Exoplanet pair orbits two stars

Outer orb sits in habitable zone of binary star system

Web edition : 6:34 pm

BEIJING ? And then there were two. The Kepler spacecraft has spied the first pair of planets passing in front of the binary star system they orbit. Adding spice is that the outer planet ? a potential Neptune-like world ? inhabits the life-friendly zone around the two stars.

?It receives about 88 percent the amount of energy the Earth receives from the sun,? says William Welsh of San Diego State University, who is scheduled to report the finding August 29 at the International Astronomical Union meeting. ?And it?s a multiple planet system. It?s hard enough to imagine how you get one planet in the binary; now we have two.?

The system, called Kepler-47, could have even more planets: A tantalizing but unconfirmed hint of an additional world lurks in the blinking starlight produced when the planetary companions pass between the two stars and Earth. The additional blink has been seen clearly just once, so more observing time would be needed to confirm a third planet. Kepler-47 is in the constellation Cygnus.

The results also appear online August 28 in Science.

So far, scientists know that the outer planet, Kepler-47c, is roughly 4.6 times wider than Earth and that it goes around the stars every 303 days. The inner planet is three times wider than Earth, and whips around the stars every 49 days. One of the stars is similar to the sun, and the other is much smaller and dimmer. The two stars orbit one another about every seven days.

Determining the boundaries of the habitable zone in binary systems usually isn?t simple because the moving stars create a shifting region in which liquid water could survive on an orbiting planet. But this pair of stars cooperated. ?It?s a sunlike star and a real wimpy star,? Welsh says. In the case of the smaller one, ?you just ignore it.?

Kepler-47c is probably too big and gassy to host life, but if it had an Earth-sized moon, that could serve as a potential exo-incubator. ?It?s pure speculation,? Welsh says. ?But being in the habitable zone, if it had a big moon around it, it?s in the right place to have the conditions you would need for life.?

There is no evidence of such a moon, but smaller ones, more like Saturn?s moon Titan, could be present. ?If this object had a moon the size of Titan, that could be very interesting,? Welsh says.

Found in: Astronomy and Atom & Cosmos

Source: http://www.sciencenews.org/view/generic/id/343336/title/Exoplanet_pair_orbits_two_stars

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