The Hungarian Automated Telescope (HAT) is a compact observatory that operates without human intervention. It began searching for planetary transits in 2003.

“Out of 11 known transiting planets, we now have two that are substantially bigger and lower in density than theory predicts,” said co-author Robert Noyes. “We can’t dismiss HD209458b as a fluke. This new discovery suggests something could be missing in our theories of how planets form.”

Theorists had already considered a number of possibilities to explain the large size of HD 209458b, but so far without success. The only way to puff up these giant planets beyond the size calculated from planetary structure equations would be to supply additional heat to their interiors. Simple heating of the surface due to the host star’s proximity would not work. (If it could, all close-in transiting giant planets should be expanded, and not just the two.)

One way to inject energy into the planet’s centre is by tipping it on its side, like Uranus. 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 (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”.

Said co-author Dimitar Sasselov: “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”.

* The HAT network consists of six telescopes, four at the Smithsonian Astrophysical Observatory’s Whipple Observatory in Arizona and two at its Submillimetre 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. Together with the mass, which is determined by measuring the star’s wobble as the planet orbits it, researchers are able to calculate a planet’s density.