Thursday, October 27, 2011

Kepler-16 b, Planets that Orbit Two Suns

       In September the media were filled with  news and illustrations of the “Tatooine  planet,” Kepler-16b, which orbits not a  single star but a close binary pair. (It was  nicknamed for Luke Skywalker’s home  world in Star Wars.) Circumbinary planets  have been strongly suspected before, but this was a solid confirmation.
       Meanwhile, out of the media limelight  at the Wyoming exoplanet conference, Bill  Welsh (San Diego State University) was  presenting convincing evidence for at least  three and possibly a dozen other circum-binary planets in Kepler’s field of view. “I  would still call these ‘candidates’,” Welsh said, “but I would be extremely surprised  if they weren’t real. We will know for sure  within just a few months.” 
       Kepler-16AB, the “Tatooine” host, is an  unresolved, 12th-magnitude speck 200  light-years away in Cygnus. Its A and B components are orange and red dwarfs  with 0.69 and 0.20 times the mass of the Sun. Their size ratio is also about 3 to 1.  The pair eclipse each other every 41 days  as they revolve around their center of mass in a mildly eccentric orbit.
         In addition, Kepler found that the system’s light also dips by 1.7% and by 0.1%  in a complex timing pattern due to a small  third body crossing both stars about every  229 days. Its slight gravitational tugs on the stars show that it has nearly Saturn’s mass. The size of its silhouette reveals that  it’s a bit smaller in diameter than Saturn  and thus should be 1.4 times denser, suggesting that it’s richer in heavy elements.
         Such a system is especially valuable  because it enables very accurate determinations of the sizes and masses of all the  bodies involved. It may also help unlock  mysteries of planet formation. The three eclipsing objects orbit within 0.5° of the same plane. This indicates that they formed at the same time, from the same protoplanetary disk, and haven’t been much disturbed since.
          That’s not to say things here aren’t changing. Gravitational interactions in this three-body system are warping the planet’s orbital plane around in a precession cycle. The planet is now just grazing the dim starfrom our viewpoint, but should start missing it altogether in 2014. The planet will also miss the brighter star beginning around 2018. Not until about  2042 will the transits start up again.
           The planet orbits only three times farther out than the separation of its two suns; they can appear up to 20° apart in its sky. Astronomers didn’t expect such a system to be stable enough to exist long-term. However, orbital simulations indicate that Kepler-16’s particular intimate arrangement is stable on timescales of at least a few million years.



Post a Comment

Related Posts Plugin for WordPress, Blogger...

Design by Free WordPress Themes | Bloggerized by Lasantha - Premium Blogger Themes | Affiliate Network Reviews