The growing array of planets that scientists have spotted orbiting stars far from our sun got a little bigger yesterday, and a lot more bizarre.
Astronomers announced in Washington that they have discovered a new planet 450 light-years from Earth that's the biggest ever found. It's 38 percent bigger than Jupiter, the largest planet in the solar system - but with half the mass, making it less dense than cork.
The discovery is so weird and unexpected that scientists think the discovery may represent an entirely new class of planets.
"We are unable to explain it," said Robert Noyes, a research astrophysicist at the Harvard-Smithsonian Astrophysical Observatory and co-author of the study. Theoreticians "are scratching their heads as to what can possibly be going on."
The new planet has been named HAT-P-1, the first planet beyond our solar system discovered by the new Hungarian Automated Telescope network, six telescopes at observatories in Arizona and Hawaii.
The robotic network is designed to search the sky for stars that dim slightly when unseen planets orbit in front of them in a type of eclipse. Twelve planets circling other stars have now been discovered using this technique.
The HAT telescopes found this planet circling one member of a double star system called ADS 16402. They're identical twin stars, each one very similar to our sun, Noyes said. They're orbiting each other about 450 light-years from Earth, just visible with binoculars in the northern constellation Lacerta, the Lizard, in the autumn night sky.
The new planet was seen zipping around its star once every 4 1/2 days, in an orbit just one-twentieth as far from its star as the Earth is from our sun.
Scientists discovered the planet's surprisingly bantamweight nature using a technique called astrometry - precisely measuring changes in the star's light as HAT-P-1 periodically eclipsed it.
From the amount of dimming HAT saw, scientists could calculate the planet's physical size. Then, from the amount of "wobble" the star displayed from the effect of the planet's gravitational pull as it orbited, astronomers determined the planet's mass. From there, it was simple division to figure out its density.
To their astonishment, the Harvard-Smithsonian astronomers found HAT-P-1 had only half the mass of Jupiter. And its density was about a quarter that of water, and even less dense than cork.
"Of course we're very excited for a lot of reasons," Noyes said. "It's our first discovery of a planet, and ... this is really an oddball."
The vast majority of the 200 or so planets so far found circling other stars are high-mass planets. Some have several times Jupiter's mass.
It's not unusual to find a giant planet less dense than water. Unlike the rocky planets Mercury, Venus, Earth and Mars, our own solar system's giant planets, such as Jupiter and Saturn, are composed largely of light gases, such as hydrogen and helium. Saturn, like HAT-P-1, would float if one could set it down in a huge tub of water.
But HAT-P-1 appears to be the fluffiest planet ever discovered. In that tub of water, Noyes said, "the new planet would float very high up ... barely sinking in."
That puts it outside the boundaries of what planetary scientists thought was possible, based on their current understanding of how planets form and evolve.
"The only way we can have it is to have no rocky materials," Noyes said. Even Jupiter, and probably Saturn, too, are thought to have rocky inner cores. "We have discovered a very bizarre new object."
It may represent an entirely new class of planets, according to Gaspar Bakos, the lead author in a report of the discovery submitted for publication by the Astrophysical Journal. NASA provided most of the funding.
Japanese astronomers previously reported the discovery of another extra-solar planet, dubbed HD 209458b, that appeared to be similarly puffed up, although not to the same degree as HAT-P-1.
But scientists haven't a clue how it could happen. A number of hypotheses have been suggested, including somehow tipping the planet on its side and heating the planet's interior, causing it to expand and become less dense. But the idea still doesn't really fit any known theory.
"The circumstances required to tip over a planet are so unusual that this would seem unlikely to explain both known examples of inflated worlds," said Smithsonian astronomer Matthew Holman, who was not part of the discovery team.
So HAT-P-1's origins remain "an unsolved theoretical problem," Noyes said. "Perhaps in coming years we will get better observations. At the moment we don't have any answers."