O’Donovan’s Giant Planet

We may be down a planet with the recent demotion of Pluto (see box), but the number of giant planets discovered in orbit around other stars continues to grow steadily—around 200, at last count. Now, an international team of astronomers led by Caltech grad student Francis O’Donovan has detected a planet slightly larger than Jupiter that orbits a star 500 light-years from Earth in the constellation Draco. The planet, known as “TrES-2” (pronounced Trace-2) passes in front of a star called GSC 03549-02811 every two and a half days, causing a dimming of its light by about 1.5 percent.

TrES-2 is the first transiting planet—or planet that passes directly between its star and Earth—to be found in an area of the sky known as the “Kepler field,” a piece of celestial real estate about the size of your two hands held together at arm’s length, or twice the bowl of the Big Dipper. NASA’s Kepler mission, set to launch in October 2008, will stare at this patch of sky for four years, and should discover hundreds of planets, both giant and Earth-like. Discovering TrES-2 beforehand allows Kepler’s astronomers to plan additional observations of it, such as searching for moons.

TrES stands for Trans-Atlantic Exoplanet Survey, an effort involving the Sleuth telescope at Caltech’s Palomar Observatory, the Planet Search Survey Telescope (PSST) at Lowell Observatory near Flagstaff, Arizona, and the STellar Astrophysics and Research on Exoplanets (STARE) telescope in the Canary Islands—all three of which were built with off-the-shelf camera lenses and mostly from amateur-astronomy components. TrES-2, the second planet to be found by the survey, was first spotted by Sleuth, which was set up by Caltech postdoc David Charbonneau, now at the Harvard-Smithsonian Center for Astrophysics and a coauthor of the paper. The PSST, which is operated by coauthors Georgi Mandushev and Edward Dunham, corroborated the initial detection.

These small, automated telescopes took wide-field, timed exposures covering thousands of stars at a time over a period of about two months per field. When the software detected regular variations in the light from an individual star, it alerted the astronomers. In order to confirm that the dimming was due to an orbiting planet, and not, say, a small, faint companion star, O’Donovan and his colleagues switched to one of the 10-meter telescopes at the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii, to do detailed spectroscopic observations. Says O’Donovan, “All our hard work was made worthwhile when we saw the results from our first night’s observations, and realized we had found our second transiting planet.”

The paper announcing the discovery will appear in an upcoming issue of the Astrophysical Journal. The 15 other authors include JPL’s John Trauger and Associate Professor of Astronomy Lynne Hillenbrand. —RT