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This diagram compares our own solar system to Kepler-22, a star system containing the first "habitable zone" planet discovered by NASA's Kepler mission. The habitable zone is the sweet spot around a star where temperatures are right for water to exist in its liquid form. Liquid water is essential for life on Earth.
Kepler-22's star is a bit smaller than our sun, so its habitable zone is slightly closer in. The diagram shows an artist's rendering of the planet comfortably orbiting within the habitable zone, similar to where Earth circles the sun. Kepler-22b has a yearly orbit of 289 days. The planet is the smallest known to orbit in the middle of the habitable zone of a sun-like star. It's about 2.4 times the size of Earth.
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Kepler-22b is the first confirmed extrasolar planet which could be a Super-Earth orbiting in the habitable zone of a Sun-like star discovered by NASA's Kepler Space Telescope.
Discovery
The discovery was announced December 5, 2011. The planet was originally discovered on Kepler's third day of science operations in mid-2009. The third transit was detected in late 2010. Additional confirmation data was provided by the Spitzer Space Telescope and ground-based observations. The planet's radius is roughly 2.4 times the radius of Earth; it is 600 light years away from Earth, in orbit around the G-type star Kepler 22.
Composition and structure
To date, its mass and surface composition remain unknown. If it has an Earth-like density (5.515 g/cm3) then it would contain 13.8 Earth masses, while its surface gravity would be 2.4 times Earth's. If it has water-like density (1 g/cm3) then it would mass 2.5 times Earth's and have a surface gravity of 0.43 times Earth's. The planet may fall into the category of planets known as super-Earths, depending on what the actual mass is.
Possibility of life
The distance from Kepler-22b to its host star is about 15% less than the distance from Earth to the Sun, but the luminosity (light output) of Kepler-22b's star is about 25% less than that of the Sun. This combination of a shorter distance from the star and a lower stellar luminosity are consistent with a moderate surface temperature. Scientists estimate that in the absence of an atmosphere, the equilibrium temperature would be approximately -11°C. If the atmosphere provides a greenhouse effect similar in magnitude to the one on Earth, the planet would have an average surface temperature of 22 °C (72°F).
At 2.4 times the size of the Earth, Kepler 22b is substantially larger than Earth and may therefore have a different composition. For example, the newly discovered planet may not be Earth-like, but rather more like Neptune, which is mostly ocean with a small rocky core. Nonetheless, Natalie Batalha, one of the scientists on the project, speculated "it's not beyond the realm of possibility that life could exist in such an ocean.
Image Source: NASA
Source: Wikipedia
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