Saturday 15 June 2013

Kepler 69c; a Super Venus rather than a Super Earth?

Kepler 69 is a G-type star, similar to our Sun, 2700 light years from Earth in the constellation of Cygnus. On 17 April 2013, a team of scientists led by Thomas Barclay, of NASA's Ames Research Center and the Bay Area Environmental Research Institute published a paper on the arXiv online database at Cornell University Library, describing the discovery of two planets in the Kepler 69 system by the Kepler Space Telescope. The outermost of these, Kepler 69c, was deemed to be on the inner edge of the system's habitable zone (the zone within which a planet could potentially host life), and having approximately 1.7 times the radius of the Earth, was described as a 'Super Earth' by the authors of the paper.

In a second paper published on the arXiv database on 13 May 2013, and in The Astrophysical Journal Letters on 30 May 2013, Stephen Kane of the NASA Exoplanet Science Institute at Caltech, along with Thomas Barclay and Dawn Gelino, also of the NASA Exoplanet Science Institute, publish a reviewed model of the Kepler 69 system, revising both the orbit of Kepler 69c and the habitable zone of the system.

Kane et al. postulate two possible habitable zones for the Kepler 69 system, which they name the conservative and optimistic habitable zones. Based upon the original calculations of Kepler 69c's orbit, the planet would only graze the inner part of the optimistic habitable zone, spending most of its time closer to the star. The revised orbit of the planet keeps it within the optimistic habitable zone throughout its orbit, but Kane et al. calculate that this would still make the planet too hot to support life. Kepler 69c is calculated to have a radius 70% larger than the Earth, which must almost inevitably mean it also has higher gravity and a thicker atmosphere. This in turn will cause Kepler 69c to have a stronger greenhouse affect than the Earth, leading to higher temperatures, and pushing the zone in which the planet will be habitable further away from the star.

For this reason Kane et al, propose the planet should be thought of as a potential Super Venus, rather than a Super Earth.

The orbit of Kepler 69c, as originally calculated (top) and as revised by Kane et. al. (bottom). Kane et al. (2013).

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