planets mid-formation, you won't have to wait too long, though. A new, ambitious radio telescope array in Chile, the Atacama Large Millimeter Array (ALMA), is steadily approaching full operational strength. And this particular piece of kit, once it's up and running, will be able to provided deep, high-resolution views of the interiors of accretion disks. The upcoming James Webb Space Telescope will help, too, but you'll probably have to wait a bit longer for that, since it won't be launching any earlier than late 2015.

The point is, as beautiful as this Spitzer image is, it's only a preview of what we're likely to see and learn about our deepest planetary origins in the next decade.

Sometimes I worry that the popularity of cosmology—the study of the universe on the very largest and smallest of scales—has created a significant misconception about astronomy among the public. Specifically, in cosmology, the perceived worthiness of a question seems roughly proportional to the increasing number of light-years that separate us from its answer. In this view, most events considered interesting will occur a trillion years from now, or already took place many billions of years ago on the other side of the universe. This is exactly opposite from much of the rest of observational astronomy, where many of the most treasured objects are those that, because they are relatively nearby, offer plentiful photons for astronomers to gather and study.

This is particularly true in the search for habitable exoplanets. Planet-hunters need lots of photons from a star to confidently determine whether it harbors any planets, and they need even more photons from a planet to have any notion of whether it harbors anything alive. The relatively low number of available photons from the distant stars in the Kepler field is what will keep us from closely examining more than a handful of the many transiting, potentially habitable worlds we will glimpse there. And it's the pursuit of more photons that drives the costs and specifications of ambitious space telescopes that, once built, should be able to study the atmospheres of small, rocky worlds like ours.

What this means, and what I worry many people don't realize, is that we probably don't need to gaze across the universe or even the galaxy to find another Earth-like planet. In fact, we may only need to look right next door. It could be that, to have a good chance of gathering enough photons to find life beyond our solar system, we need to only build a very large and expensive space telescope for perhaps $5-10 billion, rather than a ridiculously large space telescope that would be an order of magnitude more expensive.

I recently wrote a feature story for Nature exploring how focusing on nearby stars could help reduce the astronomical projected costs of finding and verifying living worlds beyond our solar system, and I'm planning to talk more about the field's near- and long-term prospects later here on BoingBoing, but for now, here's a quick listing of five relatively nearby stars that could soon yield a high-value, world-changing discovery, and kickstart serious efforts to make the next giant leap in observational astronomy.

1. Alpha Centauri

I'm cheating a bit here, because Alpha Centauri is actually two Sun-like stars, Alpha Centauri A and Alpha Centauri B, orbiting each other with an average separation of a bit more than the distance between our Sun and Uranus. There's also a red dwarf, Proxima Centauri, drifting in the outskirts of the system. Both A and B have stable regions in their habitable zones where small planets could exist, though there is some debate over how easily planets can form around such binary pairs. Most importantly, at only 4.3 or so light-years away, the stars of Alpha Centauri are the closest in the sky other than our own Sun. They're very bright, and very easy to study, though no planets have yet been found. At least three planet-hunting teams are currently performing radial-velocity surveys of Alpha Centauri, with a particular focus on B, which is a more quiescent star, and thus easier to monitor. For additional context about these stars and two of the competing teams, check out my 2009 feature story in Seed magazine, The Long Shot.

This red dwarf star is relatively close by, just over 20 light-years away, and is one of the few stars of this very promising type to have been closely scrutinized for planets. Searches have discovered 4 planets and 2 candidates that await confirmation. Two of these objects, Gliese 581 d and the to-be-confirmed Gliese 581 g, are small enough and close enough to the star that they could conceivably be habitable. I wrote about Gliese 581 g, the so-called "Goldilocks" world, shortly after its discovery announcement last year, describing what its environment might be like. Since then, doubts about g's existence have received lots of media coverage, but one aspect of this story has gone largely overlooked: Even if many of the details of Gliese 581 g as reported in its discovery paper are proved incorrect, that particular orbital slot could well be occupied by another, smaller, potentially more habitable planet that has yet to be indisputably detected. Given the public uproar and resulting increased observational cadence for this system, I'm guessing we'll know soon whether Gliese 581 has a Goldilocks.

At 40 light-years away, this red dwarf star is close to the cusp of my comfort-zone in labeling it "nearby." And I'm also being a bit generous in its potential to deliver a game-changing discovery for habitable planets. The only known planet this star does have, GJ 1214 b, is a warm "super-Earth" of about 6 Earth-masses, and isn't really very Earth-like at all. But GJ 1214 b is very special, because it transits—which allows astronomers to examine its atmosphere and learn more about the planet's composition and history.
Deeper investigation of worlds like GJ 1214 b is very important, because while Kepler and other surveys are finding them seemingly everywhere they look, no planets like this exist in our own solar system. Consequently, at present it's quite difficult for astronomers to say whether planets of several Earth-masses could realistically be habitable. These worlds may almost always turn out to be more like "mini-Neptunes" than super-Earths. GJ 1214 b is the most easily-studied planet of this sort known to us, but hopefully more examples will be found soon transiting nearby stars.

HD 40307 is a K star slightly smaller and cooler than our Sun. It's even further away than GJ 1214, lying 42 light-years distant in the constellation Pictor, but it's of great interest to planet-hunters because of its confirmed planets, three worlds discovered in 2008 in scorching, nearly circular orbits, all less than ten times Earth's mass.
These worlds were discovered via the radial-velocity technique, meaning that astronomers can only estimate their mass and don't have reliable information on their density or composition. Just as with most other super-Earths, it's not entirely clear whether they're large rocky planets with thin atmospheres or bloated balls of gas with smaller rocky cores. Given the growing number of known planets of this kind, this system doesn't at first glance appear very notable. But there are tantalizing residual motions in its radial-velocity signals that hint at an additional orbiting companion further out in the system, perhaps even in the star's habitable zone. The European Southern Observatory's HARPS spectrometer, the world's premier radial-velocity instrument, is closely monitoring HD 40307 in hopes of finding additional, more habitable planets there.

61 Virginis is a star very similar to our Sun, though probably a billion or so years older, just less than 28 light-years away. Like HD 40307, it's known to harbor at least three planets in scorching, tightly packed orbits, a super-Earth and two Neptune-sized worlds that were discovered in 2009. The system also appears to have a debris disk of cold dust in its outer reaches, likely the product of cometary impacts in an analog of our own solar system's Kuiper belt. According to calculations by members of the discovery team at the University of California's Lick Observatory, stable orbits exist in 61 Virginis' habitable zone. 61 Virginis is one of the top targets for Lick Observatory's new Automated Planet Finder Telescope, a facility dedicated exclusively to radial-velocity planet-hunting that is beginning operations this year. By having the ability to observe this and other target stars each and every night instead of in infrequent chunks, the APF could rapidly discover many small, rocky planets.

Got questions about why I chose these stars instead of others? Do you have your own preferred list of most promising places to look for potentially habitable planets? Let me know in the comments!

Nevermind the Ewoks. For astrobiologists, the best part of Return of the Jedi was probably the gas-giant planet Endor and its accompanying forest moon. This bizarre concept—a habitable, Earth-like world orbiting a massive planet like Jupiter or Saturn—has proved so captivating that it has inspired not only Avatar, the highest-grossing movie of all time, but also a canonical 1997 peer-reviewed research paper published in Nature.

Besides the idea's pure novelty, there are sound reasons for scientific interest in habitable "exomoons." The growing consensus is that after the Earth, the moons of giant planets like Jupiter and Saturn appear most likely to harbor some sort of life.

