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Artist s impression of the Kepler-47 system. Credit: NASA/JPL-Caltech/T. Pyle Artist's impression of the Kepler-47 system. Credit: NASA/JPL-Caltech/T. Pyle

Multiple planets orbiting binary star system

Sen— NASA's Kepler space telescope has discovered its first circumbinary planetary system - multiple planets orbiting two stars. The binary star system, designated Kepler-47, is 4,900 light-years from Earth in the constellation Cygnus. The news comes less than a year after the discovery of Kepler-16b, a planet which, like Tatooine in Star Wars, is orbiting two stars. 

The Kepler-47 system consists of an eclipsing binary where the two stars pass in front of each other every 7.5 days as viewed from Earth. The primary star is of similar size to our Sun, but less bright. The lesser of the pair is only one third the size of the Sun, and much fainter than the primary star.

Around half of the stars in our Galaxy are in binary star systems, so studying planets around these stars is of vital importance in understanding planet formation.

"In contrast to a single planet orbiting a single star, the planet in a circumbinary system must transit a 'moving target.' As a consequence, time intervals between the transits and their durations can vary substantially, sometimes short, other times long," said Jerome Orosz at San Diego State University and lead author of the Science paper. "The intervals were the telltale sign these planets are in circumbinary orbits."

Kepler-47 system

Unlike our own Solar System, Kepler-47 has two stars. One star is similar to the Sun in size, the second is smaller measuring only one-third the size of the Sun and less than one percent as bright. As the stars are smaller than our Sun, the system's habitable zone is closer in. Credit: NASA/JPL-Caltech/T. Pyle

Kepler-47b, the inner planet, has an orbital period around the binary star of less than 50 days. This is close enough to the stars for hellish temperatures to exist on the planet, and it is possible that methane could be destroyed in the hot atmosphere which would cover the planet in a choking haze. The inner planet is the smallest circumbinary planet detected to date, with a radius three times that of the Earth.

The outer planet, which is called Kepler-47c, has a much larger orbit of 303 days, which places it in the habitable zone of the star system. This is the area where the temperature is just right for liquid water to exist on the surface of a planet, however this particular planet is a gas giant of similar size to Neptune, so it is unsuitable for life. However, if any moons existed around Kepler-47c it is possible that they could host life, due to the planet’s position in the habitable zone.

"Unlike our Sun, many stars are part of multiple-star systems where two or more stars orbit one another. The question always has been - do they have planets and planetary systems? This Kepler discovery proves that they do," said William Borucki, Kepler mission principal investigator. "In our search for habitable planets, we have found more opportunities for life to exist."

"The presence of a full-fledged circumbinary planetary system orbiting Kepler-47 is an amazing discovery," said Greg Laughlin at the University of California in Santa Cruz. "These planets are very difficult to form using the currently accepted paradigm, and I believe that theorists, myself included, will be going back to the drawing board to try to improve our understanding of how planets are assembled in dusty circumbinary discs."

Launched on 6 March 2009, Kepler’s vantage point above the Earth’s atmosphere gives it the unique capability of detecting small rocky planets around other stars. Kepler is monitoring more than 150,000 stars to search for dips in light that could indicate the presence of a transiting planet.

The Kepler mission has been extended until 2016, it was announced in April this year.

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