Saturday 17 September 2011

Kepler-16 (AB)b; the first planet discovered orbiting a binary star.

The 16 September edition of the journal Science contains a paper by a team lead by Lawrence R. Doyle of the Carl Sagan Centre for the Study of Life in the Universe, part of the SETI Institute, which describes the discovery of Kepler-16 (AB)b, the first planet found in a binary star system.

An animation of the Kepler-16 system.

The Kepler-16 system is 196 light years from the Earth in the constellation of Cygnus. It was initially discovered by the Kepler Space Telescope in 2010, when it was listed as KIC (Kepler Input Catalogue) 12644769. The initial observations revealed that the system was a variable star, which careful modeling explained as a binary system, with a larger k-type orange dwarf star, 70% of the size of our sun and 77% as hot, orbited by a smaller m-type red dwarf star, 20% the size of our sun and less than 60% of its temperature, orbiting about a mutual centre of gravity every 41 days.

Follow up observations with Kepler and the Tillinghast 1.5 m Telescope at the Fred Lawrence Whipple Observatory of the Smithsonian Astrophysical Observatory revealed the presence of a third body in the system by a 3% dimming of the variable 'star' every 229 days. This could not be explained as a third star, so the team new they were dealing with a planet - the first time a planet had been discovered in a binary star system, though this has long been theorized and is a popular concept with science fiction writers.

A binary sunset on the planet Tatooine, in the movie Star Wars.

The planet was named Kepler-16 (AB)b, implying a planet orbiting both the stars Kepler-16 A and Kepler-16 B. This is slightly misleading as 'b' implies the second body discovered in a stellar system, but it Kepler-16 c could imply a body orbiting just Kepler-16 A, which would be inaccurate as Kepler-16 A is not at the centre of the system.

Kepler-16 (AB)b has been widely compared to Tatooine, from the Star Wars movies, the most widely know binary star planet in science fiction, though it is a very different planet. It is roughly the same size as Saturn, though it is much denser, implying a large rocky or icy core, perhaps occupying 50% of the planet's volume. It orbits the centre of the Kepler-16 system at a distance of 0.7 AU, roughly the same distance at which Venus orbits our sun, though it is much cooler, due to the smaller, cooler stars, with an estimated surface temperature of 200 K, far to cold for any form of life as we understand it. Seen from the planet the stars Kepler-16 A and Kepler-16B would always appear close together in the sky, and would eclipse one another every 41 days. Kepler-16A would appear as an orange disk, roughly twice the size of our sun in the sky, Kepler-16 B as a red disk two thirds the size of our sun.

An artist's impression of Kepler-16 (AB)b.