Europa has a vast liquid-water ocean beneath its icy crust that may be enriched with nutrients from the moon's rocky deep interior. Enceladus is a water-ice slushball that seems to have pockets of liquid water beneath its surface, which betray their presence in vaporous plumes jetting from the moon's southern polar regions. And Titan boasts not only a subsurface water ocean but also a thick atmosphere, complex organic chemistry, and a global methanological cycle that mirrors the aqueous rhythms of life on Earth. These are only the most notable and scrutinized potentially habitable moons in our solar system—there are others even more mysterious.

Even generously including Venus and Mars in the inner solar system's tally of habitable places, the outer solar system still offers more worlds where life could conceivably exist. When it comes to the search for alien life in our own backyard, moons are the next great frontier, even though they are quite different from the environments we're used to on Earth. And, according to David Kipping, an astronomer at University College London, moons may also be the next big thing in the search for life beyond the solar system.

Kipping has developed a way to look for habitable exomoons using NASA's Kepler mission, and says that we could conceivably discover one before finding another convincingly Earth-like exoplanet. It's even possible, he says, that the homeworlds for the majority of life in our galaxy more resemble Endor or Pandora than our familiar planet. As you read this, Kipping and his colleagues are sifting through the new Kepler data, looking for the telltale signatures of Earth-sized exomoons circling giant planets in the habitable zones of distant stars.

Is all this too outlandish to be true? Is Kipping on to something, or has he been watching too much Star Wars? I chatted with him about his ideas and research to give you the information you need to decide.

David Kipping: Detecting exomoons is very similar to how astronomers detect exoplanets. The oldest way to try to find exoplanets is known as astrometry, which is basically just looking at stars very carefully and watching to see if they periodically wiggle and wobble about on the sky. If a star exhibits that periodic motion, that's an indication that there's a planet in orbit and tugging on the star. To look for a moon, we do the same thing, but we look at a planet instead of a star.

The way we can measure these wobbles is through the transits that we see when planets are fortuitously aligned with our line of sight. A transit lets us measure both the position and the velocity of a planet at that moment in time. So by repeating this measurement over and over, we can see if the planet's position or velocity is changing. If it is, then that means something is tugging at it, which could be a moon or an unseen perturbing planet.

DK: Oh yeah. There are two dovetailing measurements, transit-timing variation (TTV) and transit-duration variation (TDV). TTV just measures the instant that a transit occurs, which means it's very sensitive to the position of the transiting planet in its orbit. TDV, on the other hand, measures the length of the transit, which can give you the planet's velocity. If the planet is moving faster, its transit duration will be shorter, and vice versa. For moons, the velocity shift is always out of phase with the position shift. So we look for that phase shift by measuring both TTV and TDV.

It's just like a swinging pendulum: When the pendulum hits its lowest point, it's moving at its maximum velocity, and when it swings to the top, its highest position, it has a momentary velocity of zero. If we detect that pendulum-like short-period phase shift for a transiting planet, we know it's definitely caused by a moon and not something else.

DK: I had this idea in 2008, when I was working out models for the shapes of the light curves we might see from Kepler. I was studying this very carefully, thinking about the different properties we could measure for a transit and how reliable they were. And I realized you could see TTV without knowing necessarily what was causing it. I started imagining the Earth going around the Sun as if it were a transiting planet seen from far away, and it struck me that our Moon would have a big effect there, that the Moon's orbit would periodically pull our planet around and change its velocity, therefore changing the transit's duration. That led to the two papers I wrote where I proposed how to find exomoons.

DK: Kepler is the most precise instrument we have for this at the moment. I worked on a feasibility study for Kepler, to see how well it could do, how small of a moon we could detect. And we found that in the best-case scenario we can detect habitable-zone Earth-like moons down to about a fifth of the Earth's mass. A more realistic scenario might be targeting exomoons of about one Earth-mass. In that case, there are about 25,000 stars in Kepler's field of view that are bright enough to give us sufficient signal-to-noise so that we can look for such objects. If these sorts of large, Earth-like moons exist, we should be able to find some of them in the next year or two with Kepler.

DK: We have to think very carefully about selecting targets. There are two competing tensions: Whether or not a moon could be present in the first place, and how easy a moon would be to detect. In terms of detecting a moon, you want shorter orbital periods that give you more transits per unit time, which allows you to see more events and build up better statistics. You also want puffy, low-density planets, planets that cast a big shadow but aren't too heavy. That combination would give you a big transit and also a bigger signal from any accompanying moon.

But in terms of a moon's probability of existence, short-period planets aren't where you'd prefer to look. For example, "hot Jupiters" are just no good; we wouldn't expect moons to be around them for dynamical reasons. It's like Mercury or Venus in our own solar system—we think they were too close to the Sun and its gravitational influence to keep any moons they may have had.

Planets with longer-period orbits, wider separations from their stars, should have better chances of harboring moons. We already know from radial-velocity searches that there are plenty of Jupiter- or Saturn-mass planets in the habitable zones of stars. These sorts of planets are out there. We just need to look at the ones that transit and try to find some moons.

For the new Kepler data release, the sweet spot would be big, low-density planets around lower-mass stars, K- or M-dwarfs, that have already displayed several transits. You'd get the enhanced perturbation from a big moon and you'd be getting enough data to feasibly work with, but since the stars don't have much mass, their gravitational influence is less, and the moons are less likely to have been ripped away before you can detect them. That's where we might find an exomoon in the near term.

It's looking good. There's a new interesting paper from Eric Ford, where he and his team have been doing transit-timing on all the new Kepler candidates. And they found that at least 12 percent of all planetary candidates for which transit-timing could reliably be detected are actually showing TTV. Which means that these candidates are being perturbed by something. Whether that's another planet, or a moon, we have to do more work to be sure.

DK: There are lots of things that make this difficult. We prefer single, isolated candidates rather than ones in multi-planet systems, like the packed planets in Kepler-11, for example. The planet-planet gravitational interactions there make subsequent analysis harder, because you have to delete all those timing changes in search of any from a moon. We try to avoid candidates with eccentric orbits, because that can indicate disastrous early history filled with planet-scattering events that moons might not survive. Then there are variations in the co-planarity of the planet-moon system—how much they orbit in the same, shared plane can complicate your analysis. Some configurations will make it easier to discern the moon's gravitational tug, and others will hide it.

Analyzing these things takes time. A single candidate can take a month or more. We have to clean the data of spurious signals from cosmic-ray hits on Kepler's detectors and process it a few times to make sure the signals are robust. Then we generate a synthetic data set that doesn't have a moon in it, for example, and make sure we recover a null result. We have to do a lot of self-checking before we're sure anything we're seeing is real.

DK: Well, for most of the Earth-size planets we'll find in Kepler's field, it will be hard to really pin down their mass with radial-velocity measurements because their stars are so faint. It's not impossible but it's hard. And without their mass you don't know their density, which means we don't really know what these planets are made of. Moons, by contrast, automatically give you their mass and thus their density because they are detected through dynamic gravitational effects. So it's straightforward to say whether a moon is rocky or icy or gassy, whatever. That's important for quantifying habitability. And what it means is, we might learn more about small rocky bodies from studying Kepler's exomoons than from studying its exoplanets.

Unfortunately, as I said, almost all of Kepler's stars are quite faint, so it probably won't be feasible to look for atmospheric biosignatures like oxygen or methane or anything like that for Kepler's planets or moons. But if Kepler shows that plenty of moons are out there, the next-generation of transit surveys would be expected to find a certain fraction of moons orbiting transiting planets around nearby, bright stars. If a nearby exomoon has an orbital separation from its planet like the Earth and its Moon, it would be feasible to follow-up and look for biosignatures in that case using something like the James Webb Space Telescope.

