Sara Seager studies worlds beyond our solar system.
As a child, Sara Seager was convinced that the moon followed her wherever she went. The thrill of peering at it through a telescope when she was five is one of her earliest memories.
As Seager was getting that first good look at the moon in 1976, astronomers at NASA were already discussing the need for a space-based infrared telescope. When the Spitzer Space Telescope launched into orbit nearly three decades later, Seager herself would be one of the scientists using it to study the atmospheres of planets beyond our solar system–planets that no one was sure existed 10 years earlier.
By 1995, the moon had followed Seager to grad school at Harvard, where she was choosing a PhD thesis topic. That fall, Swiss scientists announced they had spotted a planet orbiting a star in the constellation Pegasus–the first of what would soon be several planets detected outside our solar system. Seager wrote her thesis on how the atmospheres of “hot Jupiters”–extrasolar planets that are giant and gaseous, like Jupiter, but much closer to their stars and thus more than 10 times hotter–are affected by radiation from their stars. Today, she’s considered a pioneer in the study of extrasolar planets, or exoplanets. “In some fields you just make incremental progress toward questions that have been there for decades,” she says. “In this field, we come up with as many questions as answers.”
With more than 200 exoplanets now documented, researchers are understandably enthusiastic about having so much new territory to explore and explain. “People are excited, and they just want to do new things but often aren’t as careful as they should be,” says Seager. “It’s like the Wild West. Things happen so fast, you just do things. Then you leave.” Eager to bring more sophistication to the field, she joined the MIT faculty in January to launch a new program in extrasolar planets and is developing an exoplanet course she’ll teach in the fall. “I came here to bring some of the knowledge and tools [of MIT atmospheric science] into exoplanet atmosphere research,” she says. As the Ellen Swallow Richards Associate Professor in the Department of Earth, Atmospheric, and Planetary Sciences, Seager now rubs shoulders with meteorologists and atmospheric scientists as well as fellow astronomers. Topographical maps of the United States and the world frame the doorway of her Green Building office, where she ponders planets well beyond our solar system.
Seager’s “lab” consists of her brain and her computer, which is stocked with data collected above Earth’s atmosphere by the Spitzer telescope. Her office contains little else. Boxes of files from her last job, as a senior researcher at the Carnegie Institution of Washington, line one wall; unpacking them would take time away from her research. A blackboard hangs on another, covered with diagrams illustrating techniques she and others developed to glean clues about exoplanets’ atmospheres.
When astronomers discovered the first exoplanet, they were shocked to find it seven times closer to its star than Mercury is to the Sun. Its proximity to a blindingly bright star made it extremely difficult to study. But Seager knew that before long, someone would find an exoplanet that, when viewed from Earth, would transit in front of its star and then disappear behind it. (The probability of a close-orbit exoplanet transiting its star is about 10 percent.) Astronomers could then use a method devised in the mid-1900s for studying eclipsing binary stars: by measuring the slight drop in the star’s light when the exoplanet passed in front of it, they could calculate the ratio of the planet’s area to that of its star.
Sure enough, in 1999 astronomers observed the seventh close-orbit exoplanet transiting its star some 904 trillion miles from Earth. A gaseous giant with temperatures that can top 1,300 °C, the exoplanet HD 209458b is classified as a “hot Jupiter.” It’s also one of just two known transiting exoplanets whose stars shine enough light through their atmospheres to give astronomers the data they need to do detailed studies.
The only way astronomers can learn about the atmosphere of another planet is to study the planet’s radiative transfer, or the propagation of light through its atmosphere. But HD 209458b is so close to its star that it completes its orbit in three and a half days, and even the Spitzer–which detects infrared light to the level of 1 part per 1,000–can’t on its own distinguish the planet’s light from the star’s. So as part of a research group based at NASA’s Goddard Space Flight Center, Seager and her colleagues used a simple calculation to isolate the light of the planet. With the Spitzer’s infrared spectrograph, the researchers measured the light of the star and planet together (when both are visible) and subtracted the light from the star alone (when it eclipses the planet).
The spectrograph, which functions like a prism, also separated the planet’s light into its component wavelengths. The researchers analyzed the light at each wavelength, looking for features that would identify molecules in the exoplanet’s atmosphere. In the February 22 issue of Nature, they reported gathering the first spectral data from HD 209458b over the course of two eclipses in July 2005–and explained their somewhat surprising findings.
To do their spectral analysis–and even to decide what to look for in the first place–the researchers used models of possible exoplanet atmospheres that Seager had developed. When starlight shines on a planet, photons encounter molecules in the atmosphere; depending on what molecule it hits, a photon might get absorbed, scattered, or reëmitted at a different wavelength. By observing the photons that emerge from the atmosphere, Seager can determine what molecules the atmosphere contains. “Photons are our currency,” she says.
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.”