Earth as an Extrasolar Planet

Somewhere in the Milky Way, astronomers have found a world that sports crucial ingredients for life. When they trained a high-resolution spectrograph on starlight reflected from the planet's moon, they picked up traces of ozone, oxygen, sodium, and nitrogen. Alas, the planet is Earth. But the researchers say a similar technique could be used to find signatures of life on planets orbiting other stars.

Astronomers have discovered hundreds of planets beyond our solar system, but they know very little about them. Telescopic surveys usually reveal just basic information about an extrasolar planet's minimum mass, its distance from its parent star, and whether it is likely to be gassy or rocky, like Earth.

To learn more about these alien worlds, astronomers have taken advantage of a phenomenon called stellar occultation. When a planet transits, or passes between its star and Earth, components of the planet's atmosphere subtract some wavelengths from the star's light and add others. By training a spectrograph on this light, scientists can tease out the composition of the planet's atmosphere. In 2001, astronomers led by David Charbonneau of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts, used the technique to detect sodium in the atmosphere of a "hot Jupiter" orbiting a star 150 light-years from Earth.

But no one knew if the approach would work with a much smaller, Earthlike world. Astrophysicist Alfred Vidal-Madjar and colleagues at the Institut d'Astrophysique de Paris decided to test the idea on the most Earthlike world they could find: Earth. To treat Earth like an extrasolar planet, the researchers looked at the sunlight filtered through Earth's atmosphere during a partial lunar eclipse in 2008. In a lunar eclipse, Earth passes between the sun and the moon. The moon's-eye view of sunlight when that happens best replicates what can be seen when an extrasolar planet passes in front of its parent star, says Vidal-Madjar.

Using SOPHIE, a high-resolution spectrograph attached to the Observatoire de Haute Provence's 1.93-meter telescope in southern France, the researchers successfully detected ozone, oxygen, nitrogen, and sodium in the reflected light from Earth's atmosphere. "The surprise was that we succeeded with extremely sparse observations under relatively bad weather conditions," Vidal-Madjar says. "But seeing how easily oxygen was seen strongly argues in favor of high-spectral-resolution searches [of Earthlike extrasolar planets]." The team reports its findings in a paper accepted for publication in Astronomy & Astrophysics.

"This is an important paper," Charbonneau says. "Many of us have great hopes for the use of this method to make the first honest search for life outside the solar system." That's going to be tricky, says planetary scientist James Kasting of Pennsylvania State University, University Park. "The [French team] has done a nice job," he says, "but it is very difficult to get a transit spectrum of an Earthlike planet in the habitable zone of its parent star." The problem, Kasting notes, is that Earthlike worlds would have thin atmospheres, which would be hard to detect, and their small size makes them harder to catch during an occultation.

Those odds may improve if the European Space Agency launches its PLATO (Planetary Transits and Oscillations of Stars) mission in 2017, which should give astronomers a complete atlas of transiting planets within some 300 light-years. Vidal-Madjar says he and colleagues will be waiting to see if they'll have as much luck with these planets as they've had with Earth.