TrES-4b

TrES-4b is an extrasolar planet, and the fourth-largest ever found, after WASP-12b, WASP-17b, and CT Chamaeleontis b (though the latter may be a brown dwarf). It was discovered in 2006, and announced in 2007, by the Trans-Atlantic Exoplanet Survey, using the transit method. It is 1,430 light-years (440 pc) away at star GSC 02620-00648, in the constellation Hercules.

TrES-4 orbits its primary star every 3.543 days and eclipses it when viewed from Earth. The planet is 0.919 times as massive as Jupiter but 1.799 times the diameter, the largest planet ever found at the time (next to WASP-17b, found on 1 May 2009), giving it an average density of only about 0.333 grams per cubic centimetre. This made TrES-4 both the largest known planet and the planet with the lowest known density at the time of its discovery.

TrES-4's orbital radius is 0.05091 AU, giving it a predicted surface temperature of about 1782 K. This by itself is not enough to explain the planet's low density, however. It is not currently known why TrES-4 is so large. The probable causes are the proximity to a parent star that is 3–4 times more luminous than the Sun as well as the internal heat within the planet.