A tale of two planets: What's "Earth-like" mean?

by Lee Billings

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.

PIA06236_modest.jpg 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.

An on-board microphone recorded the eerily familiar sound of rushing, gusting wind.

The probe touched down in what appeared to be a dry riverbed strewn with fluid-polished pebbles.

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.


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