Astronomers have identified 1,235 candidate planets beyond the solar system, including 54 where life might have a chance of gaining a foothold.
Extrapolating from the findings, which are based on observations of bright stars in a tiny patch of sky monitored by NASA’s Kepler spacecraft, some 20,000 planets may reside in other regions of the Milky Way where liquid water is stable, says Kepler chief scientist Bill Borucki of NASA’s Ames Research Center in Mountain View, Calif. He reported the results, which are based on Kepler’s first four months of operation, at a NASA press briefing on February 2.
“I’m just exhilarated we’ve found all these things already, and I’m awed that there are so many,” said Borucki. The Kepler contenders are separate from the more than 500 confirmed extrasolar planets that astronomers have discovered since 1995.
Of the 54 candidate planets in the habitable zone, five are roughly Earth-sized, and the other 49 range from twice the size of Earth to larger than Jupiter. The larger planets, which are likely gaseous, might be able to support liquid water and life on solid moons, Borucki says. Some of those moons could be the size of Earth, he adds.
Overall, the Kepler census includes 68 planet candidates roughly the size of Earth, 288 that are a few times the size of Earth, 662 with diameters about equal to that of Neptune and 165 the size of Jupiter. About 30 percent of the candidates belong to multiple-planet systems, with several planets orbiting the same star.
“This is definitely our best first look at the galactic planetary census,” says theorist Greg Laughlin of the University of California, Santa Cruz, who is not part of the Kepler team.
All of the putative planets regularly pass in front of, or transit, one of the roughly 156,000 stars monitored by Kepler, blocking a tiny amount of starlight during each passage. The amount of dimming reveals the size of each planet. The masses of the transiting objects have yet to be measured, which would confirm their existence and rule out that the dimming is a spurious signal or due to the passage of another star.
But after putting the data through a battery of tests, Borucki says he and his collaborators are confident that at least 90 percent of the candidates will turn out to be real.
At the briefing, Kepler researcher Jack Lissauer of NASA Ames unveiled a sextet of planets orbiting a star dubbed Kepler-11. The six orbs form the most compact planetary system ever detected.
By luck, as viewed by the craft, all six planets pass in front of their sunlike parent star, revealing the size of each orb. And because the five innermost planets lie so close together — all five would fit inside Mercury’s orbit about the sun — their gravitational interactions can be used to determine their masses, which range from 2.3 to 13.5 times Earth’s and make five of the six among the lowest-mass extrasolar planets ever measured.
All are less dense than Earth; in composition they resemble the icy solar system planets Neptune and Uranus. The inner two planets have the highest density, an indication that their proximity to the heat and radiation from their parent star has eroded much of the hydrogen and helium atmosphere they may have been born with.
“This discovery is as momentous” as the discovery in 1995 of the very first extrasolar planet known to orbit a sunlike star, says Debra Fischer of Yale University.
Until last year, says Laughlin, the composition of planets intermediate in mass between Earth and Uranus, the solar system’s next heaviest planet after Earth, was uncharted territory (SN: 2/12/11, p. 12).
The Kepler-11 system suggests that many such planets resemble Uranus more than they do rocky Earth, Lissauer says. The system will also allow astronomers to understand how different kinds of planets can form around the same star, he adds.