WASHINGTON, D.C.--Those hoping that today's opening plenary talk of the American Astronomical Society's meeting here would deliver a stunning revelation probably came away disappointed. NASA's Kepler mission has added five new planets to a growing roster of more than 400 beyond our solar system--and none of the newbies is remotely hospitable to life. But there's still plenty to chew on. One of the planets, for example, is as light as Styrofoam--and that has astronomers scratching their heads.
The Kepler spacecraft, launched last March, orbits the sun while scanning upward of 150,000 stars for signs of a slight dimming--a sign that a planet has crossed its face. The five "exoplanets" described today are Kepler's first finds, confirmed by ground-based telescopic observations of how the exoplanets gravitationally tug on their stars.
Like 19 earlier finds by other groups, four of the exoplanets are unexpectedly lightweight for their size. Although they are about 40% larger than Jupiter, all four are far less dense, ranging from 0.166 grams per cubic centimeter to 0.894 g/cm³. (Jupiter, with its rocky core, has a density of 1.326 g/cm³.) That makes them downright "fluffy," says astrophysicist Alan Boss of the Carnegie Institution for Science in Washington, D.C. Fluffiest of all is so-called Kepler-7b, whose extreme size-to-mass ratio gives it the same density as Styrofoam.
"This is accumulating evidence that low density is a common feature" among exoplanets, says planetary physicist David Stevenson of the California Institute of Technology in Pasadena, who was not involved in the research. The problem is that no one has come up with a mechanism that could puff up an exoplanet that way.
Jupiter was formed similarly puffed up, Stevenson notes, but it quickly shrank as it cooled. So something has kept fluffy exoplanets from cooling off. One possibility, says Stevenson, is that a bit of the heat energy from the star only a few million kilometers away might be carried deep into the planet, perhaps through winds blowing from the permanent day-side of the planet to the night-side. Or tides could be kneading and therefore heating the interior, the way Jupiter heats its rocky satellite Io into a volcanic frenzy.
The fifth exoplanet reported today--and online later this week in Science--is less of an oddity. Kepler-4b appears to be an ice giant like Neptune. Although it wouldn't be home to little green men, the team says that finding planets this size this early bodes well for detecting Earth-like extrasolar planets within 3 years.
In another talk this afternoon, Kepler team member and astronomer Jason Rowe of NASA Ames Research Center in Mountain View, California, reported that the spacecraft has spotted two other stellar companions, though it's unclear exactly what they are. Instead of dimming the light from their respective parent suns as they passed in front of them, the light dimmed when the objects passed behind the suns, suggesting that a large amount of light had been hidden. That means that the orbiters must be substantially brighter and hotter than their stars. And indeed, the data show that the objects, which are simply named KOI 81 and KOI 74 and are 90% and 40% the radius of Jupiter, respectively, have temperatures of 13,500 K and 12,000 K--whereas their stars are only about 10,000 K. That combination of size and temperature do not match any known object, and the mechanism for how these small objects get so hot is still unknown. Rowe says his next step is to determine the mass more precisely, "but regardless of the mass, these are pretty exciting," he says.