DK: The big two are the Earth-Moon system and Neptune's moon, Triton. For something like the Galilean moons of Jupiter, and even Saturn's moon Titan, the mass ratios are too small for us to currently detect, even when you add all the moons together. Kepler does have the photometric precision to detect Saturn's rings, though, and I think we'll find some ringed worlds with Kepler soon. But for the sorts of moons Jupiter and Saturn have, which are formed from the gaseous disk around their planets early in our solar system's history, that process seems to yield unfavorable planet-moon mass ratios.

Earth's moon is different. It's irregular. It formed from a Mars-sized impactor throwing debris around the primordial Earth. Triton is probably the remnant of a binary object that wandered too close to Neptune, where one of the objects got captured and the other got ejected from the solar system. For irregular cases like these, there's not really a mass limit, and you can imagine all sorts of odd configurations happening. The question is, are these moon-forming events rare or are they common? If they are common, I'm very hopeful that we'll find several exomoons with Kepler. But if it happens that the vast majority of moons form like the Galilean moons did, I think we'll need to wait for the next generation of space-based telescopes to find them.

The real point is, we still don't know how common rocky planets are in the habitable zone, let alone moons. It could conceivably be that there are more Earth-like moons in the universe than there are Earth-like planets, which, if true, gives a pretty different picture of what life might typically be like across the universe. This is so fascinating because no one has ever done it before. If we find an exomoon, that would open up an entirely new field of astronomy, just like the first detection of an exoplanet did.

Following up on yesterday's post about Dan Fabrycky's festive rendition of Kepler's candidate multi-planet systems, I'm proud to unveil an exclusive new visualization of Kepler's candidate planets, courtesy of the multitalented Jer Thorp, who has an excellent blog at blprnt.blg.

Jer is a native of Vancouver, Canada, but he currently makes his home in New York City, where he is the New York Times' Data Artist in Residence and a visiting professor at New York University. He's also a contributing editor at Wired UK. Shortly after the Kepler data release, Jer and I started talking about how he could dynamically visualize both the magnitude and the nuance of the discoveries. This is what we came up with.

What you're looking at is every Kepler candidate, arranged as if orbiting a single star. In the real world, this would result in a catastrophic gravitational destruction derby, but in our virtual world it simply normalizes the candidates and allows a proper sense of scale to be perceived. Each candidate's estimated size, orbital speed, and orbital separation is accurately depicted, and each world is color-coded according to its estimated effective temperature, with red being relatively hot and deep blue/violet being relatively cold. Mercury, Mars, Earth, and Jupiter are added for context; the high-value Kepler candidates KOI 326.01 and 314.02 are also highlighted.

The color scale is calibrated so that Earth is a pale blue dot. This is the color it would display across the gulfs of interstellar space, a hue that suggests blue oxygen-rich skies and deep liquid-water oceans. Two concentric rings plot distances of 0.5 and 1 astronomical units from the central star, and a pale blue line delineates Earth's location on two self-organizing charts. In the video posted above, the first chart in the sequence plots semi-major axis (i.e., average orbital separation) versus effective temperature, while the second plots semi-major axis versus planetary size.

Important data trends prominently emerge from this visualization. The abundance of smaller candidates and relative sparsity of larger ones clearly indicates that there are many tiny, meek worlds for every giant planet. But curiously there is a relatively stark drop-off in the frequency of Kepler candidates at or below approximately Earth-size. My gut says this is probably an observational bias that later data releases will smooth out, but perhaps it's not, and Mother Nature actually prefers slightly chubbier planetary progeny, with Earth being a slender outlier.

Similarly, Kepler's nascent sensitivity to smaller planets with larger orbital separations is also suggested from the pile up of small, hot, fast-moving planets in very short-period orbits. The tight clustering of worlds large and small, cold and hot, all well within the orbit of Mercury testifies to the fact that most of Kepler's haul thus far comes from small, cool stars. Only later data releases will probe the habitable zones of larger, hotter stars like our Sun. Hopefully, the best is yet to come.

The visualization also highlights some things that are just plain weird. For instance, several other apparently promising candidates accompany both KOI 326.01 and 314.02 in terms of estimated habitability. Why weren't they assigned higher values based on Greg Laughlin's equation? I'm guessing part of the answer is that they orbit very distant and dim stars that offer extremely marginal chances of follow-up observations, something that Laughlin's formula substantially penalizes. The rest can be explained by the fact that any candidate large enough to be easily noticeable in this visualization is several times the size of the Earth, and hence of dubious habitability. The prevalence of apparently promising pale blue dots is an artifact of just how great the disparity is between the largest and smallest candidates.

Speaking of which, the largest candidate, an enormous pale blue orb, strikes me as extremely strange. It's several times larger than our solar system's heavyweight champion, Jupiter, yet it's also relatively distant from its star, and estimated to be rather cool. If I recall correctly, planets puffed-up to such large sizes get that way through being extremely hot and close to their stars. Perhaps this candidate will turn out to be something distinctly unplanetary, like a brown dwarf.

What do you see in this visualization? Do any clarifying insights or unsolved puzzles leap out at you?

How do you think it could be improved?

Let me know in the comments, and stay tuned—there may be an updated, interactive version of this coming soon.

Last week's data release from Kepler appears to have temporarily overwhelmed both professional and amateur exoplanet enthusiasts. After the initial flurry of basic overview posts on the Kepler data, I noticed a conspicuous hush fall over many of my favorite astronomy blogs, presumably caused by their authors turning en masse to parse the new treasure trove.

The list of more than 1200 candidate planets will likely yield more than a thousand actual confirmed worlds once culled of false positives. One system, Kepler-11, contains six confirmed transiting planets, and another system, KOI 157, has five candidates. Eight systems have been found with four candidate planets, along with 45 triple-planet and 115 double-planet systems. It's a lot to digest, and only represents the first four months of data from a 3.5-year mission.

Fortunately, the incapacitating shock of the breadth and depth of the new dataset seems to be wearing off, and researchers are beginning to reveal some of their initial explorations. In particular, Daniel Fabrycky, a University of California-Santa Cruz astronomy post-doc and member of the Kepler team, has created an impressive visualization of projected orbital motions for all the multi-planet systems Kepler has discovered to date. Within each system, planets are color-coded according to size, with the redder planets being larger and bluer planets being smaller.

It's hard to overstate the magnitude of the insights that can potentially be extracted from novel presentations of Kepler's raw data and its present and future planetary ensemble. Though each planetary system constitutes an essentially static snapshot of only one outcome from eons-long stochastic processes, lurking in the aggregate are lessons about how exactly planets form, how orbital configurations change over time, the relative distributions of planetary size, and frequency and how a star's age, size, and mass determine the sorts of planets it produces.

Sometimes these rules and the relationships between them may clearly manifest through a simple chart or graph of two key variables, but in other cases they may only reveal themselves through more dynamic presentations and multivariate analyses that better leverage the pattern-recognition capabilities of people. In this way, Kepler's large, diverse data sets may stimulate not only a more robust understanding of stellar and planetary science, but also significant progress in the effective design and usage of scientific data visualization.

For example, novel visualizations of stellar light curves from Kepler's first batch of data, released in June 2010, allowed members of the citizen-science project Planethunters.org to preliminarily identify 83 candidate planets that were only confirmed in last week's data release.

The same visualizations, which plot dips in the brightness of the more than 150,000 Kepler monitors, also yielded what may prove to be 47 additional candidate planets that slipped through the Kepler team's automated pipeline. Many fainter, subtler signals of smaller planets in habitable orbits around larger stars are certainly present unrecognized in the most recent dataset—borderline events that won't trigger a flag in a software routine but will catch the human eye. More people should be looking—there is a not-insignificant chance that with a bit of luck and careful observation, you could discover a potentially Earth-like world in Kepler's data even before the mission's scientists do.

