A newfound planet estimated to be lighter than a giant ball of cork is baffling astronomers. They’re guessing that some unknown mechanism heats this type of world internally, puffing it up.

Using a network of small automated telescopes, researchers at the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass., identified the planet. Designated HAT-P-1, they said it orbits one of a pair of distant stars in the direction of the constellation Lacerta.

“We could be looking at an entirely new class of planets,” said the center’s Gaspar Bakos, who designed and built the telescopes, known as the HAT network. He is also the lead author of a paper submitted to the Astrophysical Journal describing the find.

About 1.38 times wider than Jupiter, HAT-P-1 is the biggest known planet, yet weighs only as much as half of Jupiter, the Bakos added. That would make it one fourth as dense as water.

“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.”

It circles its host star every 4.5 days in an orbit one-twentieth of the Earth-Sun distance, the astronomers said. Once per orbit, they added, it passes before its parent star. This dims it by about 1.5 percent to our view. Planets that darken their parent stars in this way are called transiting planets.

This star is part of a double-star system called ADS 16402, visible with binoculars.

Although perhaps stranger than any known world outside our solar system, HAT-P-1 is not alone in its low density, researchers said: the first transiting planet ever found, HD 209458b, also is puffed up about 20 percent larger than theory predicts, while the new one is 24 percent larger.

“Out of 11 known transiting planets, now not one but two are substantially bigger and lower in density than theory predicts,” so the previous find can no longer be dismissed as a fluke, the center’s Robert Noyes said.

“This new discovery suggests something could be missing in our theories of how planets form,” he continued. One way to explain the find is that some unknown mechanism provides certain planets with more internal heat than previously believed, he added.

The HAT network consists of six telescopes that conduct robotic observations each clear night. They seek out transiting planets. Astronomers measure a planet’s size from the amount of the star’s dimming.

Combined with the mass—determined by measuring the amount of the star’s wobble as the planet orbits it—researchers then calculate a planet’s density. The newfound world is estimated to be 450 light-years away; a light-year is the distance light travels in a year.


- Courtesy Harvard-Smithsonian Center for Astrophysics
and World Science staff

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