About phase. While orbiting its parent star (not shown), exoplanet CoRoT-1b alternately shows it bright and dark face.

Leiden Observatory, Leiden University

Illuminating an Extrasolar Planet

Since prehistoric times, people have watched the moon waxing and waning through its phases. Four hundred years ago, Galileo was the first to observe similar phases of the planet Venus. Now, for the first time, astronomers have seen the changing phases of a planet outside of our solar system. They expect similar observations in the future to reveal important details about the atmosphere, cloud cover, and climate of planets orbiting other stars.

Most extrasolar planets (or exoplanets for short) can't be seen directly, especially if they are very close to their parent stars; astronomers see only the combined light of the star and the planet. Sometimes, they happen to see the orbits of exoplanets edge-on. In those cases, the planet alternately shows more of its dayside and then more of its nightside as it circles its star. These phases should show up as tiny variations in the intensity of the observed light, researchers have predicted.

Astronomers Ignas Snellen of Leiden Observatory in the Netherlands and colleagues have now demonstrated that such optical observations are feasible. The researchers scanned publicly available data on a star known as CoRoT-1, which is 1600 light-years away. In the numbers, they found minute periodic variations in brightness that they believe are caused by the changing phases of its gaseous giant planet, known as CoRoT-1b.

Because it is less than 5 million kilometers from its parent star, the planet's dayside temperature is about 2000°C, and part of the planet's light is probably thermal glow instead of reflected star light. But, says Snellen, "even if we assume that the observed brightness variation is fully due to reflected star light, we find that the planet's reflectivity is probably lower than 10%." That's much less than the reflectivity of Jupiter, indicating that CoRoT-1b probably lacks extensive clouds. The team reports its results tomorrow in Nature.

"It's a trailblazing result," says astrophysicist Sara Seager of the Massachusetts Institute of Technology in Cambridge. In this pioneering case, the measurements aren't precise enough yet for a detailed analysis of CoRoT-1b, she says, but knowing a planet's reflectivity is key to learning about its cloud cover and climate. "It heralds an interesting new way to characterize planets."

NASA's Kepler satellite, launched in March, will probably reveal many more phase-changing exoplanets, and so might the newly refurbished Hubble Space Telescope and even large, ground-based telescopes. "If it can be done for this planet, you can do it for other systems too," says astronomer Heather Knutson of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts. "Someday, this kind of observation will become commonplace."