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.

Kepler's habitable planet

Kepler-22 b, a planet known to comfortably circle in the habitable zone of a Sun-like star, is more than twice the size of Earth and orbits a Sun-like star at a distance very comparable to our planet's. Click here for a larger version.

NASA / Ames / JPL

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.

Kepler's celestial targets

The fields of view of Kepler's 42 detectors sample a rich area of the Milky Way where mission scientists hope to monitor the light from 100,000 stars to distances of 3,000 light-years. Click here to see a larger version.

NASA / Carter Roberts

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.

In the 2½ years since its launch, NASA's Kepler spacecraft has identified more than 2,000 candidate planets around distant stars. With time, the spacecraft is finding more candidates with small sizes and longer orbital periods (bigger orbital distances).

NASA / Ames / JPL

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."

Allen Telescope Array

The Allen Telescope Array, developed specifically to search for alien radio transmissions, is part of the University of California's Hat Creek Radio Observatory. To date, 42 of the array's planned 350 dishes have been built.

SETI Institute

Regular readers of S& 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.


Image of BMayfield


December 6, 2011 at 8:03 pm

Excellent news, exoplanet fans! Remember that Kepler can only detect the relatively tiny percentage of systems with orbits closely alined with our system. So, for a crude approximation ... Total Planet Count = Total Stars x Kepler Planets / (Kepler Dectablity Rate x Kepler Stars). Let the planetary predictions begin, and UP with Pkepler!

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T Fleming

December 8, 2011 at 7:18 am

The graphic places Keppler-22 near the inner limit of the habitable zone yet lists the temperature range well outside the freezing point of water. Either the zone is mapped incorrectly or the planet position is misplaced. A mistake this obvious would have been caught so I'm looking for an explanation of the gap in my understanding. Yes, the right atmospheric conditions could make it habitable but planet position relative to the cited temperature should place 22 near the further edge of the zone. Also this representation places the earth near the inner edge but other sources place it more centrally.
Discussion is invited

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Sam Storch

December 9, 2011 at 1:33 pm

Those who recall the first detection of an extrasolar planet (October 1996, 51 Pegasi, if I recall correctly) will appreciate the significance of the findings reported here. The confirmation of Kepler 22b as well as its teasingly small mass compared with the early exoplanet discoveries are reasons to exult.

However, there is much work to be done in correcting the vastly exaggerated reports in the media- the assumptions come "one upon the other!" With no evidence yet for an atmosphere around the planet, the temperatures reported are speculative based on reasonable but unverified assumptions, and so on. Surely IF (note the dissonance... ) there is an atmosphere, there is water vapor, support for life, and so on. The only sure things here are the reported findings (called data) and the huge media hype.

This is a far more exciting time for those who appreciate the real work done by Kepler. Compare that with the "news of the moment" and then please kiss an astronomy educator!

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John Swatek

December 10, 2011 at 9:22 am

Another bandwagon? Seems to me everyone are jumping again.

"Confirmed" means a confirmed candidate - not a fact. Is this not true?

As long as there remains a large uncertainty in the distance to the host star, all other characteristics such as habitability, possible surface features, what the world would be like, to name but a few, must remain extremely suspect.

I do not recall reading two different descriptions of the candidate planet - one if the closer distance is considered and the other if the farther distance is accepted. This would be refreshing. It would highlight the fundamental uncertainties in the planet search projects.

It would humanize the scientists as infallible. This is probably why I don't see it in the public literature.

"Scientists" can't be trusted. Otherwise, there wouldn't be a need for peer reviewed journals to announce discoveries and research results. This is often overlooked.

Another point that is often overlooked is that, since our Sun serves as the model, the scientist can not know more about another star than we do about our own Sun. To claim otherwise is not scientifically sound. I believe it would violate the Scientific Method. Don't you agree?

Therefore, the cyclic repetitive variations in a star's brightness MAY be caused by something the scientist don't know about yet - not necessarily a planet! Until "ground truth" is obtained, all "confirmed" discoveries must remain suspect - until proved otherwise, in other words.

Thank you for this opportunity to express some of my opinions - even if they happen to be unpopular with some of your readers.

John Swatek

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December 10, 2011 at 1:09 pm

John Swatek, interesting comments and words of caution concerning claiming NASA Kepler has found an Earth-like exoplanet. I noticed that Sky & Telescope report here appears more cautious too. However other media reports hype this stuff up for sure. Perhaps there is a philosophy of origins at work in much of science that is seeking to view the Earth as not unique but just a cosmic accident including the origin of life.

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December 17, 2011 at 5:49 pm

Mr. Swatek's comments are an interesting mix of almost valid concerns and odd misconceptions. There is an uncertainty associated with almost all measurements, but that does not mean they can't be trusted. One has to assess and discuss them, which is what happens in peer reviewed journals. All the uncertainties associated with Kepler discoveries are well documented and treated. The presence of such journals greatly INCREASES the trust one can place in results (Mr. Swatek does not note that most other endeavors do not have this sort of quality control). Of course, if Mr. Swatek will not believe in other planets unless he is standing on them, that is his choice...

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Tom Kellogg

January 29, 2012 at 9:13 pm

Kepler stares at 150,000 stars and a few thousand have had detectable dips in brightness. The only planetary systems that are detected by Kepler are those whose plane of planetary rotation is in alignment with our direction of viewing them. It seems like a 1 percent detection rate is high enough that may indicate that all stars may have planets.

If we were viewing our solar system from lightyears away the only places would be directly in line with the ecliptic. A very slight angular distance away from the ecliptic would not be detected by brightness variation.

What percentage of directions would detect Earth if we were the only planet in our solar system? If that percentage is less than 1 percent then Kepler results seems to be indicating a very high percentage of stars have planets.

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