TrES-4 is biggest and lowest-density exoplanet ever discovered

William Atkins
Wednesday, 08 August 2007 20:15

Science - Space

TrES-4 was discovered by an international team of astronomers with the Trans-Atlantic Exoplanet Survey (TrES). What makes it an unusual type of planet it that TrES-4 is just under twice the diameter of Jupiter (the largest planet in the solar system) but only approximately three-quarters the mass.           

Like any object with this density, if dropped into water, it would float. Its lead author, Georgi Mandushev, Lowell Observatory (Arizona, United States), states that “its density of 0.2 grams per cubic centimeter is somewhere between a wine cork and balsa wood”.

The star (catalogued as GSC 02620-00648) in which TrES-4 orbits is about 1.2 times as massive as the Sun. However, it has already depleted its reserves of hydrogen (thus, is considered a subgiant), so it is rapidly turning into a red giant star. Because of its closeness to its parent star—TrES-4 orbits only about 4.4 million miles (7 million kilometers) away—scientists estimate that within about one million years the star will grow large enough so that TrES-4 will be consumed.

Currently, however, the exoplanet TrES-4 orbits its star in only about 3.55 earth-days, as compared to about 365 days for the Earth to orbit once around the Sun.

An exosolar planet, or exoplanet, is any planet that orbits a star other than the Sun. As of July 2007, according to the Interactive Extra-solar Planets Catalog, the number of known exoplanets is 248. A later count will, no doubt, raise that number by at least one.

TrES-4 is about 1,400 light-years from the Sun. It is located in the direction of the constellation Hercules. It is considered a gas planet, like Jupiter, but it much hotter, at about 2,400 degrees Fahrenheit (1,315 degrees Celsius). It is composed mostly of hydrogen gas.

Mandushev states that, “There is probably not a really firm surface anywhere on the planet. You would sink into it.” He goes on to say that scientists, “can’t understand why these so-called fluffy planets are so fluffy. It really is a mystery, just how they can be so low-density.”

Thus, the astronomers studying TrES-4 are puzzled by its huge size but low density. By finding out more about such planets, they hope to learn more about the planets and make up our solar system and the general nature of planet formation throughout the Milky Way galaxy and beyond.
 
In fact, Edward Dunham, one of the scientists with the discovery, states, "TrES-4 appears to be something of a theoretical problem. It is larger relative to its mass than current models of superheated giant planets can presently explain. Problems are good, though, since we learn new things by solving them."

The discovery was first announced on Monday August 6, 2007, in the Astrophysics Journal. The paper is entitled, “TrES-4: A Transiting Hot Jupiter of Very Low Density". The astronomers used telescopes at the Lowell Observatory, Palomar Observatory (California, United States), and on the Canary Islands (Spain). Specifically, the Lowell Observatory’s Planet Search Survey Telescope (PSST) first observed the planet. It was later verified by the Sleuth telescope at Palomar Observatory.

The authors of the paper are: Georgi Mandushev and Edward Dunham (Lowell Observatory); Francis T. O’Donovan and Lynne Hillenbrand (California Institute of Technology); David Charbonneau, Guillermo Torres, David Latham, Gáspár Bakos, Alessandro Sozzetti, and José Fernández (Harvard-Smithsonian Center for Astrophysics); Mark Everett and Gilbert Esquerdo (Planetary Science Institute); Markus Rabus and Juan Belmonte (Instituto de Astrofísica de Canarias in Tenerife, Spain); and Timothy Brown (Las Cumbres Observatory Global Telescope).