The promise of NASA's Kepler mission has always been to find a plurality of worlds like our own: relatively compact orbs circling stars like the Sun, at distances where liquid water could exist abundantly.
Today, its scientists announced that they've found one such place. It's called Kepler-22 b, the designation for a planet found circling an 11th-magnitude star about 600 light-years away, in Cygnus near its junction with Lyra and Draco. With a spectral type of G5, this star is very slightly cooler than our Sun. Kepler-22 b, about 2.4 times the diameter of Earth, circles in a 290-day-long orbit very slightly smaller than our planet's. Read all about it here.
Astronomers don't yet know the mass of this new find nor, therefore, its density. So it's too soon to say whether it might have a composition dominated by rocky matter, liquid, or gas. A determination of its mass might emerge next summer, once ground-based telescopes have a chance to observe the system high in a dark sky.
Nonetheless, the orbital specs are enough to suggest that Kepler-22 b should have a temperature between 225 and 262 K, which is below water's freezing point. But as pointed out by William Borucki, the mission's principal investigator, it wouldn't take much greenhouse warming due to an atmosphere to raise the surface temperature to a shirtsleeve-like 72°F.
Thus, the project team is trumpeting Kepler-22 b as the mission's first confirmed planet occupying the habitable zone of its host star.
The key here is "confirmed." Kepler spots candidate companions by staring at roughly 156,000 stars from 9th to 15th magnitude in an area of sky covering 105 square degrees. When a planet crosses directly in front of one of these stars, the passage registers as a slight dip in the star's apparent brightness. The Kepler team doesn't consider a candidate viable until three transits are seen.
To date, Kepler's confirmed planets have all hugged their stars closely, causing repetitive light-curve dips every few days or weeks. But to record three passages by Kepler-22 b, each just 7½ hours long but spaced nearly 10 months apart, required a bit of luck. The spacecraft noted the first transit on May 15, 2009, just a few days after it started taking data in earnest. The third came in the days before Christmas last year, just prior to a week's-long hiatus caused when the spacecraft hiccuped and took itself offline. "If there had been any change when these occurred, we would have missed them," Borucki explains. "So we think of this as our Christmas planet — it was a great gift."
But after detection comes the painstaking work of proving the dips aren't due to some other cause. For example, this past summer astronomers made 16 observations of Kepler-22 using one of the Keck telescopes just to ensure that its companion wasn't a second star masquerading as a planet.
"We take three approaches to rule out 'false positives'," explains Natalie Batalha, Kepler's deputy science-team leader. Ground-based telescopes make radial-velocity observations, which measure the star's periodic wobble due to its companion. Or analysts can look for slight variations in the transit timing, an indication that the companion is small enough to be perturbed by the gravity of other planets in the system. If these fail, she says, the team works through a comprehensive list of things it couldn't be — until all those are ruled out. It's a labor-intensive process involving lots of observing time and computer modeling — which explains why so few of Kepler's candidates have achieved "confirmed" status.
And this vetting process is getting even farther behind, because the spacecraft has been discovering new candidates at a brisk clip. To kick off the Kepler Science Conference, which began today at NASA's Ames Research Center in California, Borucki and Batalha announced that Kepler has added another 1,094 candidates to the pile, nearly doubling the total count (so far) to 2,326. Multiple planet candidates have been found in 367 systems, about 20% of the total.
Among all candidates, Batalha notes, are 207 objects considered Earth-size (no more than 1¼ times our planet's diameter) and another 680 classified as "super-Earths (1¼ to 2 times). Meanwhile, 48 planet candidates orbit in their respective stars' habitable zones. Of these, 10 appear to closely match Earth in size — though five are considered suspect. Multiple planet candidates have been found in 367 systems, about 20% of the total.
"The parameter space is spreading," Batalha says. "We're pushing to smaller planets and longer orbital periods." Some of the candidates even appear to be smaller than Earth.
Listening for "Techno-Signatures"
If the announcement of another thousand planet candidates was the "cake" at today's press briefing, the "icing" was that Kepler-22 b and the mission's other tantalizing planet finds will be scrutinized for coherent radio signals by the Allen Telescope Array in northern California. "For the first time," exults Jill Tarter, director of the Center for SETI Research at the SETI Institute, "we can point our telescopes at stars — and know that those stars actually host planetary systems."
Regular readers of S&T.com will recall that funding woes forced scientists to mothball the ATA last April. However, at today's briefing Tarter announced that the phalanx of 42 radio dishes has resumed operation as of today. The revival, good for at least two years of observations, was enabled by more than $230,000 raised privately by the "SETI Stars" effort and additional funding from the U.S. Air Force, which wants to assess ATA's capabilities for precision satellite tracking.
Tarter explains that the ATA will be used to monitor planetary systems at 9 billion individual frequencies from 1 to 10 gigahertz. These include the most likely interstellar "hailing frequencies": the ubiquitous 1.4-GHz emission (wavelength: 21 cm) from atomic hydrogen and an 8.7-GHz (3.5-cm) line from helium-3 ions.
By the way, Kepler is far from done. Batalha says we can expect at least one more big dispatch of candidate planets in the months ahead. And Borucki is still hoping that NASA managers will give him the funds needed to keep looking after the mission nominally draws to a close late next year. As I detailed back in July, the target stars fluctuate in brightness much more than expected, and the spacecraft needs to observe several additional years to identify all the Earth-size planets in its search field.