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Kepler Confirms First Planet in Habitable Zone of Sun-Like Star

Artist's conception of Kepler-22b. Credit: NASA/Ames/JPL-Caltech

Scientists from the Kepler mission announced this morning the first confirmed exoplanet orbiting in the habitable zone of a Sun-like star, the region where liquid water could exist on the surface of a rocky planet like Earth. Evidence for others has already been found by Kepler, but this is the first confirmation. The planet, Kepler-22b, is also only about 2.4 times the radius of Earth — the smallest planet found in a habitable zone so far — and orbits its star, Kepler-22, in 290 days. It is about 600 light-years away from Earth, and Kepler-22 is only slightly smaller and cooler than our own Sun. Not only is the planet in the habitable zone, but astronomers have determined its surface temperature averages a comfortable 22 degrees C (72 degrees F). Since the planet’s mass is not yet known, astronomers haven’t determined if it is a rocky or gaseous planet. But this discovery is a major step toward finding Earth-like worlds around other stars. A very exciting discovery, but there’s more…

It was also announced that Kepler has found 1,094 more planetary candidates, increasing the number now to 2,326! That’s an increase of 89% since the last update this past February. Of these, 207 are near Earth size, 680 are super-Earth size, 1,181 are Neptune size, 203 are Jupiter size and 55 are larger than Jupiter. These findings continue the observational trend seen before, where smaller planets are apparently more numerous than larger gas giant planets. The number of Earth size candidates has increased by more than 200 percent and the number of super-Earth size candidates has increased by 140 percent.

According to Natalie Batalha, Kepler deputy science team lead at San Jose State University in San Jose, California, “The tremendous growth in the number of Earth-size candidates tells us that we’re honing in on the planets Kepler was designed to detect: those that are not only Earth-size, but also are potentially habitable. The more data we collect, the keener our eye for finding the smallest planets out at longer orbital periods.”

Regarding Kepler-22b, William Borucki, Kepler principal investigator at NASA Ames Research Center at Moffett Field, California stated: “Fortune smiled upon us with the detection of this planet. The first transit was captured just three days after we declared the spacecraft operationally ready. We witnessed the defining third transit over the 2010 holiday season.”

Comparison of the Kepler-22 system with our own inner solar system. Credit: NASA/Ames/JPL-Caltech

Previously there were 54 planetary candidates in habitable zones, but this was changed to 48, after the Kepler team redefined the definition of what constitutes a habitable zone in order to account for the warming effects of atmospheres which could shift the zone farther out from a star.

The announcements were made at the inaugural Kepler science conference which runs from December 5-9 at Ames Research Center.

See also the press release from the Carnegie Institution for Science here.


Paul Scott Anderson is a freelance space writer with a life-long passion for space exploration and astronomy and has been a long-time member of The Planetary Society. He currently writes for Universe Today and Examiner.com. His own blog The Meridiani Journal is a chronicle of planetary exploration.

Comments on this entry are closed.

  • Anonymous December 6, 2011, 5:14 PM

    This is about 13.8 times the mass of Earth and its surface gravity would be 5.75 times Earth’s. This planet could also turn out to be not a rocky super-Earth, but a Neptunian type of gas sub-giant. So at 600 light years away and with its physical dimensions homesteading is probably out of the question. If there are oceans there life could flourish of course, but on land everything would be short and squat.


    • William Sparrow December 7, 2011, 12:45 AM

      LC, Where did you arrive at the mass figure. AFAIK, the mass has yet to be determined, or am I missing something?

      • Anonymous December 7, 2011, 3:03 AM

        I assumed this planet is a rocky planet like Earth and that the density is about the same. So the mass would be 2.4^3 = 13.8 times that of Earth by volume increase. If the planet is a gas sub-giant then the density would be a lot less, maybe a quarter that of Earth, so the mass would be less.


    • magnus.nyborg December 8, 2011, 2:23 PM

      Dont forget the effects of core-compression for such a large rocky planet. A back-of-the-envelope calculation based on an earth comparison indicates you need to pour more than 2.5 times the material on a rocky planet before it will reach 2.4 times the earth radius. Proportionally more gases or volatiles significantly reduces that though, but then it would no longer be an earth-analogue.

  • Torbjörn Larsson December 6, 2011, 8:27 PM

    This is even sweeter than one can think at first. If you haven’t done so, you should head over and watch the press conference!

