A newly discovered solar system is intriguing astronomers because it has two suns – and one of its planets orbits them in the habitable zone meaning it could harbour alien life.

Two planets have been spotted in a study of the double star using NASA’s Kepler space telescope. They revealed themselves by the fade in starlight as they passed in front of, or transited, the binary which has been labelled Kepler-47.

Astronomers describe it as the first transiting circumbinary multi-planet system, a cumbersome phrase that means two planets orbiting around a pair of stars. Last year a single planet was found that has two suns just like Tatooine in Star Wars.

Kepler-47 lies about 5,000 light-years away from us in the northern constellation of Cygnus, the swan. It was found by a team from San Diego State University, USA, led by Jerome Orosz.

He said: “In contrast to a single planet orbiting a single star, planets whirling around a binary system transit a moving target. The time intervals between the transits and their duration can vary substantially, from days to hours, and therefore the extremely precise and almost continuous observations with Kepler space telescope were fundamental.”

The Kepler observatory is constantly watching 100,000 stars in the direction of Cygnus and neighbouring constellation Lyra to watch for blinks that give away the presence of planets in transit. Despite this being one relatively small patch of sky, the telescope has been mopping up planetary systems since it was launched in March 2009.

Orosz added: “Since about one third of all stars are either binary or multiple star systems, finding planets in binary star systems has very important implications not only for estimating the total numbers of planets that exist, but for how star–planet systems form as well.”

The two stars in the Kepler-47 system orbit around each other once every 7.5 days. On of the stars resembles our own Sun while its companion is only a third the size and 175 times fainter.

Kepler observations reveal that the inner planet, labelled Kepler-47b, is only three times the diameter of Earth and orbits the binary star in 49 days. The outer planet, Kepler-47c, is around 4.5 times the size of Earth, making it slightly larger than Uranus, and has an orbital period of 303 days.

That 303-day long “year” is the longest known for a transiting planet. There will surely be others even longer yet to be discovered but the rarity of their transit events obviously makes them more difficult to spot that rapidly orbiting worlds.

What is intriguing about the outer planet is that it lies inside this star system’s so-called habitable zone where conditions will be right for an Earth-like planet to have liquid water on its surface. Sadly Kepler-47c itself does not seem to be a rocky world, but it might have rocky satellites. And as Skymania has reported before, astronomers are developing techniques to detect alien moons.

William Walsh, co-author of a paper announcing the find, said: “While the outer planet is probably a gas giant planet and thus not suitable for life, large moons, if present, would be interesting worlds to investigate as they could potentially harbour life.”

The discovery, announced at a meeting of the International Astronomical Union (IAU) in Beijing, is published this week in the journal Science. The San Diego team worked with astronomers at the McDonald Observatory, Texas.

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