What exists there between two stars

Satellite observations show that popular science fiction motifs also exist in reality

Kepler-16

Mountain View (USA) - A popular motif in science fiction films: Two suns in the sky of an exotic planet. For the first time, an international team of researchers has found direct evidence of the existence of a planet orbiting two suns. As seen from Earth, the planet passes alternately in front of the two stars and thus causes telltale fluctuations in brightness of the double star, the scientists report in the journal "Science".

"So far there has been no direct evidence for the existence of planets orbiting two stars," write Laurance Doyle of the SETI Institute in California and his colleagues. There are indirect indications of planets in some close binary stars, but the companions suspected there have so far eluded observation. It is different with Kepler-16: Doyle and his colleagues were able for the first time to use the light curve of the double star to directly demonstrate how the planet passes in front of the two stars of the system, weakening their radiation

Kepler-16 is one of the 155,000 stars whose brightness is monitored fully automatically by the Kepler satellite telescope. Kepler is on the hunt for planets: If the orbit of a planet is just such that it regularly passes in front of the star when viewed from Earth, these "transits" as small, periodic diminutions in brightness of the star reveal the existence of the companion. The Kepler data initially showed that Kepler-16 is a binary star. The two stars are smaller than our sun - they have a mass of 69 and 20 percent of the solar mass respectively - and orbit each other with a period of 41 days.

As they move past each other on their orbit, they periodically weaken their overall brightness by 13 and 1.6 percent, respectively. However, Doyle and his team encountered further, not entirely regular attenuations of the brightness of 1.7 and 0.1 percent. The only plausible explanation for these additional fluctuations is a planet the size of Saturn, which orbits the double star with an orbital period of 229 days on an almost circular orbit. The slight irregularity of the planetary transits can be explained by the fact that the two stars themselves are in motion and are therefore not always in the same place when the planet passes in front of them.

Is such an orbit around a binary star stable? Doyle and his colleagues investigated this question with computer models. The simulations show that, at least over two million years, there are only insignificant changes in orbit that do not endanger stability. However, the changes lead to a shift in the planet's orbit, so that the transits of the companion are not always visible from Earth. The transitions before the brighter of the two stars from 2018 to 2042 cannot be observed. So the researchers are lucky to catch Kepler-16 at a favorable time.