Astronomers discover planet with three suns


A gas giant 340 light-years from Earth in a complex star system has one of the widest orbits of any exoplanet spotted so far. Bill Condie reports.


An artist's impression of the triple-star system HD 131399 from close to the giant planet orbiting in the system.
European Southern Observatory / L. Calçada

Astronomers have found a strange world where the seasons last longer than a human lifetime. In some seasons it is always light, while in others there are three sunrises and sunsets a day.

The planet, HD 131399Ab, is a gas giant, 340 light-years from Earth in the constellation Centaurus and is a part of a complex three-star system, called HD 131399.

Two of the stars are binary system while HD 131399Ab orbits the third, brighter star in the widest known orbit within a multi-star system.

The planet is massive – about four times Jupiter’s mass – with a temperature of 850 kelvin (580 °C).

But, at only about 16 million years old, it is it one of the youngest exoplanets discovered so far.

“HD 131399Ab is one of the few exoplanets that have been directly imaged, and it’s the first one in such an interesting dynamical configuration,” said Daniel Apai of the University of Arizona.

His team observed the system using direct imaging at ESO's Very Large Telescope (VLT) in Chile as part of the Nexus for Exoplanet System Science (NExSS) project, dedicated to the search for life on planets outside our solar system.

The discovery was published in the journal Science.

The system appears to be centred on a star about 80% more massive than the sun, which itself is orbited by the two remaining stars, denominated B and C, at about 300 AU (one AU, or astronomical unit, equals the average distance between Earth and the sun).

B and C twirl around each other like a spinning dumbbell, separated by a distance roughly equal to that between our sun and Saturn.

Planet HD 131399Ab travels around the central star, A, in an orbit about twice as large as Pluto’s in our solar system.


“For about half of the planet’s orbit, which lasts 550 Earth-years, three stars are visible in the sky, the fainter two always much closer together, and changing in apparent separation from the brightest star throughout the year,” said Kevin Wagner, the paper's first author.

“For much of the planet’s year, the stars appear close together, giving it a familiar night-side and day-side with a unique triple-sunset and sunrise each day.

“As the planet orbits and the stars grow farther apart each day, they reach a point where the setting of one coincides with the rising of the other – at which point the planet is in near-constant daytime for about one-quarter of its orbit, or roughly 140 Earth-years.”

HD 131399Ab is the first exoplanet to be discovered with SPHERE – the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument installed on the VLT.

SPHERE is sensitive to infrared light, making it capable of detecting the heat signatures of young planets, along with sophisticated features correcting for atmospheric disturbances and blocking out the otherwise blinding light of their host stars.

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Bill Condie is publisher of Cosmos.
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