Hot on the heels of my earlier conversation with Greg Laughlin about his valuation formula for Kepler's exoplanets, Laughlin crunched the numbers for Kepler's new list of 50 candidates orbiting in the habitable zones of their host stars:

The total value of the planets in Kepler paper's Table 6 is USD $295,897.65. As with most distributions of wealth, this one is highly inequitable—the most valuable planet candidate in the newly released crop is KOI 326.01, to which the formula assigns a value of USD $223,099.93. Assuming 5g/cc density, this planet has a mass of ~0.6 Earth masses, which is actually a little on the low side as far as the valuation formula is ensured. Nevertheless, USD $223,099.93 is a huge increase in value over Gl 581c, which charts at USD $158.32.

Back in 2009, I wrote that (in my opinion) the appropriate threshold for huge media excitement is USD 1M. With the planets in Table 6 of the paper, we are starting to get very close to that.

Here are the planets in the table with a formula valuation greater than one penny.

So there it is, folks. Get used to hearing a lot more about the quarter-million-dollar candidate world, KOI 326.01, at least until something better comes around. And don't forget about the runner-up, KOI 314.02, which is still worth a cool $71732.15.

An enterprising mind might think to go ahead and snatch up relevant domain names and search strings. But, incidentally, don't get too attached to those names. They're provisional labels in lieu of actual designations, which will be applied if and when these candidates are confirmed. KOI stands for "Kepler Object of Interest," and the three-digit number is the designation of the star in the Kepler Input Catalog. The two-digit number following the decimal point encodes the order in which transit candidates were identified around the star; so KOI 326.01 was the first identified transit candidate around star KOI 326, while KOI 314.02 was the second identified transit candidate around KOI 314.

OKLO.org: A Quarter Million Dollar World

Back in March of 2009, less than a week after the $600-million Kepler planet-hunting spacecraft rode a pillar of fire into orbit on a mission to make history, Greg Laughlin, an astrophysicist at the University of California-Santa Cruz, quietly posted a curious equation on his blog, oklo.org.

This equation's initial purpose, he wrote, was to put meaningful prices on the terrestrial exoplanets that Kepler was bound to discover. But he soon found it could be used equally well to place any planet—even our own—in a context that was simultaneously cosmic and commercial. In essence, you feed Laughlin's equation some key parameters--a planet's mass, its estimated temperature, and the age, type, and apparent brightness of its star--and out pops a number that should, Laughlin says, equate to cold, hard cash.

At the time, the exoplanet Gliese 581 c was thought to be the most Earth-like world known beyond our solar system. The equation said it was worth a measly $160. Mars fared better, priced at $14,000. And Earth? Our planet's value emerged as nearly 5 quadrillion dollars. That's about 100 times Earth's yearly GDP, and perhaps, Laughlin thought, not a bad ballpark estimate for the total economic value of our world and the technological civilization it supports.

At first blush, placing monetary values on mostly unexplored planets for which our sum total of knowledge consists of a few numbers in a database seems like the height of folly, or of hubris. But Laughlin argues that the point of his exercise is simply to measure the perceived potential that we collectively bequeath to a planet. Using his equation, you could begin to define something akin to their market value. And appreciating their dynamic range, the soaring highs and abyssal lows that define both the best and worst of all possible planets, just for a moment you might feel sensations vast, cool, and unsympathetic as you weigh and evaluate the worth of a world.

I spoke with Laughlin last week about his equation, how it works, and whether it deserves to be taken seriously.

Greg Laughlin: I've just always thought that the concept of an "Earth-like planet in the habitable zone" was pretty vaguely defined, and I wanted a metric that I could plug a planet into to see whether its value was high enough to warrant media hype. This is just a way for me to be able to quantify how excited I should be about any particular planet.

GL: Sure. "V" is the output, the total dollar value of the planet that you're evaluating; the "6 x 10⁶" is six million, which is normalizing for a baseline value. That value was obtained by taking the total cost of the Kepler mission—$600 million—and dividing by the number of Earth-like planets—about 100—that they expected to find. That means that society has valued a Kepler-observed Earth-like planet at 6 million dollars of real money. There's nothing arbitrary about this assignation.

After that, you have the age of the star, divided by half a billion years. This term is designed to favor stars that are older. If you're looking for life you don't want to look around stars that are just 100 million years old. You probably should be most interested in stars that are as old or older than the Sun, stars that offer more time for their planets to develop life.

This relates to the next term, the one in parentheses, that's the mass ratio of the planet to the star, raised to the ^1/3rd power. The more massive a star is, the shorter its existence, and the less opportunity it affords life to arise.

The exponent at the end there is giving this a weak dependence, and is the first of several exponents I added that multiply different chunks of the equation. If they're larger than 1, then that is making what's being multiplied more valuable relative to an "average" Earth-like planet found by Kepler. If the exponent is less than 1, then that's making whatever is multiplied less valuable. So the term we're discussing here is meant to favor lower-mass stars, although not by a huge amount.

Besides being better suited for harboring potentially Earth-like planets, lower-mass stars are also much easier to study, so that any promising planets they hold can be characterized. They give you a much better transit signal, and if you're using radial-velocity you're looking at an intrinsically larger signal, as the signal strength depends on the mass of the planet compared to the mass of the star.

GL: Each of those "exp - " terms represents 'e to the negative quantity²'.

Everything in this whole section is basically creating a Gaussian distribution, a bell-curve with the central peak being Earth's mass (M_⊕). This bestows planets that are nearer to Earth's mass with higher values, and devalues planets either less massive or more massive than Earth.

And that's admittedly a rather aggressive function. But I think it's reasonable. For habitability I think one would need to be very strongly biased toward Earth-mass planets, because we really don't know very much at all about planets that lie between Earth and Uranus in mass. None exist in our solar system. We're just barely getting our first pieces of concrete information about these worlds. And to talk about their ability to support habitable environments is premature to say the least.

GL: Right. The "exp - " here is doing the same thing, making a Gaussian that maximizes the value of the Earth, but with the amount of light and heat that the planet is getting from its star rather than the planet's mass.

We only have one example of a habitable planet, the Earth. So this favors planets that are similar in the amount of energy they receive from their star. If a planet is likely to get considerably less—like Mars—or considerably more—like Venus—the equation doesn't treat it well. The equation wants to see planets that get an Earth-like flux of energy from their star.

T_eff is a planet's calculated effective temperature, and 273 is, in kelvins, the melting point of ice at standard atmospheric pressure, and is also fairly close to Earth's average effective temperature. You can calculate an effective temperature basically by considering a spot on the equator of the planet that is painted black, with no overlying atmosphere. It's just a way of calibrating to the Earth. The division by 30 says that, if you're more than 30 degrees away from 273 you're getting a significant devaluation; it's scaling how fast you devalue as you move away from a value defined by the Earth.

GL: Yes. This term places a value on immediacy. Kepler launched in 2009, so that's the baseline. The closer to 2009 that Kepler discovers a planet, the more it's worth. The way to think about this is, if a planet like 51 Pegasi b or HD 209458 b were discovered today, it would barely merit a paper in a major journal, whereas 10 or 15 years ago those planets forged careers and reputations. There is enormous value associated with early discoveries, and that term reflects this. Though perhaps not even as much as it should.

