NASA’s Kepler telescope has discovered five giant planets that whip around their stars on tight orbits (Illustration: NASA/JPL)

A giant planet with the density of Styrofoam is one of a clutch of new exoplanets discovered by NASA’s Kepler telescope. The planets are too hot to support life as we know it, but the discoveries, made during the telescope’s first few weeks of operation, suggest Kepler is on the right track to find Earth’s twins, researchers say.

More than 400 planets have now been found orbiting other stars, but Earth-sized planets – which may be the best habitats for life – have remained elusive.

NASA’s orbiting Kepler telescope is designed to find them. It has been scrutinising 100,000 stars since April 2009, searching for telltale dips in starlight created when planets pass in front of their host stars.

During its first six weeks of observations, it found five new planets. All are giants – four are heavier than Jupiter and one is about as massive as Neptune. They all orbit their host stars so closely that their surfaces are hotter than molten lava. “Looking at them might be like looking at a blast furnace,” says lead scientist William Borucki, who presented the results on Monday at a meeting of the American Astronomical Society in Washington, DC.


Puffed up

The newly found planets are all less dense than expected based on models of how giant planets coalesce from gas.

One, called Kepler 7b, is about as dense as polystyrene. It is about 1.5 times as wide as Jupiter, but only about a tenth as dense, making it one of the most diffuse planets yet found.

Heat is thought to have caused this bloating, but the heating mechanism is not yet understood – it might be stellar radiation, gravitational tugs from the star, or something else.

“We think this has something to do with the fact that they are all very close to their host stars … but we haven’t found the smoking gun,” says team member Dimitar Sasselov of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Massachusetts.

Hotter than their hosts

Kepler’s images also revealed two objects that are curiously hotter than their host stars, with temperatures of roughly 10,000 °C. These temperatures suggest the objects are diminutive stars called white dwarfs.

White dwarfs are typically about as large as Earth, but Kepler’s observations suggest the objects are much larger – on the order of Jupiter’s size.

One possible explanation is that the objects are white dwarfs that lost much of their mass early on, lowering the tug of gravity that keeps the stars compact, says Ronald Gilliland of the Space Telescope Science Institute in Baltimore, Maryland.

Even though the new planets are inhospitably hot, mission members say finding them bodes well for Kepler’s ability to detect Earth’s twins. “[They are] certainly no place to look for life. That will all come later,” says Borucki.

Definitive detections of exoplanets require at least four dimming events, which means that detections of Earth-like planets on year-long orbits are still years away.