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Kepler sees changing phases of giant planet

Posted: August 06, 2009

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NASA’s exoplanet seeking spacecraft, Kepler, has been confirmed to be up and running, and in doing so has captured the light of a gas giant orbiting a star over a thousand light years away.

Kepler was launched in March on a mission to search for planets ranging from gas giants to Earth-sized worlds orbiting around 100,000 stars in the direction of the constellation Cygnus. Kepler won’t be imaging the planets directly; instead it will watch for transits, as planets cross in front of their suns, blocking some of the starlight. The resulting dip in starlight can be as small as 0.01 percent, but now calibration tests on the spacecraft have proven that its photometers (light collectors) are fully functional and operating to the required sensitivity. In the process, Kepler has observed the light of a gas giant orbiting just 5.9 million kilometres from its star.

An artist's impression of a hot jupiter like HAT-P-7b. Image: ESA/C Carreau.

The planet, HAT-P-7b, was already known about having been discovered in 2008. Its orbital properties were well-known, making it an ideal test for Kepler. A light curve (a graph showing how the light of the star rises and falls during a transit) of the star and its planet taken from the ground has a lot of scatter, brought about by the distorting effects of our own atmosphere. The dip in the light as the planet passes in front of its star can be seen, but none of the real details of the light curve.

Comparing the light curves of HAT-P-7b taken from the ground and by Kepler in space. Image: NASA.

Kepler’s light curve of the star HAT-P-7 and its planet was crystal clear, and when magnified a 100 times, a breathtaking discovery was made. There was a smooth, rather than sharp, fall and rise in brightness during the transit, caused by Kepler detecting the varying light from the phases of the planet as it orbited its star. The gradient of this curve shows that the temperature of the planet on its day-side is a whopping 2,300 degrees Celsius. But more was to come. Around a day after the transit, with the light curve magnified a hundred times, a second dip in the light occurs, but this is not a second transit. Rather, it is the occultation of the planet as it passes behind its star. Not only was Kepler able to detect the light of the gas giant, determine its phases and know when it had vanished from view behind its sun, but this second, tiny dip in light is equivalent to that expected from the transit of an Earth-sized planet.

“Kepler’s detectors are working so well it proves that we can find Earth-like planets,” says Kepler’s principal science investigator, William Borucki of NASA’s Ames Research Center. “It is the first time anyone has ever seen the light from this planet.”

Kepler's light curve of HAT-P-7b, magnified by seven and a hundred times. At a 100x magnification, the gradual rise and fall of the main transit, and the second dip, can be clearly seen. Image: NASA.

Though its science mission has only been underway a couple of weeks, Kepler has already identified hundreds of candidate planets, but it will take some time to confirm them. All kinds of phenomena could cause a star’s light to dip, from starspots to eclipsing binary systems. To rule these out, three consecutive transits have to be observed to prove that it is a planet, and then follow them up with large ground-based telescopes to gather additional data. Borucki hopes to be able to announce the first set of results, chronicling the discoveries of hundreds or maybe even thousands of so-called hot jupiters – gas giants close to their stars orbiting in a matter of days or weeks – early next year. We’ll have to wait a few years longer though for news on potentially habitable Earth-like planets at distances from their stars where it is warm enough for liquid water; to observe three transits of them will take three years (if they are at the same distance from their sun as Earth is). “It will be 2012 before we can tell whether Earth is common, or whether we’re alone,” says Borucki.

“The headline today is that Kepler works,” adds exoplanet hunter Dr Alan Boss of the Carnegie Institution in Washington. “Now we just have to let it get on with the job.”

For an animation showing the light curve and the phases of the planet, visit the Kepler website