Science & Environment

Tatooine-like double-star systems can host planets

Binary system
An artist's impression depicts the two relatively small planets orbiting a binary in the constellation Cygnus

A new study shows that planetary systems can form and survive in the chaotic environment around pairs of stars.

A team reports in Science the discovery of two planets orbiting a pair of stars - a so-called binary.

Gravitational disturbances generated by stellar pairs are thought to be very severe for any orbiting planets.

Nasa's Kepler space telescope found two small planets around a pair of low-mass stars.

Such systems have particular significance for science fiction fans. In the Star Wars films, Luke Skywalker's home planet of Tatooine orbits a binary star.

The planetary system, known as Kepler-47, is located roughly 5,000 light-years away, in the constellation Cygnus.

It contains a pair of stars whizzing around each other every 7.5 days. One star is Sun-like, while the other is about one-third the size of its neighbour and emits just one 175th as much light.

Circling the stars is an inner planet about three times larger in diameter than the Earth, and an outer planet that is just slightly larger than Uranus.

The inner planet - dubbed Kepler-47b - takes 49 days to complete an orbit, while the outer planet - Kepler-47c - takes 303 days.

The orbit of the outer planet places it in the so-called "habitable zone", the region around a star where it is neither too cold nor too hot for liquid water to persist on the surface of a planet.

While the outer world is probably a gas-giant planet and thus not suitable for life, its discovery establishes that these "circumbinary" planets can, and do, exist in habitable zones.

A handful of single circumbinary solar systems have been discovered before, but this is the first known example of more than one planet around a pair of stars.

It is thought that gravitational perturbations generated by the binary could either toss planets out of the system, into one of the stars, or fling them into devastating head-on collisions.

"Kepler-47 shows us that typical planetary architectures, with multiple planets in co-planar orbits, can form around two stars," said co-author Joshua Carter, from the Harvard-Smithsonian Center for Astrophysics.

"We've learned that circumbinary planets can be like the planets in our own Solar System, but with two suns."

Prof William Welsh, from San Diego State University, who presented the work at the International Astronomical Union (IAU) meeting in Beijing on behalf of the Kepler Science Team, said: "The thing I find most exciting... is the potential for habitability in a circumbinary system.

"Kepler-47c is not likely to harbour life, but if it had large moons, those would be very interesting worlds."

The Kepler space telescope discovered the planets by measuring the tell-tale drop in brightness they cause when they transit (eclipse) their host stars.

Spectroscopic data from telescopes at McDonald Observatory in Texas enabled the absolute sizes of the planets to be measured.

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