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What we know (and don't know) about exoplanet Kepler-22b

This artist's conception illustrates Kepler-22b, a planet known to comfortably circle in the

The discovery of Kepler-22b, an exoplanet orbiting Kepler-22 (otherwise known as UCAC3 276-148830, a sun-like G5 star about 600 light years from Earth) within the "habitable zone," the region where liquid water could exist on a planet's surface, was confirmed on December 5, 2011.

Kepler-22b is about 2.4 times the radius of Earth. Its orbital period is 289.9 days, which sets the semimajor axis of its orbit at 0.85 Astronomical Units. Scientists don't yet know if the newly discovered planet has a predominantly rocky, gaseous, or liquid composition, but its discovery is a step closer to finding Earth-like planets.

AAAS MemberCentral had the opportunity to ask AAAS member Alan Boss of the Department of Terrestrial Magnetism at the Carnegie Institute for Science about Kepler-22b and the status of the quest for exoplanets.  Here are his comments.

AAASMC: Can you briefly describe Kepler-22b and its home star?
Alan Boss:
The planet is a super-Earth, that is, a planet with a mass perhaps in the range of 10 to 15 times that of the Earth. We do not know of what it is composed, but given its size, about 2.4 the diameter of the Earth, we expect it to be made up of rock, iron, ice, and water. Most likely it has an ocean covering most of its surface.  If the planet has an atmosphere, as we expect it does, the average temperature on the surface should be about 72 degrees Fahrenheit.

The host star is a star remarkably similar to our sun -- if we were living on the planet and looked up at the star, it would look very much like our own sun. It has just about the same mass and size, though it is a little bit fainter.

AAASMC: What observational methods and techniques have so greatly changed the exoplanetary landscape?  Is this new momentum likely to continue?
51 Peg b, discovered in 1995, is considered the first bona fide planet found around a sun-like star.  Since then, most of the confirmed planet candidates have been found by Doppler spectroscopy, which measures the wobble of the star around the center of mass of the star-planet system. Ground-based transit surveys have found the next largest number of exoplanets. Kepler has now found over 2000 exoplanet candidates, by doing a transit survey from space, so that the Earth's atmosphere does not interfere with the observations. Kepler will continue to discover large numbers of new exoplanets, especially if NASA grants a mission extension for Kepler.

AAASMC: Kepler-22b was discovered by observing its transit between its star and us.  This makes the atmosphere (if any) surrounding the planet available for observational analysis.  Is there currently any sign that Kepler-22b has an atmosphere, and if so, what is known about it?
Exoplanetary atmospheres are studied by how the light of the host star is absorbed by passing through the planet's atmosphere.  An atmosphere on Kepler-22b has not been detected to my knowledge, and it is unlikely to be detected with any current instrumentation.

AAASMC: What upcoming technique and/or missions may tell us more about the nature of Kepler-22b? And what sort of characteristics might we be able to discover, if present?
Kepler-22 may be too far away for even the yet to be launched James Webb Space Telescope to say anything about the atmosphere of its planet. We need to find planets that are much closer to Earth for us to do a proper follow-up.

AAASMC: The Drake equation attempts to quantify the number of SETI-discoverable civilizations in the galaxy. Two of the multiplicative factors in Drake's equation characterize solar systems in ways to which the current spate of exoplanetary discovery is relevant --  fp is the fraction of stars that have planets, and ηe is the average number of planets that can potentially support life per star that has planets. What effect has the recent spate of exoplanetary discoveries had on the Drake equation?
Boss: It means that ηe is going to turn out to be fairly close to one, though we won't know for sure what it really is until Kepler finishes an extended mission, perhaps four or five years from now.

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Brian Dodson, Ph.D.

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