In a first, astronomers see two planets orbiting binary stars
For the first time, astronomers have detected two planets in stable orbits about a binary star system, suggesting that binary stars could have complex planetary systems much like our own. One of the planets in the so-called circumbinary system exists in what astronomers call the habitable zone -- a region where water would remain liquid and thus could support life. The planet itself is Uranus-sized, however, and is thus most likely a gas giant. But if it has a large moon, that moon could conceivably support life.
The system was discovered by NASA’s Kepler mission, which continually scans a portion of the sky searching for exoplanets -- planets orbiting distant stars -- and binary star systems. The objects are revealed when they pass between a star and the Earth, causing the star’s light to dim briefly. So far, Kepler has detected more than 2,300 candidate exoplanets and 2,100 binary stars. Four of those binary systems have been found to have a single planet.
Astronomer Jerome A. Orosz of UC San Diego and his colleagues first observed the circumbinary system, called Kepler-47, then enlisted astronomer Michael Endl and his colleagues at the University of Texas at Austin to help study it with that university’s 9.2-meter (362-inch) Hobby-Eberly Telescope. William Walsh of UC San Diego presented the team’s findings Tuesday at an International Astronomical Union meeting in Beijing. The findings will be published online Thursday by Science Express.
Kepler-47 lies 5,000 light-years away in the constellation Cygnus (the Swan). The primary star in the system is about the same size as Earth’s sun; the second is a faint M-dwarf star about a third the primary’s size. The two orbit each other every 7.45 days.
The radius of the smaller planet is about three times that of the Earth and the planet orbits the binary pair every 49.5 days. It is the smallest planet orbiting a binary pair to be found so far. The larger planet’s radius is 4.6 times that of Earth, and the planet orbits the pair every 303 days, giving it the longest orbital period of any known planet orbiting a binary pair. That puts it in the habitable zone where water could be liquid.
The planets are quite different from Tatooine, Luke Skywalker’s home planet in the “Star Wars” saga, Endl said. That epic showed Tatooine having relatively constant illumination during its daytime. But the two new planets move quickly in their orbits close to and away from the binary stars, and their daytimes would thus have very uneven illumination, he noted.
The most important aspect of the discovery is simply that multiple planets in one orbital plane can form in a binary system, said co-author Joshua Carter of the Harvard-Smithsonian Center for Astrophysics in Cambridge, Mass. “We’ve learned that circumbinary planets can be like planets in our own solar system, but with two suns,” he said.
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