Searching for Earth's History Among Earth-like Worlds

Mercury Winter 2007 Table of Contents

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by Lisa Kaltenegger

It is only a matter of time before astronomers find an Earth-sized planet orbiting a distant star. We will naturally ask whether the planet is habitable or bears life. But will we be able to tell if there are bacteria, roaming dinosaurs, or even more advanced life?

While the first images of any putative exoplanet will be only visual smudges a single pixel in size, even a low-resolution spectral picture of the planet will be able to tell us a great deal. For examples: from spectra we should be able to infer whether the world, like Earth four billions years ago, was enveloped in a steamy, oxygenless atmosphere and covered completely by an ocean; or, in a Jurrasic-park-like epoch, a distant planet’s atmosphere consists of about three-fourths nitrogen and one-fourth oxygen, with a small percentage of other gases like carbon dioxide and methane.

In the past twelve years, scientists have discovered more than two hundred large planets orbiting other stars, yet finding Earth-like planets is a fascinating and substantially more technologically demanding endeavor. And, once we finally do find them, what are clues to life’s presence on them? Indeed, how do we need to design our instruments so we don’t miss life’s spectral signature?

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