As reported in the New York Times:
Kepler Planet Hunter Finds 1,200 Possibilities
By DENNIS OVERBYE
Published: 3 February 2011
Astronomers have cracked the Milky Way like a piñata, and planets are now pouring out so fast that they don’t know what to do with them all.
In a long-awaited announcement, scientists operating NASA’s Kepler planet-hunting satellite reported Wednesday that they had identified 1,235 possible planets orbiting other stars, potentially tripling the number of known planets in the universe.
Of the new candidates, 68 are one-and-a-quarter times the size of the Earth or smaller — smaller, that is, than any previously discovered planets outside the solar system. Fifty-four of the possible exoplanets are in the so-called habitable zones, where temperatures should be moderate enough for liquid water, of stars dimmer and cooler than the Sun; four of these are less than twice the size of Earth, and one is even smaller.
Astronomers said that it would take years to confirm that all these candidates are really planets — by using ground-based telescopes to try to measure their masses, for example — and not just double stars or other strange systems. Many of them might never be vetted because of the dimness of their stars and the lack of telescope time and astronomers to do it all. But statistical tests of a sample of the list suggest that 80 to 95 percent of the objects on it were real, as opposed to blips in the data.
“It boggles the mind,” said William Borucki of the Ames Research Center, Kepler’s leader.
At first glance, none of them appears to be another Earth, the kind of cosmic Eden fit for life as we know it, but the new results represent only four months worth of data on a three-and-a-half-year project, and have left astronomers enthused about the chances they will ultimately reach their goal of finding Earth-like planets in the universe.
“For the first time in human history we have a pool of potentially rocky habitable zone planets,” said Sara Seager of M.I.T., who works with Kepler. “This is the first big step forward to answering the ancient question, ‘How common are other Earths?’ ”
Mr. Borucki noted that since the Kepler telescope surveys only one four-hundredth of the sky, the numbers extrapolated to some 20,000 habitable-zone planets within 3,000 light-years of Earth. He is the lead author of a paper that has been submitted to The Astrophysical Journal describing the new results.
In a separate announcement, to be published in Nature on Thursday, a group of Kepler astronomers led by Jack Lissauer of Ames said they had found a star with six planets — the most Kepler has yet found around one star — orbiting in close ranks in the same plane, no farther from their star than Mercury is from the Sun.
This dense packing, Dr. Lissauer said, seems to violate all the rules astronomers thought they had begun to discern about how planetary systems form and evolve.
“This is sending me back to the drawing board,” he said.
Summarizing the news from the cosmos, Geoffrey W. Marcy of the University of California, Berkeley, a veteran exoplanet hunter and a mainstay of the Kepler work, said, “There are so many messages here that it’s hard to know where to begin.” He called the Borucki team’s announcement “an extraordinary planet windfall, a moment that will be written in textbooks. It will be thought of as watershed.”
Kepler, launched into orbit around the Sun in March 2009, stares at a patch of the Milky Way near the Northern Cross, measuring the brightness of 156,000 stars every 30 minutes, looking for a pattern of dips that would be caused by planets crossing in front of their suns.
The goal is to assess the frequency of Earth-like planets around Sun-like suns in the galaxy. But in the four months of data analyzed so far, a Kepler looking at our own Sun would be lucky to have seen the Earth pass even once. Three transits are required for a planet to show up in Kepler’s elaborate data-processing pipeline, which means that Kepler’s next scheduled data release, in June 2012, could be a moment of truth for the mission.
Habitable planets, in the meantime, could show up at fainter stars than our Sun, where the habitable, or “Goldilocks,” zone, would be smaller and closer to the star and planets in it would rack up transits more quickly.
Attention has been riveted on Wednesday’s data release since June, when Kepler scientists issued their first list, of some 300 stars suspected of harboring planets, but held back another 400 for further study. In the intervening months, Mr. Borucki said, some of those candidates have been eliminated, but hundreds more have been added that would otherwise have been reported in June this year.
One of the sequestered stars was a Sun-like star in the constellation Cygnus that went by the name of KOI 157, for Kepler Object of Interest. It first came to notice in the spring of 2009 when the astronomers saw that it seemed to have five candidate planets, four with nearly the same orbital periods, and in the same plane, like an old vinyl record, Dr. Lissauer recalled. Two of them came so close that every 50 days one of them would look as large as a full moon as seen from the other, Dr. Lissauer calculated.
“I got very interested in this system,” Dr. Lissauer said. “Five was the most we had around any target.” Moreover, the planets’ proximity to one another meant that they would interact gravitationally. In the fall, a sixth planet — the innermost — was found.
By measuring the slight variations in transit times caused by the gravitational interference of the inner five planets with one another, Dr. Lissauer and his colleagues were able to calculate the masses and densities of those planets. These confirmed they were so-called super-Earths, with masses ranging from two to 13 times that of the Earth. But they were also puffy, containing a mixture of rock and gas, rather than being pure rock and iron like another super-Earth, Kepler 10b, a hunk of lava announced last month at a meeting in Seattle.
Dr. Lissauer said, “It suggests that most super-Earths may be more like Neptune than Earth-like.”
Alan Boss, a planetary theorist at the Carnegie Institution of Washington, said the Kepler 11 system, as it is now known, should keep theorists busy and off the streets for a long time. “This system,” he wrote in an e-mail message, “certainly belongs in the pantheon of exoplanet systems: six planets lined up in a plane pointing toward us, waiting patiently for billions of years for humankind to develop sufficient technical capabilities to detect them.”
Mr. Borucki said the growing ubiquity of small planets as revealed by Kepler was a welcome relief from the early days of exoplanet research, when most of the planets discovered were Jupiter-size giants hugging their stars in close orbits, leading theorists to speculate that smaller planets might be thrown away from those environs by gravitational forces or even dragged right into their stars.
“Those little guys are still there,” he said, “and we’re delighted to see them.”