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Planetary News: Extrasolar Planets (2006)

Jupiter-Sized Extrasolar Planet Found Using Novel Small Telescope Network

 

September 11, 2006
'Hot Jupiter' at the star HD 209458
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Credit: G. Bacon (STScI/AVL)

An international team of astronomers has detected a planet slightly larger than Jupiter that orbits a star 500 light-years from Earth in the constellation Draco. The newly discovered planet is known by "TrES-2" and passes in front of the star "GSC 03549-02811" every two and a half days.

Discoveries like these are getting more and more common (the first extrasolar planet was detected in 1995, and now we know of nearly 200 planets orbiting stars other than our Sun). What's especially noteworthy about this new planet is how it was discovered -- through a network of small automated telescopes, consisting of mostly amateur-astronomy components and off-the-shelf 4-inch camera lenses. A group of professional astronomers used these humble telescopes to search for transiting planets (planets that pass in front of their home star). The discovery of TrES-2 is the third transiting planet found using telescopes similar to those used by many amateur astronomers.

By definition, a transiting planet passes directly between Earth and the star it orbits, which causes a slight reduction in the light in a manner similar to that caused by the Moon's passing between the Sun and Earth during a solar eclipse. According to Francis O'Donovan, an Irish graduate student in astronomy at the California Institute of Technology, "When TrES-2 is in front of the star, it blocks off about one and a half percent of the star's light, an effect we can observe with our telescopes.

"We know of about 200 planets around other stars," says O'Donovan, lead author of the paper announcing the discovery in an upcoming issue of the Astrophysical Journal, "but it is only for nearby transiting planets that we can precisely measure the size and mass of the planet, and hence study its composition. That makes each new transiting planet an exciting find. And because TrES-2 is the most massive of the nearby transiting planets, it sets a new limit to our understanding of how these gas planets form around stars."

TrES-2 is also noteworthy for being the first transiting planet discovered in an area known as the "Kepler field," which has been singled out as the targeted field of view for the upcoming NASA Kepler mission. Using a satellite-based telescope, Kepler will stare at this patch of sky for four years in hopes of discovering hundreds of giant planets and Earth-sized planets. Finding a planet in the Kepler field with the current method allows astronomers to plan future observations with Kepler that include searching for moons around TrES-2.

And finally, the research team hails the discovery as the second transiting "hot Jupiter" found with the Trans-Atlantic Exoplanet Survey (TrES), an effort involving the "Sleuth" telescope at Caltech's Palomar Observatory in San Diego County, 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. The name of the planet, TrES-2, is derived from the name of the survey.

Artist's Rendering of Transit of HD209458
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Artist's Rendering of Transit of HD209458
In 1999, astronomers witnessed for the first time a distant planet passing in front of its star. The passage of a planet between a star and the Earth is called a "transit." If such a dimming is detected at regular intervals and lasts a fixed length of time, then it is very probable that a planet is orbiting the star and passing in front of it once every orbital period. Credit: Lynette Cook

To look for transits, the small telescopes are automated to take wide-field timed exposures of the clear skies on as many nights as possible. When an observing run is completed for a particular field -- usually over an approximate two-month period -- the data are run through software that corrects for various sources of distortion and noise.

The end result is a "light curve" for each of thousands of stars in the field. If the software detects regular variations in the light curve for an individual star, then the astronomers do additional work to see if the source of the variation is indeed a transiting planet. One possible alternative is that the object passing in front of the star is another star, fainter and smaller.

In order to confirm they had found a planet, O'Donovan and his colleagues switched from the 10-centimeter TrES telescopes to one of the 10-meter telescopes at the W. M. Keck Observatory on the summit of Mauna Kea, Hawaii. Using this giant telescope, they confirmed that they had found a new planet. O'Donovan says, "Each of us had spent countless hours working on TrES at that point, and we had suffered many disappointments. 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."