Planetary three-sun baffles boffins

A Caltech researcher has identified a planet that has three suns. We wondered immediately if it was in a galaxy far, far away, but sadly for headline writers everywhere, the four-body system lies a mere 149 light-years hence, well within our own Milky Way.

The system, known as HD 188753, is located in the constellation Cygnus. The planet, whose existence was indirectly determined by measuring tiny Doppler shifts in the spectra of the stars, is thought to be slightly larger than Jupiter.

A relatively large central star holds court. The planet orbits just eight million kilometres away from it, whipping around roughly once every 80 hours. Further out, at about the distance from our Sun to Saturn, the other two stars are locked in a binary orbit of each other, and whirl around the main stellar body.

It transpires that in this region of our galaxy, binary and multiple star systems are more common than singletons like our Sun. But the presence of a planet in this triple system has foxed astronomers, causing some to suggest that we need to rethink theories of planetary formation.

Author of the research, Caltech's Dr Maciej Konacki, said that working out how a planet formed in this setting was "very puzzling". What is agreed on is that the planet is unlikely to have formed in the "normal" (as we understand it) way. Conventional theories of planet formation hold that gas giants form far away from their star, with gases coalescing around an icy core.

Astronomers speculate that some then migrate inwards, dragged by the remaining material in the accretion disk around the star. This would account for the many gas giants discovered in very close orbits around their stars.

But in this system, the two smaller stars orbit in exactly the gas giant-forming zone, and would have demolished any potential planet-forming material.

Dr. Konacki suggests that it might be better to think of the planet as a failed fourth star that just didn't have the mass needed to get going.

Meanwhile, Artie Hatzes, an astronomer at the Thuringia State Observatory in Germany told Nature.com: "You shouldn't see it, but you do see it. Maybe nature found a way."

Konacki's paper is published in the current issue of Nature. ®

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