You'll notice the immediacy value has a fall-off of 4 years, during which these planets devalue significantly. And even that might be optimistic—they might go down even faster than that. The first planets of a type are a huge deal, and then subsequent ones rapidly lose excitement among the public as you go further out in time. Discovering an Earth-mass planet in the habitable zone of a Sun-like star in the year 2060 is not going to be a big deal, in all likelihood.

GL: This term is basically saying that the brighter the star appears to you, the more valuable the planet is, though it's weakly dependent. The "V" values for actual astronomical objects range from V=-26.7, for the Noonday Sun as seen from Earth, down to V=+30, for the dimmest galaxies that Hubble Space Telescope can spot. The magnitude scale can be traced back to the ancient Greek system for ranking stars visible to the eye. It's a logarithmic scale, because the human eye has a logarithmic, nonlinear response to light. The difference between a bright value of 0 and the dimmest value of 5 on this scale is a factor of 100 in the actual flux, the energy, arriving on Earth.

This is important because we rely on photons to characterize these exoplanets, and it's just much easier to characterize them and extract their intrinsic value if they're nearby and giving us more photons. So it's basically a proximity term, and other than the $600 million baseline provided by Kepler, it's the most important part of the equation.

GL: Think of it this way: If we're sitting here on Earth, our sun is extraordinarily bright in the sky. The brightness of the Sun thus makes that term enormous if we run this equation for the Earth. If we evaluate this equation for the Earth, we get an answer of about 5 quadrillion dollars. And that's basically the value of all our infrastructure, accumulated through history.

This isn't a man-who-sold-the-Earth kind of statement, but rather it's pinning things down on the other end. Being there. How much is a habitable planet worth if you're actually there? Well, this says it's worth quite a bit.

If, for instance, there is a planet orbiting in the habitable zone of Alpha Centauri B, part of the closest star system in the sky other than our Sun, that planet's worth about $6 billion using this scale. But then if you voyage there, Alpha Centauri B appears brighter, and brighter, and brighter, until it is your Sun in your sky and you're on the planet's surface. So in going there you have this ability to intrinsically increase value. And that's an exciting thing because it ultimately provides a profit motive for perhaps going out and making a go of it with these planets.

This is saying that something that is several billion dollars on Earth, could be, if you go there, a quadrillion-dollar payoff.

It's also interesting to note that the $6 billion evaluation for a promising planet around Alpha Centauri B is very similar to the cost of a direct-imaging mission that could conceivably characterize such a planet. It's not hard to imagine that if you have a really alluring Earth-mass planet in an Earth-like orbit in Alpha Centauri, that even in the current climate there might be the political and public will to launch an ambitious mission to characterize it.

The public would be perhaps willing to spend that much, on the order of billions of dollars. The public would not be willing to spend trillions of dollars to build that space telescope for that task. And the public would certainly be willing to spend more than millions of dollars to do it. So that number is more or less in the ballpark of what is realistically conceivable for NASA, for us, to do.

GL: Well, the formula says Mars is worth nearly $14,000. Which is sobering, but at the same time realistic in the sense that you would last 5 seconds if you showed up there.

And you can apply it to previously discovered exoplanets, giving the benefit of the doubt to radial-velocity planets and their uncertain masses. When you do that, the most valuable and undisputed planet that has been announced as of right now, the end of January, remains Gliese 581 c, which is valued at only I think $160 or so. But the value of the much-discussed candidate planet Gliese 581 g, based on the properties listed in its discovery paper, yields a value of approximately $60,000.

What this is telling us is that despite all of the media excitement that has surrounded the detection of planets supposedly lying in the habitable zone, these aren't the ones to really get excited about yet. These aren't the planets we're looking for. It's going to get a lot better, a lot sooner than people realize. The planets that have been found to date don't really register on this scale as calibrated by Kepler's budget and its projected harvest of Earth-like planets.

This scale is not something that press officers would like in pushing marginal planets into press conferences and so forth. It treats the planets in a fair and objective way. Objects like Gliese 581 c, you can write press releases and say they're habitable, but this formula reveals that they're not very Earth-like. But they are out there. Alpha Centauri aside, some radial-velocity searches could easily yield a planet any day now that would hit $10 million, maybe even $100 million, using this formula.

GL: I will say that an important caveat for this formula is that it considers nothing related to habitability other than the planet's most basic orbital and mass properties, the orbital distance, and the parent star. It's not assigning more value to a planet if it has water or an oxygen-rich atmosphere or anything like that. We're still quite a ways away from being able to properly evaluate what exoplanets are actually like in those terms.

This is strictly a speculative value being assigned. This is rather like putting a value on something like Facebook, which has scant meaningful earnings. It's all in potential. In the dot-com bubble of the late 1990s, you saw things like pets.com and @home.com getting huge valuations. Seeing those valuations and then looking at another internet startup company that had no actual profits, you'd be able to value that company based on the similar valuations that you'd seen for other dot-coms.

That valuation is not a mark of intrinsic worth, but rather a valuation that is in line with prices that are being paid at the time. In that case it was for those stocks, and in this case it's for the outlay you need to make the observations to discover the planets. It's a very real thing that pets.com or @home.com were trading for many dollars per share, and people spent real money for those shares and got them in return. That's exactly the nature of my valuation here.

The question of intrinsic value, which hinges on the rate of occurrence of truly habitable planets, is not addressed at all. That's why I take it seriously and would argue that this is in no way controversial or even particularly forward looking. Given that society is willing to pay for speculative investigations looking for habitable planets, given the amount of money we've been spending, this is how you would value planets that will be coming in.

GL: Yeah. Venus is a great example. It does pretty well in the equation, and actually gets a value of about one and a half quadrillion dollars if you tweak its reflectivity a bit to factor in its bright clouds. This echoes what unfolded for Venus in the first half of the 20th century, when astronomers saw these bright clouds and thought they were water clouds, and that it was really humid and warm on the surface. It gave rise to this idea in the 1930s that Venus was a jungle planet. So you put this in the formula, and it has an explosive valuation. Then you'd show up and face the reality of lead melting on the surface beneath sulfuric-acid clouds, and everyone would want their money back!

If Venus is valued using its actual surface temperature, it's like 10^-12 of a single cent. @home.com was valued on the order of a billion dollars for its market cap, and the stock is now literally worth zero. Venus is unfortunately the @home.com of planets.

It's tragic, amazing, and extraordinary, to think that there was a small window, in 1956, 1957, when it wasn't clear yet that Venus was a strong microwave emitter and thus was inhospitably hot.

The scientific opinion was already going against Venus having a clement surface, but in those years you could still credibly imagine that Venus was a habitable environment, and you had authors like Ray Bradbury writing great stories about it. At the same time, the ability to travel to Venus was completely within our grasp in a way that, shockingly, it may not be now. Think what would have happened, how history would've changed, if Venus had been a quadrillion-dollar world, we'd have had a virgin planet sitting right next door. Things would have unfolded in an extremely different way. we'd be living in a very different time.

The Kepler teleconference ended a couple of hours ago. I tried my best to live-tweet salient details, so you can get your fill on my Twitter page. Here's the very compressed big picture: Kepler is working nearly flawlessly, and it's finding oodles of *candidate* transiting exoplanets, some of which appear to be rocky worlds orbiting in the habitable zones of their stars.

The Kepler team has announced more than 1200 new candidates.

Of those, 68 are approximately Earth-sized (equal to or less than 1.25 Earth radii). More than 50 candidates of all sizes are located in the habitable zone of their host stars, including 5 that are less than twice the size of Earth. The evidence suggests that smaller planets occur more frequently around smaller, cooler stars than hotter, larger stars, of which our Sun is one example. Nearly 15 percent of the stars with candidate planets harbor more than one candidate, suggesting that multi-planet systems are fairly common.

