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Probing Distant Atmospheres
by Paul Gilster, gilster_at_centauri-dreams_dot_org

Hunting for terrestrial planets is not going to be easy, and even when we start getting images of such worlds, there will be plenty of questions to answer. How to detect life on a terrestrial planet was one of the subjects that came up last September at the Pale Blue Dot workshop at Adler Planetarium in Chicago. Cassini's recent picture of Earth from Saturn space, much like Voyager's 'pale blue dot' image of 1990, reminded everyone at the conference of our fragile place in the cosmos. It also forced the question of how we might find other such worlds.

And finding a blue planet in a star's habitable zone isn't enough. As laid out in a JPL backgrounder, the key will be to gather enough spectral data to make a judgment call that could change how we view our place in the universe. Breaking down the light from a distant planet should tell us much about its chemical composition. Carbon dioxide and water vapor, for example, are both clues to life, their dual presence suggesting both an atmosphere and an ocean.

But even liquid water isn't sufficient to make the call. In reality, we'll need a combination of things. Oxygen is useful because it suggests plant life or some kind of living cycle to produce it and keep it in the atmosphere. Methane likewise suggests life processes at work, though by itself it's not sufficient (we can certainly find the stuff in places where life seems less likely, as witness Titan). So here's the take from the JPL story:

Scientists say that oxygen is a more reliable sign of life than methane, but if they found large quantities of both, they'd be more convinced. "Finding two of these molecules together would be much better than one. The more, the better," said Dr. Victoria Meadows of NASA's Spitzer Science Center, Pasadena, who served as chair of the third Pale Blue Dot conference. "For example, if we found carbon dioxide, oxygen and water vapor, in addition to methane, then we'd be pretty convinced that we were looking at an environment like our own."

Inevitably, the hunt for extraterrestrial life looks first for the kind of life we find on Earth. But we may have to widen that view, and the key is to make as few assumptions as possible. For if we've learned one thing from the 200+ extrasolar planets found thus far, it's that solar systems around other stars can be utterly different from anything we had imagined. Finding that alien blue dot with the right mix of chemicals in its atmosphere would be profoundly suggestive, but it doesn't rule out more bizarre abodes of life that we don't yet know how to categorize. Not all those pale, living dots are going to be blue.

This editorial first appeared on Centauri Dreams, and is used here by the kind permission of the author.

Disclaimer: The opinions expressed in editorials are those of the individual authors, and do not necessarily reflect the position of The SETI League, Inc., its Trustees, officers, Advisory Board, members, donors, or commercial sponsors.

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