    On Kepler-22b:

    – The planet is well within the habitable zone, as opposed to the other two known habitable candidates which are marginal.
    – The planet is just shy being a general Earth analog (0.5 – 2.0 Earth radii; not the Kepler definition).
    – Borucki shows a diagram where it is either a water planet or rocky, but definitely not a gas Neptune! (They don’t know the mass yet.) If rocky, it is still drenched in water. Maybe no continents but definitely set for abiogenesis – temperature, water, minerals.*

    On Kepler 3d release:

    – The increase in planets is mostly due to a better data pipeline. They expect another large increase with further improvement in the update pipeline to the data pipeline!
    – The number of small planets increased most due to the better data pipeline.
    – The number of habitables went up from ~ 50 to ~ 130 before taking the atmosphere into account. So the number of habitables increased with nearly 200 % as well in the old raw measure. What happened then was that the old number of ~ 50 went down to ~ 25 before we landed on ~ 50 again.
    – The ratio of “multis”, Batalha’s term for multiplanetary systems, has remained steady. A slight increase from ~ 17 % to 20 %.

    On the Kepler conference:

    – Several speakers will release numbers on η, the ratio of exoplanet systems/stars.
    – HARPS has η ~ 50 % while Kepler has ~ 30 %. A key speaker claims he can reconcile that. [My guess is that HARPS has the better estimate.]

    * Unless of course the water is so deep that there is some form of ice shielding the crust.

  • Anonymous December 7, 2011, 2:20 AM

    If this planet has several moons, is their mass included in the estimated mass? Would the existence of moons mean the planet could be smaller than the estimate? Would the moons (or rings for that matter) affect the transit shadow, making it appear larger? If the planet is a ‘small Neptune’, could it have an Earth size moon? Thanks for keeping us informed!

    • squidgeny December 7, 2011, 2:11 PM

      A quick calculation suggests that if the planet had a single Earth-like moon (by which I mean, a moon of comparable proportion), the parent planet could not be more than about 5% smaller for the duo to cast the same silhouette area as it transits.

      The moon however would be only 1.3 times the size of Earth! We don’t understand enough about our own Moon’s formation to say whether such a moon is common among terrestrial planets or just a freak accident, but it seems like a possibility :)

      An exoplanet’s moon would always add to the silhouette, unless it was by astonishing coincidence in front of or behind the planet at the time. In theory we could detect a moon by making a distinction between the first dip in starlight, as the planet enters the disc, and the second dip as the moon follows (or vice versa). But we’re not able to detect moons yet – there’s too much noise in the data.

      • Torbjörn Larsson December 7, 2011, 2:46 PM

        Jon Voisey: Forget Exomoons. Let’s talk Exorings.

        “Fortunately, a study this year by Schlichting & Chang demonstrated that, even if the planet’s spin is aligned with the plane of orbit, it’s quite possible that the rings will be significantly warped due to gravitational interactions with the star.


        With such precision, the team suggests that Kepler should be more than capable of detecting a ring system similar in size and nature to those of Saturn. However, other transit finding telescopes, such as CoRoT, would mistake the rings for a slightly larger planet.”

      • Anonymous December 7, 2011, 3:19 PM

        Thanks! I know all this is only speculation, but thinking about a moon, Earth sized or slightly larger, orbiting a large planet inside the habitable zone, makes the mind burst with questions which we can’t yet answer. I’m praying Kepler will answer some, but we will have to wait and see.

  • Online Logo Design December 7, 2011, 5:25 AM

    This is really a great info. Thanks for sharing it with us and thank you for putting up such a helpful and useful information on your blog.

  • Anonymous December 8, 2011, 8:41 AM

    Hello, I have a question about this.
    In this article for example, you see the image of the new planet, at 600 light years away.
    Does this mean we are seeing the planet 600 years ago in the past or as it is right now?

    • squidgeny December 8, 2011, 1:11 PM

      That’s an artist’s conception – we don’t know what it really looks like.

      But yes, the light we are receiving from the system was cast about 600 years ago.

  • Anonymous December 8, 2011, 8:43 AM

    I have a question here.

    Is the image and information we see and have of this planet the actual recent information or, is it at 600 light years away, the planet as it was 600 years ago?

  • thespeedofdark December 11, 2011, 3:24 PM

    Hi Earthlings! We’ve been watching you for months now. Your planet is a mess! Who does the planetwork here? The land masses have got forests growing all over them. Yuk! There’s snow all over the mountains and nobody’s brushed the deserts for aeons. Your eating habits are vile! The heterotrophs eat each other. They even eat the autotrophs! I can’t wait till I get back to Kepler 22b (as you like to call it).