Much more work remains to be done, and indeed the follow-up observations required to confirm that all these candidates are actually planets will likely take many years. We still don't know if life exists elsewhere in the universe, but we've now taken another major step into the asymptotic frontier, and life's cosmic abundance appears more inevitable than it did yesterday.

This moment has been coming for a long, long time.

Most commentators will point to Kepler's immediate origins in a 1992 mission proposal called FRESIP from Kepler's eventual Principal Investigator, William Borucki. But I prefer to trace the defining moment back forty years more, in the overlooked musings of a brilliant Russian-American astronomer, Otto Struve.

In this paper, first published in The Observatory in 1952, Struve lays out the basic case for hunting for planets using both high-precision radial-velocity spectroscopy as well as transit photometry. He was a remarkably prescient man, and his story is worth telling, but that will be for another time. Suffice to say, I think Struve deserves far more credit than he has received for his early contributions to the wildly successful modern era of planet-hunting.

Of course, the trail goes back further still. Some 2,500 years ago, in Ionian Greece, a man named Leucippus, of the town of Miletus, first theorized that everything in the universe was made of tiny, indivisible atoms.

His disciple, Democritus, extended these ideas to state that endless configurations of atoms and void created infinite worlds that exist apart from our own, and that the Milky Way's soft glow emerges from countless faraway suns. Two centuries later, the philosopher Epicurus best summarized these ideas in a letter to a certain Herodotus (not to be confused with the historian of the same name): "There are infinite worlds both like and unlike this world of ours ... We must believe that in all worlds there are living creatures and plants and other things we see in this world."

It's not a stretch to say that, with today's announcement, the Kepler team has, in one swift stroke, made more progress toward solving this ancient mystery than has been made in the entirety of previous human history on Earth.

Think about that, and then realize that the most exciting steps—confirming these planets, finding ones even more Earth-like around nearby stars, and studying them for signs of life—still lie in our future. With any luck, and a hefty helping of public engagement, these things will happen before you, me, and everyone we know are only memories like Struve and Democritus.

Read More: Here's a nice round-up of coverage from Emily Lakdawalla of the Planetary Society. Nature has an excellent overview of the new discoveries, including a smashing Kepler feature story by Eugenie Samuel Reich, and an accompanying piece from yours truly discussing cost-effective technical and technological developments that are poised to deliver potentially habitable worlds for prices even a rabid deficit-hawk could love. I'll probably discuss some of those developments in more detail in coming blog posts.

I hadn't realized (until checked my news feed this morning) that today was Groundhog Day, the annual holiday celebrated in the United States and Canada where a chubby, furry rodent—a groundhog—is made to emerge from its burrow, and then given a choice: Either stay out, or go back in. The story goes that if the groundhog emerges under cloudy skies, it will hang around outside, and wintry weather will soon cease. If the groundhog comes out into sun, it will retreat, and winter will endure for six more weeks. The crux of the rite is whether or not the groundhog sees its shadow.

It makes me smile, wondering whether the scientists and administrators for NASA's Kepler mission knowingly chose today for their next data release based on its tenuous resonance with a bizarre cultural tradition. What we will see later this afternoon at NASA's 1pm EST press conference is fairly similar in its essentials. Researchers, of course, play the groundhogs. Bright-and-bleary-eyed from excitement mixed with lack of sleep and too many hours burrowing into their light curves and RV plots on computer monitors, they will emerge from their isolation and tell the world whether they saw shadows, silhouettes of alien worlds transiting distant suns. The biggest difference is that the more shadows they see, and the smaller they are, the more likely a spring awakening will occur in exoplanetology.

This is because Kepler's primary goal is not, despite frequent misleading statements to the contrary, to discover Earth-like planets—living worlds. Rather, its official mission is to provide a good estimate of the frequency of all varieties of planets and planetary systems that may exist in our galaxy. Planets like ours, places like home, will be equivalent to cherries atop Kepler's far larger smorgasbord of discoveries that astronomers and the interested public will devour.

The point, from the very beginning, has been to leave everyone hungry for more. Kepler alone cannot tell us whether we live in a crowded, living universe. For that, astronomers unavoidably need more, and more expensive, telescopes on the ground and in space. They need people to care about the question they're trying to answer, and to believe that answering the question is possible. They need public support and interest. Otherwise there is scant hope that the costly hardware required to take the next bold steps will be built, and the goal of finding life beyond the solar system may slip beyond our lifetimes.

Which, at risk of running my cultural comparison into the ground, brings me back to Groundhog Day—not the holiday per se, but the excellent Bill Murray film, where he plays a jaded newscaster who cycles endlessly through a closed loop of time, experiencing the same events over, and over, and over.

The Kepler conference hasn't even happened yet, but I already have the worrisome gut feeling that I've seen this show before. It happened in 1996, with the premature declaration of Martian life in an ancient meteorite. It happened in 2004 with controversial detections and interpretations of methane, a potential biosignature, in the Martian atmosphere. It happened in 2010 with the sensationalized announcement of arsenic-munching bacteria, and with the disputed discovery of a habitable planet, Gliese 581 g, thought to orbit a nearby red dwarf star. There are many other lower-profile examples.

I'm not rejecting these previous claims as necessarily false, but I am questioning the wisdom of the manner in which they were often communicated to the public. It is irresponsible and inherently self-defeating for scientists, press officers, and journalists to not highlight key uncertainties when revealing scientific results on a topic as explosively profound as the existence of life elsewhere in the universe. The truth is, at least until we can actually go off-planet to farflung places for first-hand investigations, the quest for extraterrestrial life is an asymptotic frontier, approaching, but never reaching, certainty that someplace else is just like home, that something else lives or thinks just like we do. Evidence will accrue, conclusions and rebuttals will battle, and progress will be made, but there are limitations to our knowledge that must be acknowledged rather than dismissed.

When you picture an astronomer looking for planets around other stars, what do you see?

I'm guessing many of you are summoning images of Galileo Galilei sketching on parchment and gazing upward through a hand-held spyglass, or Edwin Hubble sitting all night in a cavernous, cold observatory dome, pipe clenched in teeth, peering through the eyepiece of a gargantuan telescope and exposing photographic plates. But that's rather like imagining a dentist extracting a tooth with string and a strongly slammed door, or a mechanical engineer performing calculations with an abacus. These days, rare is the professional astronomer who actually looks through a telescope at all, and rarer still those who make profound discoveries doing so. The human eye simply isn't up to the task of discerning subtle details—and the human hippocampus is inadequate for recording them.

This is part of the reason why most exoplanets don't swim fully-formed into view through a telescope's lens so much as they gradually take shape from a mist of statistics and inference. But an even bigger contributor to this observational dislocation is that, in comparison to stars and the vast distances between them, planets are so faint and small that actually seeing them at all is incredibly challenging. Of the six main methods for detecting exoplanets, only one, direct imaging, hews to the antiquated notion of observing and studying an object by taking its picture. The other five—astrometry, radial-velocity spectroscopy, microlensing, transits, and timing—discover planets by observing stars and searching for the subtle effects induced by any accompanying worlds.

You'd think that direct imaging would be the easiest to understand and explain, but it actually requires so much complex, cutting-edge, and just plain cool technology that I'm saving it for another post. Direct imaging using specialized space telescopes also deserves separate discussion because it may be the only way to deliver the results everyone wants: compelling evidence of a complex biosphere on an Earth-sized planet orbiting another Sun-like star. For now, suffice to say that both ground- and space-based direct-imaging techniques can only directly image very large, hot exoplanets orbiting far from their stars.

