Bloated planet

Imagine a planet nearly twice the diameter of giant Jupiter, yet puffy enough to float on water. Discovered by a team led by Georgi Mandushev of the Lowell Observatory in Flagstaff, Ariz., this alien orb is both the largest and the lowest-density planet yet found in the cosmos.

As seen from Earth, the planet periodically passes in front of its parent star, blocking about 1 percent of the starlight. Observations of several of these minieclipses, known as transits, along with the timing of the orbit, reveal that the planet has a diameter 1.67 times that of Jupiter but only 84 percent of its mass, putting its density close to that of balsa wood. Jupiter, the solar system’s largest planet, has about six times that density. The bloated body, dubbed TrEs-4, lies some 1,435 light-years from Earth.

Residing closer to its parent star than one-tenth Mercury’s distance from the sun, TrEs-4 is heated by intense starlight to a blistering 1,600 kelvins, the team reports in an upcoming Astrophysical Journal Letters. Further study will reveal whether the planet has an extended atmosphere like that of the much-studied, transiting planet HD 209458b. Because of TrEs-4’s weak gravity, some of the planet’s atmosphere may be escaping in a cometlike tail.

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