A planet with three suns

For the first time ever, a team led by the University of Arizona astronomers has reported sighted a planet orbiting a triple star system in a study published online in Science. “We found the planet using sophisticated imaging equipment on the Very Large Telescope in Chile.

This allows us to correct for the image distortion (twinkle), introduced by the Earth’s atmosphere, and also to block out most of the starlight so that we may see the faint planet next to it. This technique is called direct imaging, and is one of the most challenging observations possible in modern astronomy,” said Kevin Wagner, first author of the paper, in an email interview.

The sighting is unexpected because despite the inherent instability of the complicated dynamics of the three-star system, the planet, known as HD131399Ab, has stuck on to its stars.

Isaac Asimov’s famous novel Nightfall described a world with four suns. Now planet hunters have found this system which comes close.

“We were very excited when we confirmed the planet, but mostly we knew it was time to get to work. We observed this star for about a year, allowing it enough time to move with respect to the background stars and for us to observe that the planet indeed moves with it (that the planet is gravitationally bound to the star),” said Kevin Wagner.

Considering astronomical distances, the planet HD131399Ab is not too far from us. It is located about 320 light years away in the constellation of Centaurus, which contains the nearest star neighbour to the sun – Proxima Centauri.
The discovery of this system shows that wide, and possibly unstable, orbits can be outcomes of the planet formation and migration processes.

“It broadens our understanding of what types of planet are out there and in what sort of configurations. Piecing together how planets form in general can help us to understand how our own solar system formed and became habitable, and can inform us about where else habitable planets could exist in our own galaxy,” says Dr Wagner.

The most massive star of this triplet, HD131399A, which is about 80 per cent more massive than the sun, is orbited by the other two, less massive, stars B and C at a distance of 300 astronomical units (1 au is equal to the average distance of the earth from the sun.) While orbiting star A, stars B and C also move around each other, like a twirling dumb bell, at a distance of approximately 10 au.

Given this picture, the planet HD131399Ab revolves around the most massive star A in an orbit whose radius is approximately twice that of Pluto’s orbit. This will bring it to a region about one-third the separation between star A and the pair of stars B and C.

For most of its 550-earth-year orbit, the planet will have a familiar night-side and day-side, with a triple sunset every day. But it will eventually reach a point in its orbit where the setting of one sun coincides with the rising of another. It is estimated that the planet will be nightless for a quarter of its orbit, roughly 140 earth years.

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