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Huge, Lightweight Planet Discovered

A new, lightweight planet has been discovered orbiting a star 450 light-years away in the constellation Lacerta. This unusual planet is larger than Jupiter, but it has only half its mass; astronomers estimate it has the same density as cork. The planet, named HAT-P-1, orbits its host star every 4.5 days. A network of automated telescopes detected how the planet dims its parent star by 1.5% when it passes in between the star and the Earth. Why this planet is so swollen is still a mystery to astronomers.


“We could be looking at an entirely new class of planets,” said Gaspar Bakos, a Hubble fellow at the Harvard-Smithsonian Center for Astrophysics (CfA). Bakos designed and built the HAT network and is lead author of a paper submitted to the Astrophysical Journal describing the discovery. That paper is available online at http://arxiv.org/abs/astro-ph/0609369.

With a radius about 1.38 times Jupiter’s, HAT-P-1 is the largest known planet. In spite of its huge size, its mass is only half that of Jupiter.

“This planet is about one-quarter the density of water,” Bakos said. “In other words, it’s lighter than a giant ball of cork! Just like Saturn, it would float in a bathtub if you could find a tub big enough to hold it, but it would float almost three times higher.”


HAT-P-1’s parent star is one member of a double-star system called ADS 16402 and is visible in binoculars. The two stars are separated by about 1500 times the Earth-Sun distance. The stars are similar to the Sun but slightly younger – about 3.6 billion years old compared to the Sun’s age of 4.5 billion years.


“Out of eleven known transiting planets, now not one but two are substantially bigger and lower in density than theory predicts,” said co-author Robert Noyes (CfA). “We can’t dismiss HD209458b as a fluke. This new discovery suggests something could be missing in our theories of how planets form.”


One way to inject energy into the planet’s center is by tipping it on its side, similar to Uranus in the solar system. A planet in that state orbiting close to its star would be subjected to tidal heating of the interior. But according to Smithsonian astronomer Matthew Holman (who was not a member of the discovery team), “the circumstances required to tip over a planet are so unusual that this would seem unlikely to explain both known examples of inflated worlds.”

According to co-author Dimitar Sasselov (CfA), “Another explanation for HD 209458b’s large size was tidal heating due to an eccentric orbit, but recent observations have pretty much ruled that out.”

The scientists will continue observing HAT-P-1 to see if such an explanation could hold in this case, but “until we can find an explanation for both of these swollen planets, they remain a great mystery,” Sasselov said.

The HAT network consists of six telescopes, four at the Smithsonian Astrophysical Observatory’s Whipple Observatory in Arizona and two at its Submillimeter Array facility in Hawaii. These telescopes conduct robotic observations every clear night, each covering an area of the sky 300 times the size of the full moon with every exposure.

HAT searches for planets by watching for stars that dim slightly when an orbiting planet crosses directly in front of the star as viewed from Earth – a sort of mini-eclipse. Transits offer astronomers a unique opportunity to measure a planet’s physical size from the amount of the dimming. Combined with the mass, which is determined by measuring the amount of the star’s wobble as the planet orbits it, researchers then calculated a planet’s density. Measurements of the wobble of HAT-P-1’s parent star were led by co-author Debra Fischer of San Francisco State University.

Major funding for HATnet was provided by NASA. More information about HAT is available online at http://www.cfa.harvard.edu/~gbakos/HAT/.


Original Source: CfA News Release



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