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Extrasolar Planet Kepler-22 b

The exoplanet Kepler-22 b became well-known after it was promoted to confirmed exoplanet status and advertised for being the only planet outside of our solar system orbiting a sun-like star, that was in a potential habitable zone, and had a radius measurement (in addition to a mass measurement). Knowing both the mass and radius is important for calculating an average density and thereby putting some constraints on a planet's physical composition. To quote from the announcement paper by Borucki et al. (2012):

“Although there is no evidence that Kepler-22b is a rocky planet, it is the first confirmed planet with a measured radius to orbit in the Habitable Zone of any star other than the Sun.”

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The star Kepler-22 is about 619 light years from Earth, and both its mass and radius are withing 3% of the respective values for our Sun. The planet itself has a mass of about 35 Earth masses, which is about twice the mass of Neptune. The radius of the planet is about 2.3 times that of Earth. That makes the average density about 2.86 times that of Earth, and the surface gravity is very large, at about 6.81 times the value for Earth. The surface gravity value means that a person weighing 150 pounds (68 kg) on Earth would weigh 681 pounds (463 kg) on Kepler-22b. The implied physical conditions are thus very different to Earth and the high density is hard to explain.

The orbit of Kepler-22 b has maximum star-planet distance (semimajor axis) of about 85% of the Earth-Sun distance (0.85 astronomical units (AU)). The orbital period is about 290 Earth days.

In July 2012 Kepler-22 b was listed as one of top 5 potentially habitable exoplanets by the Planetary Habitability Laboratory (PHL). However, given the caveats discussed in the article How many habitable planets have been found?, such propositions must be regarded as tentative.

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File under: Kepler-22 b; Potential habitable zone exoplanet; Exoplanet with an unusually high density and surface gravity.

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