The remainder of this post will discuss the other five techniques, all of which hold promise for discovering potentially habitable worlds, but have somewhat less to offer in terms of deeply investigating them. They are perhaps less glamorous, but they form the foundation of planet hunting, and are crucial for understanding all the exciting discoveries that will be announced in coming years, months, and days. Let's get started, there's a lot of ground to cover.

Wibbles and Wobbles

Two of the methods, astrometry and radial-velocity spectroscopy, look for "wobbles" in a star's motion caused by the gravitational to-and-fro tug of orbiting planets. A wobble's periodicity—how often it repeats—indicates a planet's orbital period. And a wobble's strength—whether it corresponds to a kilometer or a centimeter of motion, for instance—indicates a planet's mass.

Astrometry looks for wobbles by measuring a star's motion within the two-dimensional plane of the sky, as judged against the positions of relatively static, more distant stars. So if, from our perspective on Earth, a star is orbited by a planet that tugs it to the right and left, or up and down, the astrometric signal is detectable.

Astrometry is actually the oldest planet-hunting technique—it dates back to the mid-19th century. But it's also the most underutilized, perhaps because all of the planets it has discovered have later proved illusory. See, for instance, the story of Peter van de Kamp, or the recent retracted astrometric detection of the exoplanet VB 10 b. To detect the astrometric signal of an Earth twin around a nearby Sun-like star, you'd need a measurement precision of something like one-millionth of an arcsecond, which, if my math is right, is akin to discerning the thickness of a dime from more than half a million kilometers away. Astronomers have plans for spacecraft that could perform such precise measurements, but currently lack the funding to build them.

If astrometry is the overlooked and difficult wallflower of planet hunting, then its sibling, radial-velocity spectroscopy (RV), is the all-star varsity quarterback who makes straight A's and dates the prom queen. As of this fine February 2nd morning, RV has detected more exoplanets than any other technique, but that status may change later today. Unlike astrometry, RV detects wobbles perpendicular to the plane of the sky—in other words, it detects motion toward and away from us here on Earth, not back and forth in the sky.

It does this spectroscopically, by separating a star's light into its constituent colors, just like how sunlight becomes a rainbow when fed through a prism. When a planet tugs its star toward us, the starlight shifts toward the blue end of the spectrum; when a planet tugs its star away, the starlight shifts toward the red. The effect is exactly the same as when the sound of an ambulance siren increases in pitch as it approaches, then decreases in pitch as it whizzes away. Interestingly, this means that the ideal exoplanetary system for RV detection, in which the planet's orbits are inclined edge-on to Earth, would be invisible to astrometry. And vice-versa: An ideal astrometric planetary system, with its orbits aligned precisely within the plane of the sky, would be invisible to RV.

RV most easily detects massive planets in scorchingly close orbits of their stars, which is why most of the exoplanets found by early RV searches were so-called "hot Jupiters." Our Earth's motion around the Sun causes our star to shift its position by 1 centimeter per second over the course of a year. That wee signal is what RV searches for alien Earths have to detect across the light-years. Ground-based RV surveys aren't quite reaching that degree of precision yet, but they're getting very close, primarily by combining years of observations to amplify faint signals.

The biggest downside to RV as a technique is probably that, because it can't reliably measure the inclination of an exoplanet's orbit, it can generally only provide an estimate of a planet's mass with a fudge factor of around 15 percent. That may sound small, but it could be the difference between a planet being a comfortable place to live or a frozen, desiccated chunk of lifeless rock. Astrometry provides a true measurement of mass, but is rather more difficult to perform.

Quantity and Quality

Beyond the "wobble" methods, microlensing and transits use photometry—measuring the quantity of starlight—to find planets.

Microlensing is unabashedly weird, and rather counter-intuitive, a product of living in a universe that operates using the relativistic rules of Einstein. General relativity dictates that the more massive an object is, the more spacetime curves around it. Very massive objects like galaxies can curve spacetime so much that they act as gravitational lenses, amplifying light from background objects. Astronomers reserve the term "microlensing" for stars, which create correspondingly smaller gravitational lenses. If a foreground star with accompanying planets passes in front of a far more distant, precisely aligned background star, the gravitational fields of the planets can further enhance the lensing star's magnification effect. Astronomers detect this as a transient brightening in the lensing event over a period of hours or days, and can typically obtain a lensing planet's mass and orbital separation.

Microlensing alignments are rare, so astronomers must monitor very large numbers of stars (for instance, those in the galactic bulge) to have a reasonable chance of detecting events. But the technique is very powerful: Space-based microlensing observations can easily reveal the presence of Earth-mass planets in the habitable zones of stars.

Unfortunately, each lensing event occurs only once, and typically reveals planets many thousands of light-years away from our solar system. This means that any tantalizing planets discovered via microlensing will almost certainly be beyond the reach of meaningful follow-up observations. Consequently the technique is most useful for studying the architecture of planetary systems and the statistical distribution of planetary masses and orbital separations. For anyone hoping to find evidence of life on other planets, microlensing-based detections are a little unsatisfying.

Transits, on the other hand, are the jackpots of planet-hunting. A transit occurs when a planet crosses the face of its host star as seen from our solar system, fractionally diminishing the star's light as it passes. Transits are, by their nature, rare occurrences, since for any particular line-of-sight the chance of a precise planet-star alignment is small, dictated by the ratio of the diameter of the host star to the diameter of the planet's orbit. Thus, larger planets on short-period orbits of small stars are more likely to transit, and large numbers of stars must be surveyed for any transits to be found.

Since they are silhouettes, transits yield a planet's diameter, and reveal a planet's orbital period by their recurrence. However, follow-up measurements using the RV technique can usually give an estimate of a transiting planet's mass, provided the planet is within several hundred light-years of Earth. Pairing the planet's mass and the diameter yields its density, which helps astronomers pin down whether a planet is, for instance, made mostly of gas or of rock.

Sometimes, astronomers can even study a transiting planet's atmosphere as starlight filters through or reflects off it at key points in the planet's orbit. In this way, planet-hunters can obtain information about a transiting planet's atmospheric composition, its temperature, and even what sort of weather it has. Such measurements have already been performed for very large transiting planets around nearby stars. NASA's James Webb Space Telescope, launching no earlier than 2015, might be able to perform similar studies for a handful of smaller, more Earth-like planets that transit small, dim, red stars called M-dwarfs.

Seen from interstellar distances, the transiting Earth would dim the Sun's light by one part in 12,000; that's less than a current-generation iPad dims when it shorts a single pixel that's like detecting a clump of 65 dead pixels out of the nearly 800,000 on a current-generation iPad display. Yet astronomers have already built space telescopes capable of detecting such minute changes in starlight: NASA's Kepler mission uses transits in its 3-year quest to pin down the frequency of Earth-size planets in our galaxy. The 3-year running time is key, as this allows a transit of an Earth-size planet in a yearly orbit of a star to recur 2 or 3 times, ensuring that the diminutive dip in starlight is actually caused by a planet. We'll probably be talking a lot more about Kepler throughout the remainder of my guestblogging tenure at BoingBoing, because the mission is slated to release its next treasure trove of discoveries later today.

The final technique, timing, is really a grab-bag of methods that all use gravity-induced variations in the timing of various astrophysical phenomena to indirectly detect planets. The first exoplanets to be discovered were found by timing the minuscule offset their masses caused on the clockwork regularity of the rotations of neutron stars. Others have been found by the timing variations they induce in the pulsations of the outer layers of stars.

But timing methods really come into their own when they are applied to a transiting planet: Subtle variations in the periodicity or duration of a planet's transit can reveal the gravitational influence of otherwise-unseen companions, potentially even the presence of large accompanying moons. And given that the Kepler mission has the sensitivity and precision to detect such variations in its probable yield of many hundreds of transiting exoplanets, transit-timing appears poised to provide several notable discoveries in the near future.

