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NASA mission piles on the planets

The science team for NASA's Kepler planet-hunting mission nearly doubled their list of confirmed planets beyond our solar system in one fell swoop today, announcing the discovery of 26 planets spread among 11 star systems. Their sizes range from just a little bit bigger than Earth to super-Jupiter-size, but they're all closer to their parent stars than Venus is to our own sun.

The accelerating pace of discovery is matched by the diversity seen in the worlds discovered so far, one of the Kepler mission's co-investigators, Harvard astronomer Dimitar Sasselov, told me today.

"There is more diversity out there than our limited imaginations could come up with, which is good," he said.

The $600 million Kepler mission, launched in 2009, now has a list of 61 confirmed planets, and another 2,326 planetary prospects that have yet to be confirmed. At this rate, Kepler's worlds could soon account for the majority of the exoplanets detected beyond our solar system — a tally that now stands at more than 700.

"Prior to the Kepler mission, we knew of perhaps 500 exoplanets across the whole sky," Doug Hudgins, Kepler program scientist at NASA Headquarters, said in a news release. "Now, in just two years staring at a patch of sky not much bigger than your fist, Kepler has discovered more than 60 planets and more than 2,300 planet candidates. This tells us that our galaxy is positively loaded with planets of all sizes and orbits."

The Kepler space telescope searches for other worlds by staring at more than 150,000 stars in that fist-sized patch of sky, straddling the constellations Cygnus and Lyra. Kepler's instruments can detect the faint dips in starlight that occur on a regular basis as a planet passes over the disk of its parent star, as seen from Earth. By analyzing the patterns of those passes, also known as transits, Kepler's scientists can figure out the orbit and the size of a potential planet — but not its mass.

An alternative method has to be used to confirm that Kepler is actually looking at a planet rather than something else, such as mutually eclipsing binary stars. The mission's early discoveries were confirmed by looking at the candidates' stars with ground-based telescopes and checking for the telltale gravitational wobbles that are caused by big, close-in planets.

Transit timing variations

Most of the planets added to the list today were confirmed using a different backup method. The Kepler mission's astronomers analyzed subtle changes in the intervals between the transits, caused when multiple orbiting planets exert gravitational pull on each other. The resulting data on acceleration and deceleration can be used to confirm the planets' existence and calculate their masses.

"By precisely timing when each planet transits its star, Kepler detected the gravitational tug of the planets on each other, clinching the case for 10 of the newly announced planetary systems," said Dan Fabrycky, an astronomer at the University of California at Santa Cruz and the lead author for a paper confirming four of the planetary systems, known as Kepler-29, 30, 31 and 32.

Other newly confirmed planetary systems include Kepler-25, 26, 27 and 28, described in a paper with Fermilab's Jason Steffen as lead author; and Kepler-23 and 24, which was the focus of research led by the University of Florida's Eric Ford. In today's release, Ford said the transit timing variation method "dramatically accelerated" the pace of planetary discovery.

Yet another study, led by Jack Lissauer, a planetary scientist at NASA's Ames Research Center, detected five planets around Kepler-33, a star that is older and more massive than the sun. Those five planets range in size from 1.5 to five times the width of Earth, and they're all closer to their parent star than Mercury is to our own sun.

"The approach that was used to verify the Kepler-33 planets shows that the overall reliability of Kepler's candidate multiple transiting systems is quite high," Lissauer said in today's release. "This is a validation by multiplicity."

Five of the newly confirmed planetary systems (Kepler-25, 27, 30, 31 and 33) contain a pair of worlds that are bound together in a 1:2 resonance. That means the inner planet makes two circuit for every one circuit made by the outer planet. Four other systems (Kepler-23, 24, 28 and 32) have two planets that are linked in a 2:3 resonance, like Pluto and Neptune in our own solar system.

"These configurations help to amplify the gravitational interactions between the planets, similar to how my sons kick their legs on a swing at the right time to go higher," Steffen said.

Fifteen of the 26 planets announced today are Neptune-size or smaller, and the orbital periods range from six to 143 days. The planets' distances from Earth range from 623 light-years (for Kepler-25) to 4,064 light-years (for Kepler-29).

Great expectations

Sasselov, who has just come out with a book about the Kepler quest titled "The Life of Super-Earths," marveled that so many of the newfound worlds are in multiple-planet systems. He recalled that when the Kepler mission was proposed to NASA, more than a decade ago, "there was one little sentence that said maybe two or three of the systems will have multiple transiting planets."

None of the planets announced today would be conducive to life as we know it, because their orbits are so close to their parent stars. It's likely to be just a matter of time before Kepler achieves its main goal — confirming the existence of Earth-size planets in Earthlike orbits around sunlike stars. Unfortunately, it may be a matter of more time than initially expected.

Funding for the Kepler mission is due to run out in November, but the mission's scientists "don't have enough to statistically complete the core goal of the mission," Sasselov said. It turns out that the data collected by the telescope is "noisier" than expected. That means more observations will be required to confirm the mission's trickiest planetary finds.

The Kepler team has applied for a four-year extension, and is currently waiting for a decision from NASA executives.

More about the planet search:

The planetary confirmations are described in four research papers:

Alan Boyle is's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.