Astronomers searching for extrasolar planets have discovered a real corker: a gigantic world more than twice the size of Jupiter but only half as massive, which means it's literally about as dense as a cork. Announced today at a press conference in Washington, D.C., the puffball is the largest planet yet discovered and the second largest and lightest exoplanet ever found. The finds leave theorists with some explaining to do. According to current models of planet structure and evolution, such large, lightweight orbs shouldn't exist.
More than 200 planets orbiting distant stars have been discovered. Typically, researchers detect the telltale wobble in a star's position caused by the gravitational tug of the orbiting planet. But 11 exoplanets have been spotted another way, when they cross in front of their stars, which happens only if observers on Earth see a planet's orbit edge-on. From the amount of light blocked, researchers can tell how big a planet is. In 2000, the first planet detected this way turned out to be unaccountably large and light, although the next nine were more normal.
Now, astronomers led by Gaspar Bakos of the Harvard-Smithsonian Center for Astrophysics (CfA) in Cambridge, Massachusetts, have used this technique to take the measure of a second large, puffy planet around a star some 450 light-years from Earth. Using a system of four small telescopes on Mount Hopkins in Arizona and two on Mauna Kea in Hawaii, the team found that the planet, which they dubbed HAT-P-1, blocked 1.5% of the light from its sun as it whizzed around it every 4.5 hours. They deduced that HAT-P-1 is more than two-and-a-half times as big as Jupiter, making it the largest exoplanet so far. By watching the wobble of the star, the researchers also determined that the planet has only half Jupiter's mass.
Therein lies the rub. Theorists' models of planetary structure and evolution cannot accommodate such low-density giants, says Robert Noyes, a team member and an astronomer at CfA. The problem isn't how such big planets form, but how they manage not to cool off and shrink, says Jack Lissauer, a planetary scientist at NASA's Ames Research Center in Moffett Field, California. Planets are born big and warm, he explains, but it takes energy to keep one that way. Just where the energy comes from, nobody knows.
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