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An avalanche of alien planets

Researchers have confirmed the existence of six new planets beyond our solar system, with hundreds of other new worlds potentially waiting in the wings.

The latest planetary prospects come from two different planet-hunting probes: the European CoRoT satellite and NASA's Kepler spacecraft. CoRoT's science team has confirmed the detection of 15 planets so far, including CoRoT-7b, a "lava planet" that's only five times as massive as Earth but traces a hellishly close orbit around its parent star.

The six new planets are all bigger than CoRoT-7b, but reflect the wide diversity that planetary scientists are finding as they sift through an avalanche of data. There's even a brown dwarf in the bunch - a celestial object that's considered too big to be a planet, but too small to be a fully functioning star.

"Each of these planets is interesting in its own right, but what is really fascinating is how diverse they are," Oxford University's Suzanne Aigrain, a co-investigator on the research team, said in a university news release about the discoveries. "Planets are intrinsically complex objects, and we have much to learn about them yet."

The head of the CoRoT exoplanet program, Magali Deleuil of France's Laboratoire d'Astrophysique de Marseille, said "every discovery of an extrasolar planetary system is a new piece in the puzzle of how these systems do form and evolve."

Here's the full rundown from the Oxford website:

CoRoT-8b: the smallest in this batch: At about 70 percent of the size and mass of Saturn, CoRoT-8b is moderately small among the previously known transiting exoplanets. Its internal structure should be similar to that of ice giants, like Uranus and Neptune, in the solar system. It is the smallest planet discovered by the CoRoT team so far after CoRoT-7b, the first transiting Super-Earth.

CoRoT-10b: the eccentric giant: The orbit of CoRoT-10b is so elongated that the planet passes both very close to and very far away from its star. The amount of radiation it receives from the star varies tenfold in intensity, and scientists estimate that its surface temperature may increase from 250 to 600 degrees Celsius, all in the space of 13 Earth-days (the length of the year on CoRoT-10b).

CoRoT-11b: the planet whose star does the twist: CoRoT-11, the host star of CoRoT-11b, rotates around its axis in 40 hours. For comparison, the sun’s rotation period is 26 days. It is particularly difficult to confirm planets around rapidly rotating stars, so this detection is a significant achievement for the CoRoT team.

CoRoT-12b, 13b and 14b: a trio of giants: These three planets all orbit close to their host star but have very different properties. Although CoRoT-13b is smaller than Jupiter, it is twice as dense. This suggests the presence of a massive rocky core inside the planet. With a radius 50 percent larger than Jupiter’s (or 16 times larger than Earth’s), CoRoT-12b belongs to the family of "bloated hot Jupiters," whose anomalously large sizes are due to the intense stellar radiation they receive. On the other hand, CoRoT-14b, which is even closer to its parent star, has a size similar to Jupiter’s. It is also massive, 7.5 times the mass of Jupiter, which may explain why it is less puffed up. Such very massive and very hot planets are rare, CoRoT-14b is only the second one discovered so far.

CoRoT-15b: the brown dwarf: CoRoT-15b’s mass is about 60 times that of Jupiter. This makes it incredibly dense, about 40 times more so than Jupiter. For that reason, it is classified as a brown dwarf, intermediate in nature between planets and stars. Brown dwarfs are much rarer than planets, which makes this discovery all the more exciting.

CoRoT and Kepler use essentially the same technique to detect planets: They look for patterns in the subtle periodic dips of light when a relatively dark planet passes in front of the disk of its parent star. This method requires a spacecraft to stare for months or years at the same patch of sky, in order to build up a record of the light variations. To confirm that the variations are really caused by the transit of a planet, astronomers need to see the pattern repeat itself at least three times.

Thus, in order to confirm the detection of an Earthlike planet in an Earthlike orbit around a sunlike star, you'd need to conduct at three years' worth of observations. It's easier to find bigger planets than smaller ones, and it's easier to find planets that whirl in tight orbits around their stars.

Hundreds of new worlds
Before CoRoT and Kepler were launched, astronomers said they expected to find hundreds of new worlds, including the first exoplanets as small as Earth. And based on this week's first big data dump from the Kepler mission, those astronomers won't be disappointed. Today the team said it has found 706 promising prospects for exoplanet discoveries so far. That's a far longer list than the current lineup of 460 extrasolar planets reported to date.

"We have the potential of readily doubling the number of known planets, once we have gone through the process of winnowing these signals down," Charlie Sobeck, deputy project manager for the Kepler mission at NASA's Ames Research Center, told me. "The key word in that is 'potential.'"

The Kepler team said the prospects include "viable exoplanet candidates with sizes as small as that of Earth to larger than that of Jupiter." Most of them are the size of Neptune or smaller, and five of the target star systems appear to have multiple planets orbiting around them, the team said.

Sobeck stressed that these prospects still had to go through an arduous confirmation process to rule out the possibility that they were false positives. The signals from Kepler have to be double-checked by other telescopes that use different methods of planet detection. (This interactive explains how the different methods work.)

The rising and falling signals may have been caused by other phenomena that Kepler picked up by mistake while it was staring at a particular star. For example, astronomers already know that Kepler is spotting some mutually eclipsing binary stars in the background. An entire study has been written up about 1,832 eclipsing binaries observed by the probe during the first 44 days of operation. "They're more common than anticipated," Sobeck said.

The data debate
To date, the Kepler team has officially announced the detection of only five exoplanets. Many more will likely come next February, when the team's next big reveal is scheduled. In preparation for that event, the astronomers on the Kepler team have held back the data about 400 of the best prospects from public release. Information about the other 306 potential planetary systems was posted to the publicly available Kepler data archive today.

The fact that some of the signals are being held back, even though it's generally NASA policy to release data in a year, has sparked a debate in astronomical circles. That debate bubbled up into public view on Nature.com and in The New York Times. "Kepler was constructed and launched with a comparatively large sum of money for a project that is run by a single team," Ben Oppenheimer, an astronomer at the American Museum of Natural History in New York, told the Times. "At this point, I have to say I do think they are being far too restrictive."

Kepler's team members said they were facing a special case, due to delays in launching the $600 million mission and the relatively short April-to-September observing season. They struck a deal with NASA to get some additional time to do the double-checking they felt was needed for their own research. "We'd like to finish that process," Sobeck said.

The data debate raises interesting questions about how widely and how quickly scientists in charge of high-profile experiments should distribute their raw scientific readings. Once they're confirmed, the revelations from Kepler (and CoRoT as well) could revolutionize the search for alien Earths.

"The Kepler observations will tell us whether there are many stars with planets that could harbor life, or whether we might be alone in our galaxy," the mission's principal investigator, William Borucki of NASA Ames, said today in a news release from the space agency.

How would you feel if you spent a decade preparing for a space mission and gathering the data - only to see the crowning discovery made by a number-cruncher piggybacking on your database?

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