If you made it this far, congratulations, you're now pretty well-prepared to grok whatever exoplanet-related finds and claims you'll encounter over the next few years—and the next few days, for that matter. The Kepler data release is almost upon us, and I guarantee it will be filled with wonderful surprises.

Want more? In addition to what I've written here, I'd direct you to the Wikipedia page on exoplanet detection, which is extremely comprehensive, as well as a 'Cribsheet' I produced for Seed magazine that visualizes the basics of three techniques. You can also see the tally of planets that have accrued to each technique in the chart I posted Monday.

This chart was assembled from data on the incomparable Extrasolar Planets Encyclopaedia , maintained and curated by the astronomer Jean Schneider. It depicts the 520 exoplanets detected between 1992 and 2010, divided up by detection technique. The accompanying data sheet includes a few notes and caveats about the assumptions I used to generate the chart. There's also an interactive version available.

Finding 520 planets in less than 20 years is an impressive testament to the skill of modern-day planet-hunters. And the rapid, recent acceleration of detections suggests that the next 20 years will see several thousands of additional planets added to our catalog. But given that there are hundreds of billions of stars in our galaxy alone, shouldn't we actually be finding more planets?

Let's assume that planets are an almost inevitable side effect of a star's formation from a collapsing cloud of gas and dust—a hypothesis that is less controversial by the day.

Now, there are still several ways for a star to be bereft of planets. A star can form simultaneously with one or more companion stars, and the group will be collectively bound together by its own gravity; in this case, the gravitational interactions of the stars may disrupt planet formation. Similarly, the gravitational nudge of a passing star could scatter planets from around their sun, casting them away forever into the inky void. Gravitational nudges could also make planets fall into their host stars, the planets' constituent atoms lending a dirty sheen to the surfaces of their suns, like oil slicks on oceans. These and other planet-stripping scenarios are unavoidable occurrences in the universe, so not all stars have planets, but it's still a good bet that most do.

Standing beneath a pristine night sky in the northern hemisphere, an unaided human can, at best, see perhaps 2,500 stars. Another 2,500 or so can be seen in the skies above the southern hemisphere. These ~5,000 stars are visible by virtue of their being very bright or very close to us, so that they flood our eyes with photons, particles of light.

Photons are like currency for astronomers: In general, the more you have, the more you can do. Consequently, the stars we can see with the naked eye on a dark night are, on balance, the easiest for astronomers to study. So if most of these stars have planets, and the stars are so easy to observe, why are there only 500-odd exoplanets known today, and less than 100 known around naked-eye stars, rather than 5,000 or more?

The answer comes in three parts: First, just as there are more small pebbles than giant boulders in the world, small planets are probably far more common than the large ones that are easier to detect.

Second, we haven't been looking long enough or hard enough to detect all these smaller planets around nearby stars; the first exoplanets were only discovered in 1992, and only in the past couple of years have we gained the capability to reliably detect tinier, more prevalent worlds.

Finally, and most importantly, each detection method we use has its own unique observational biases that can blind it to the presence of exoplanets large and small.

It's been my experience that most misunderstandings of exoplanetary discoveries are caused by a lack of familiarity with the capabilities and limitations of each detection method. If you want to be savvy about the search for life beyond our solar system, if you want to be immune against exoplanetary hype and flim-flam, then you need to know the basics of how astronomers find planets in the first place. Tune in tomorrow for a beginner's guide to planet hunting!

"Science—knowledge—only adds to the excitement, the mystery, and the awe of a flower. It only adds. I don't understand how it subtracts."

That's one of the first comments the late, great physicist Richard Feynman makes in a wide-ranging interview from the 1981 television documentary, The Pleasure of Finding Things Out. I recommend you watch it, if you have the time. The title comes from Feynman's description of the visceral thrill that accompanies discovery, a thrill that intensifies in direct proportion with the discovery's profundity and certitude.

I was reminded of Feynman's documentary and quote one day in 2009, during a hike on a telescope-studded Chilean mountaintop with the astronomer Debra Fischer. Fischer is a planet-hunter, one of a handful of individuals around the globe who have discovered dozens of alien worlds, and who are bent on finding more planets like our own. She was using a telescope there to search for terrestrial planets around Alpha Centauri, the nearest neighboring stellar system to our own Sun.

It was a clear, sunny afternoon, with only a single condor spiraling in the sky—a good omen for telescope work, since more soaring condors would have meant hot, rising air, atmospheric disturbance, and muddy views of the night sky through the telescope. We also found an apricot tree, improbably laden with fruit in the midst of what was essentially high desert. Barely touched, the ripe apricots fell into our open hands, and as we paused and ate, we speculated on how the tree had come to be there, and how it had managed to grow and bloom and bear fruit. (The explanation turned out to involve a beneficent groundskeeper who had planted the tree and hooked up an irrigation line, but bear with me.)

You can, of course, trace such a question back to the rarefied abstraction of why there's anything at all rather than nothing whatsoever, but I prefer the more concrete consideration that, without a rocky, warm, wet planet to support a complex biosphere, neither Fischer's apricot nor Feynman's flower could have existed in the first place, let alone be savored and appreciated.

Science is filled with big questions, and astronomy and its subfields are blessed with some of the biggest. For example, where did the universe come from? How is it that its expansion is accelerating? Why is it that time only moves in one direction? These are great, worthy mysteries ... but no matter how many billions of dollars we throw at them, I'm not at all convinced that we'll be definitively answering any of them anytime soon—or that we even know how to properly address them yet.

Looking for other habitable or inhabited planets is different, partially because we already have such a well-characterized template to guide us: Earth, and its defining, life-enabling properties.

Many of our planet's most salient features—its liquid-water ocean, its atmospheric composition, and its global population of living things—appear relatively straightforward to remotely detect across the vast distances of interstellar space. That's largely why I believe that, quite possibly for the remainder of my lifetime, the most profound question that can be answered with reasonable certainty—the most pleasurable thing that can be imminently found out—is the frequency of living worlds around other stars.

I'm admittedly biased (just look at my Twitter feed—it's clear what my interests are), but my argument rests on facts: The research architectures and observational capabilities required to find Earth-like planets in our region of the galaxy, and determine whether or not some of them harbor life, are already reasonably well-defined. Public interest in (if not knowledge of) the search for alien life is high, and nearly universal. And, in comparison to tasks like finding the Higgs boson, establishing the precise nature of dark energy, or experimentally validating string theory, completing much (though not all!) of this "planetary census" simply isn't that expensive.

Imagine if we eventually discover tens, even hundreds of potentially habitable planets within a few hundred light-years of our solar system. Or, instead, imagine that we somehow find no worlds remotely suitable for life as we know it. Either result, and all those in between, would constitute a shocking revelation.

What if we are cosmically alone, on a planet as anomalously unlikely and fertile as a fruit tree flourishing in an arid wasteland, or a flower blooming in a desert? What if worlds like ours are common as grains of sand? Does the universe hum and throb with life, or does eternal silence and sterility reign outside of our small planet? The truth is, no one really knows. But that will soon change. And when it does, this knowledge can only fill our lives, our world, and our future with more excitement, mystery, and awe.

This week, NASA is releasing a new batch of discoveries from its premier planet-seeking spacecraft, Kepler. The editors at BoingBoing have kindly invited me to come aboard to blog about these discoveries, and other insights from exoplanetary research. My goal is to take you on a journey through the past, present, and future of the search for habitable exoplanets, so that you can better understand—and, potentially, even get involved in—this exhilarating scientific frontier.