To create her models of what a "hot Jupiter" might look like, Seager adapted a model not of another planet but of a cool star, like our sun. First she altered it to make its temperature closer to that of Jupiter. Then she considered which atoms and molecules would be found on a hot planet in chemical equilibrium. Because sodium, for example, seemed a likely candidate, she added its properties to her model to create a spectral signature indicative of sodium's presence. When HD 209458b was discovered in 1999, Seager, then a newly minted PhD, entered the available data on the planet into her models and predicted the presence of sodium (among other things) in the atmosphere it was presumed to have. Using her models, astronomers designed experiments that had the Hubble Space Telescope look for sodium. In 2001, those experiments yielded the first detection of an extrasolar planet's atmosphere--and confirmed Seager's prediction.

When Seager and her colleagues had the Spitzer observe HD 209458b in 2005, they expected to find evidence of water molecules in the atmosphere. But no such evidence appeared. The researchers did, however, observe what they believe is the spectral signature of silicate clouds, beneath which water vapor may be trapped. ­Seager also hypothe­sizes that on the exoplanet's day side, temperature may be constant throughout the atmosphere, in which case there would be an equilibrium: any evidence of water absorption would be canceled out by evidence of water emission.

Seager isn't surprised when experimental data don't match most of the hundreds of models she's built so far. "That's how nature is more creative than we are," she says, glad it's possible to say with any certainty at all what elements exist on a planet some 150 light-years away. "We can actually characterize the exoplanet atmospheres," she says. "Four years ago, nobody would've believed you could do it."

Eager for more data to plug into her models, Seager is part of an MIT-led effort to develop and, by 2009, launch a private satellite called TESS, which will expand the search for exoplanets. "Spitzer can only look at things we already know and can only see one star at a time," she explains. "This is going to look at literally millions of stars, looking for this little drop in brightness indicative of a planet transit."

Seager hopes to find rocky planets--ideally, orbiting bright stars so there will be enough light to study them. "Gas-giant planets are boring because they have all the gases they were born with," she says. Earth, however, has evolved; for instance, early volcanoes spewed gases, and plants produce an abundance of oxygen. Also, gas giants are too hot for life. "We want to be able to find planets that can support life," says Seager, who "absolutely" thinks there's life beyond our planet.

"We're not going to see any little green people," she cautions, adding that she's not interested in meeting any aliens herself. Most likely, she says, we'll find bacteria. But even that could be revealing. "If we can find life in other places, it may be a clue to where we came from," she says.

Seager believes there's a good chance that we'll detect signs of life on other planets in her lifetime. "But," she quips, "I hope to live a long time."