Circumbinary planet Kepler-16 (AB)-b

Astronomers using Kepler data have found a planet orbiting two stars. I did see the news on Twitter last night but it was only when I read the journal paper this morning that I got really interested in this announcement.

Artists impression of the Kepler-16 system. CREDIT: NASA/JPL-Caltech/R. Hurt
Over the past week we've had a slew of planet announcements including: 50 candidates discovered with HARPS (on the VLT) on Monday; 23 discovered with WASP on Tuesday; and another 7 announced since yesterday. The discoveries this week have been due to astronomers saving up announcements for the Extreme Solar Systems conference in Wyoming. They've taken the total number of exoplanet candidates up to 684! Will we reach 1000 by the end of the year?

This new system is remarkable because it is the first planet found to orbit a double star system. This is really helpful because it provides many different types of transits (each of the stars transiting each other as well as transits involving the planet) and these allow absolute values for the properties of the system. Normally a transit provides the relative sizes of the star and planet and you then have to use stellar evolution models to estimate the mass of the star and get the absolute sizes.

The paper establishes that the system contains 0.2 and 0.7 solar mass stars that orbit around a common centre of mass every 41 days. They are, in turn, orbited every 229 days by a planet with about 0.33 the mass of Jupiter and about 0.75 the radius of Jupiter.

The stars are separated by about 0.2 AU and the planet's orbit is 0.7 AU so the planet will experience temperature variations depending on which star is closer. Based on models of the star types, it is estimated that the temperature ranges from about 170 - 200 K so this will be a pretty cold world unlike Tatooine. They calculate a mean density close to water but say (based on models of planetary interiors and estimates of the planet's age) it is likely to be about half gas (hydrogen/helium) and half heavier elements (rock/ice?).

They were fairly lucky to spot this system when they did. The inclination of the planet's orbit varies. If their models of the orbit are correct, we will not see a transit of the second star from 2014-2049 and we won't see the planet transit the larger star from 2018-2042. So, if Kepler had suffered a 10 year delay to launch it would never have seen this system during its 3.5 year mission and may have had to wait until the mid 21st century to know about it.

Congratulations to everyone who has announced planet discoveries in the past week or two.

Update 13:56 BST: Rob Simpson has pointed out that citizen scientists on Planet Hunters may have spotted this planet 4 months ago. Very cool.

Posted in astro blog by Stuart on Friday 16th Sep 2011 (12:34 BST) | Permalink
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