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Astronomers Find Largest Exoplanet to Date

PASADENA, Calif.—An international team of astronomers has discovered the largest-radius and lowest-density exoplanet of all those whose mass and radius are known. It is a gas-giant planet about twice the size of Jupiter, and is likely to have a curved cometlike tail. It has been named TrES-4, to indicate that it is the fourth planet detected by the Trans-atlantic Exoplanet Survey (TrES) network of telescopes.

TrES-4 is in the constellation Hercules and is the 19th transiting planet discovered so far. It orbits the star catalogued as GSC02620-00648, which is about 440 parsecs (1,435 light-years) away from Earth.

A transiting planet is one that passes directly in front of its host star as seen from Earth. When a transiting planet passes between its star and Earth, the planet blocks some of the light from the star in a manner similar to that caused by the moon's passing between the sun and Earth during a solar eclipse. In the case of TrES-4, this reduces the starlight by one percent, a tiny, yet detectable, effect.

TrES-4 is noteworthy for having a radius 1.67 times that of Jupiter, yet a mass only 0.84 times Jupiter's, resulting in an extremely low density of 0.222 g cm-3. In comparison, Jupiter has a density of 1.3 g cm-3. The density of TrES-4 is so low that the planet would float on water.

The transiting planet also causes the star to undergo a small orbital motion, but measuring this effect (from which we can tell the mass of the planet) requires much larger telescopes, such as the Keck 10-meter telescope in Hawaii, as was used in the case of TrES-4. Measuring the mass of the planet is a vital step in confirming that the transiting object is indeed a planet and not a star.

"We continue to be surprised by how relatively large these giant planets can be", says Francis O'Donovan, a graduate student in astronomy at the California Institute of Technology who operates one of the TrES telescopes. "But if we can explain the sizes of these bloated planets in their harsh environments, it may help us better understand our own solar system's planets and their formation."

The study's lead author, Georgi Mandushev of Lowell Observatory in Arizona, noted the challenges such big planets present for theories of planet formation and evolution: "This find presents a new puzzle for astronomers who model the structure and atmospheres of giant planets. It highlights the diversity of physical properties among giant planets around other stars and indicates that we can expect more discoveries of unusual and enigmatic exoplanets in the near future."

TrES is a global network of three small telescopes utilizing mostly amateur-astronomy components and off-the-shelf four-inch camera lenses: Sleuth telescope at Caltech's Palomar Observatory in San Diego County; the Planet Search Survey Telescope (PSST) at Lowell Observatory; and the STellar Astrophysics and Research on Exoplanets (STARE) telescope in the Canary Islands.

Planet TrES-4 makes a complete revolution around its parent star every 3.55 days, so a year on this planet is shorter than a week on Earth. The planet is about seven million kilometers away from its star—over ten times closer than Mercury is to the Sun—and so it is heated by the intense starlight to about 1600 degrees Kelvin (about 2300 degrees Fahrenheit).

In terms of mass and distance to its sun, TrES-4 is similar to HD209458b, and like that planet, it may have an extended outer atmosphere. Astronomers hypothesize that the outer atmospheric layers may be able to escape the planet's gravity and form a curved cometlike tail.

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 approximately 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 the 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 accurately measure the size and other properties of TrES-4, astronomers used the 0.8-meters telescope at Lowell Observatory, the 1.2-meter telescope at the Whipple Observatory (both in Arizona) and the 10-meter Keck Telescope in Hawaii.

Observations were carried out from September 2006 to April 2007.

The paper about the discovery of this extrasolar planet, "TrES-4: A Transiting Hot Jupiter of Very Low Density," has been accepted for publication by the Astrophysical Journal.

The paper's authors are Georgi Mandushev and Edward Dunham of Lowell Observatory; Francis O'Donovan, a graduate student at Caltech; Lynne Hillenbrand, an associate professor of astronomy at Caltech; David Charbonneau (Alfred P. Sloan Research Fellow), Guillermo Torres, David W. Latham, Gáspár Bakos (Hubble Fellow), Alessandro Sozzetti, José Fernández and Guilbert Esquerdo of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts; Mark Everett of the Planetary Science Institute in Tucson, Arizona; Timothy Brown of Las Cumbres Observatory Global Telescope Network; and Markus Rabus and Juan A. Belmonte of the Instituto de Astrofísica de Canarias in Tenerife, Spain.

This research is funded by NASA through its Origins program. The paper is available online at

Written by Robert Tindol

Caltech Media Relations