(2 of 2)
Our planet, seen by extraterrestrials, would have looked very different depending on the point at which it was being observed. Take a look at us 3.9 billion years ago, and we would have had a brown, globe-girdling ocean and an atmosphere made mostly of hydrogen sulfide, carbon dioxide and nitrogen. Not exactly the rain forest. Check in 2.4 billion years ago, and Earth's atmosphere was mostly nitrogen, carbon dioxide and methane; blue-green algae were blooming in the seas. Not long after that, photosynthesis began flooding the atmosphere with oxygen, leading to an explosion of modern forms of life.
Every bit of this could have been observed by faraway civilizations studying Earth with a technique known as spectral analysis. Since light coming from a planet breaks down in different wavelengths depending on its chemical composition, all you need to know is which elements are represented by which spectra and you can figure out what's going on in the atmosphere. We could make the same observations about other worlds. "We've determined how this spectral fingerprint looks for a young and an older Earth," Kaltenegger says. "We use that as an alien ID chart for other planets."
Kaltenegger is actually ahead of the curve this year. Telescopes can't yet resolve exoplanets visually; their existence and nature are inferred mostly by how they cause their parent stars to wobble and by the amount of starlight they block as they pass in front of them. In 2017, though, NASA will launch the Transiting Exoplanet Survey Satellite (for which Kaltenegger is a mission scientist), specifically looking for exoplanet atmospherics. Next is NASA's James Webb Space Telescope and then the European Extremely Large Telescope in the Chilean desert.
In the meantime, Kaltenegger is not waiting. In 2014 she and her team will specifically model 100 alien worlds potentially harboring different kinds of life forms to determine what they would look like from Earth. They will also model potentially habitable moons orbiting gas-giant planets. "With billions of rocky worlds," she says, "life would have to be extremely picky not to be able to evolve out there, wouldn't you say?"