Planet-Hunting Spacecraft Shows Its Stuff by Detecting a Known Exoplanet

Kepler's sensitivity to the orbit of a catalogued exoplanet bodes well for its ability to find Earth-like worlds

By John Matson

NASA's Kepler spacecraft, which may soon help scientists put our planet in its galactic context by showing how common Earth-like worlds are throughout the Milky Way, is off to a good start.

The space telescope, which was launched in March and began its science mission in May, will spend more than three years observing a patch of 100,000 stars near the northern constellations Cygnus and Lyra. If those stars have planetary systems aligned with Kepler's line of sight, the spacecraft's photometer should be able to detect the periodic dimming caused by the planets as they transit, or pass in front of, their stars.

Hundreds of exoplanets—planets beyond our solar system—have already been detected from the ground and from other spacecraft via transit searches and other methods. But the current exoplanet catalogue primarily reflects the low-hanging fruit—extremely large planets in tight orbits, whose visible or gravitational effects on their stars are more pronounced. Many of these planets are known as hot Jupiters.

Kepler's mission is to seek out smaller worlds more like our own, ideally in comfortable, life-enabling orbits in their respective stars' so-called habitable zone.

A paper in this week's Science, using 10 days of early data gathered by Kepler, demonstrates the spacecraft's ability to spot large planets and provides encouragement that Earth-size bodies are within its reach.

In the study, the Kepler team shows that the data clearly reveal the dimming caused by the periodic transit of HAT-P-7 b, an exoplanet nearly twice the mass of Jupiter that orbits the star HAT-P-7, about 1,000 light-years away. The exoplanet, discovered last year by ground-based observatories, orbits so close to its star that it completes a loop in just 2.2 days—making it a very hot Jupiter.

"It is so hot, in fact, that it glows, like the heating element in your oven or toaster," says Kepler deputy principal investigator David Koch of NASA Ames Research Center at Moffett Field, Calif. In addition to the dip in starlight as HAT-P-7 b passed in front of its star, Kepler was able to detect the exoplanet's glow, which appears in the data as increased emission from the star when the planet is visible alongside of it.

Crucially, Kepler also detected a slight dip in luminosity, much less dramatic than the dimming associated with the planet passing in front of the star, when HAT-P-7 b passed behind its star—the spacecraft was seeing only the star's light, without the reflection and glow from the exoplanet.

"When the planet is orbiting the star, when it goes in front of the star, of course, you see the transit—the planet is blocking the light from the star," Koch says. "When the planet goes behind the star, the star is now blocking the light from that glowing, red planet, and that causes what's called an occultation."

Occultation is a much less pronounced phenomenon than a planetary transit, so Kepler's ability to track the occultation of a large planet indicates that it will be able to detect the transit of a smaller one.

The key observation from the new research is that the small dip in the HAT-P-7 b light curve when the planet passes behind its star "is roughly equivalent to the signal of an Earth-size planet when it passes in front of its parent star," says Paul Kalas, an astronomer at the University of California, Berkeley, who is not part of the Kepler team.

Detecting smaller, cooler planets is a long process. The only truly Earth-like planet we know of—ours—takes more than 150 times as long as HAT-P-7 b does to circle its star, so collecting data on similar planets across multiple orbits will take years.

Even the new hot Jupiters and other close-orbiting planets that Kepler finds will take extensive follow-up observations from the ground to confirm. (As Koch points out, a star's periodic dimming can be explained by one star eclipsing another in a binary star system.) Koch says he expects the first large exoplanets discovered by Kepler to begin rolling out early next year.

In the meantime, the early evidence that Kepler will be able to detect Earths is "absolutely convincing," Kalas says. "Essentially, they are offering a window into the scientific future: Kepler will soon detect an Earth-sized planet outside of the solar system."