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.
Voyager 1, our civilization's furthest and fastest emissary into space. Traveling at 17 kilometers per second, Voyager 1 still would take some 73,000 years to reach the nearest star.
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.
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.
Alpha Centauri, the Sun's nearest neighboring star system, seen by the Cassini orbiter above the limb of Saturn. / NASA/JPL/Space Science Institute
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.
An artist's rendition of a habitable moon orbiting a gas-giant planet. / David A. Aguilar, Harvard-Smithsonian Center for Astrophysics
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.
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.
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.
In early 2005, a sophisticated metal-and-plastic package known as the Huygens Probe entered the atmosphere of Titan, Saturn's largest moon, and drifted on parachutes into alien terrain. The probe carried a diverse payload, including a camera, a microphone, instruments to sniff Titan's atmosphere and prod its surface, and even a DVD disc encoded with more than 600,000 digitized signatures from people on Earth.
It was very far from home, further than any lander had ventured before, separated by more than 7 years and a billion kilometers from its launchpad in Cape Canaveral, Florida. And ironically, in making the most distant landfall our civilization has ever achieved, Huygens also hinted that Titan could well be the closest we would ever come to visiting another reasonably Earth-like world.
Layers of haze and photochemical smog cloud Titan's upper atmosphere. Similar atmospheric chemistry may have held sway billions of years ago on the early Earth. NASA/JPL/Space Science Institute
As it descended, Huygens' cameras captured views of what looked like shorelines and deltas, and a suite of instruments sampled air that was at a temperature near the triple point of methane—the small subset of atmospheric conditions in which methane can exist simultaneously as a solid, a liquid, or a gas. The air was dense with nitrogen and laced with complex organic compounds.
Later, observations from Huygens' sister spacecraft, the Cassini Orbiter, would discover Titanian lakes and seas, formed from seasonal hydrocarbon rains pooling in the moon's basins, valleys, and craters. Cassini also found evidence of a subsurface water ocean, and volcanoes that spew molten water instead of magma. Chilled to nearly -200°C by remoteness from the Sun, water at Titan's surface behaves more like rock. Some scientists suspect simple life already exists there, though not in any form familiar to us.
All this, of course, only makes Titan, at best, a frozen chimera of Earth, not a mirror-image.
Titan may now resemble our planet in its earliest history, when Earth was a rather alien world shrouded in an anoxic haze of nitrogen, carbon dioxide, water vapor, and hydrocarbons. Mars and Venus are presently our closest planetary siblings. What the Huygens and Cassini observations clarify, however, is how in the future all this will change.
In roughly 7 billion years, our star will deplete its supply of hydrogen and begin fusing its more energy-dense helium, reddening and ballooning to more than 250 times its current size in the process. Titan and its water-laden mother lode of frigid organic feedstock will thaw. For a scant few hundred million years, it will in all probability be the most Earth-like spot in the solar system, remarkably similar in temperature and atmospheric pressure to our vanished planet, which, if not already engulfed and destroyed by the expanding Sun, will have been scorched to a cinder.
Maybe then more complex, almost "terrestrial" life forms will arise on Titan. Maybe, in their worldly wanderings, these organisms will encounter a strange, corroded mass of metal and plastic, and the rainbow-speckled disc it bears, filled with unknowable memories from a forgotten former world.
Clearly, "Earth-like" can encompass a host of meanings and uses. Does Titan's potential habitability in the distant future mean it should be considered more "Earth-like" than present-day Mars or Venus? For that matter, does Earth's rather unearthly past somehow change our conception of its key identifying features? Given the diversity of planetary environments possible in space and the potential magnitudes of their fluctuations over time, what should "Earth-like" even mean? The term makes for convenient shorthand, but absent a strictly standardized universal definition, it risks causing more problems than it solves. Then again, interpreting the meaning too narrowly raises its own concerns, notably the possibility that astronomers will find nothing at all meeting such exacting criteria.
It seems that as we discover worlds ever-closer to our own in size, mass, orbit, and so on, as we unveil more of the universe's planetary mysteries, it will become more and more crucial to either precisely delineate what we mean when we say "Earth-like," or to abandon the phrase entirely. A prescient editorial in this week's Nature eloquently makes these same arguments, and offers up a working definition: To be Earth-like, a planet should be of similar size to Earth, orbiting in the habitable zone of any star, and not tidally locked.
It's a start, and perhaps the best broad definition possible at this time. But the temptation to link habitability to familiar, comfortable, everyday things beckons, no matter how impossible they are to observe remotely. Both an infrared flux and the glint of morning dew on grass can speak to water's triple-point. The roar of wave-pounded surf comes from oceans, and the whisper of wind-rustled leaves come from plants, as do spectroscopic measurements of chlorophyll or specularly reflected starlight. The taste of wine, bread, or cheese is unquestionably a more pleasurable manner of confirming a planet's microbial fever than measuring atmospheric methane and nitrous oxide, and taking in the rising and setting of a sun will always be the ideal gauge of a day. The most powerful way to measure a planet's potential for life is simply to live there.
What do you think? How many equally valid definitions of "Earth-like" exist, and which one should astronomers use as they search for habitable worlds beyond our solar system?
Lee Billings is a science writer and editor whose work has appeared in publications like Seed and Nature. A few of his favorite topics are space and planetary science, video games, deep time, and hard science fiction.
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.
A hazy view of a $5 quadrillion binary-planet system, the Earth and the Moon, as seen from Mars, a $14,000 world. Taken by Mars Global Surveyor on May 8th, 2003 at 9:00 AM, Eastern Daylight Time.
[Maggie's Note: Yesterday, guest blogger Lee Billings introduced us to new batch of planets and planet candidates found by NASA's Kepler mission. If you missed that post, go back and read it first. Today is all about how much those new planets are worth to us.]
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.
An artist's rendition of Kepler-11, a newly announced system of 6 confirmed transiting exoplanets that will be a laboratory for planet-formation theories for years to come. If these 6 worlds were somehow transplanted into our own solar system, all of them would lie within the orbit of Venus, and 5 would lie within the orbit of Mercury. How this "packed" planetary system was formed is a puzzle for astronomers. NASA/Tim Pyle
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.
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.
The 2004 transit of Venus across the Sun, as viewed by NASA's Transition Region and Coronal Explorer (TRACE) satellite. The faint red ring around Venus is a consequence of sunlight scattering off its atmosphere.
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.
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?
"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.