Keck Confirms Largest Exoplanet To Date

Mauna Kea (August 6th, 2007) 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 comet-like tail. It has been named TrES-4, as the fourth planet detected by the Trans-atlantic Exoplanet Survey (TrES) network of 10-cm telescopes.

TrES-4 is in the constellation Hercules and is the 20th transiting planet discovered so far. It orbits the star catalogued as GSC02620-00648, about 440 parsec (1435 light-years) away from Earth.

A transiting planet is a planet that passes directly in front of its host star as seen from Earth. When a transiting planet passes between its star and the 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 1 percent, a tiny effect, yet detectable even with the small TrES telescopes.

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-m 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.

TrES-4 is noteworthy for having a radius 1.67 times Jupiter, and a mass only 0.84 times the mass of Jupiter, leading to the extremely low density of 0.222 g cm-3. As a 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.

“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 planets and their formation.”

The study’s lead author, Georgi Mandushev from Lowell Observatory, 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; Planet Search Survey Telescope (PSST) at Lowell Observatory; and 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 7 million kilometers away from its star – over ten times closer than is Mercury 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 this 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 comet-like 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 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 accurately measure the size of the TrES-4 planet, astronomers used the 0.8-m telescope at the Lowell Observatory in Arizona, the 1.2-m telescope at the Whipple Observatory, also in Arizona, and the 10-m Keck telescope in Hawaii. The latter has proven to be essential for the confirmation of all four of the TrES planets.

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 submitted for publication by the Astrophysical Journal.

The paper’s authors are:
Georgi Mandushev and Edward Dunham of the Lowell Observatory (Flagstaff, AZ);
Francis O’Donovan and Lynne Hillenbrand of the California Institute of Technology (Pasadena, CA)
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 (Cambridge, MA);
Mark Everett of the Planetary Science Institute (Tucson, AZ);
Timothy Brown of the Las Cumbres Observatory Global Telescope
Markus Rabus and Juan A. Belmonte of the Instituto de Astrofisica de Canarias in Tenerife, Spain;

This research is funded by NASA through the Origins of Solar Systems Program. The paper will be available online at

Press Release courtesy of California Institute of Technology