Stranger than Star Wars - the planet with three suns

Stranger than Star Wars - the planet with three suns

A planet with three suns has trumped Hollywood by proving that astronomical fact can indeed be stranger than fiction.

Luke Skywalker's home world Tatooine in the Star Wars movies was bizarre enough, with two suns in its sky.

But the real planet, 320 light years away in the constellation of Centaurus, is even more exotic.

An observer on the planet, known as HD 131399Ab, would either experience constant daylight or triple sunrises and sunsets each day depending on the seasons, which can last longer than a human lifetime.

Scientists who made the discovery were surprised that such a world could survive at all. The instability of its orbit had been expected to eject it rapidly out of the system into deep space.

HD 131399Ab follows a wide orbital path around the brightest of the three stars. The other two stars, twirling around each other, lie outside the planet's orbit and also circle the dominant central star, which is thought to be 80% more massive than the Sun.

The planet is one of the few worlds outside the Solar System to be directly imaged by astronomers, rather than having its existence inferred from light measurement data.

Astronomer Kevin Wagner, from the University of Arizona, US, who initially identified the planet and led follow-up observations, said: "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.

"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 further 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."

The planet, described in the journal Science, is only about 16 million years old, making it one of the youngest exoplanets discovered to date.

It has an estimated mass equivalent to four Jupiters and a searing surface temperature of around 580C. Despite these extremes HD 131399Ab is one of the coldest and least massive exoplanets to be imaged directly.

The discovery was the first to be made with Sphere (Spectro-Polarimetric High-contrast Exoplanet Research), a new ultra-sensitive instrument designed to detect the heat signature of young planets.

Sphere is part of the European Southern Observatory's Very Large Telescope in northern Chile.

PhD student Mr Wagner added: "It is not clear how this planet ended up on its wide orbit in this extreme system, and we can't say yet what this means for our broader understanding of the types of planetary systems, but it shows that there is more variety out there than many would have deemed possible.

"What we do know is that planets in multi-star systems have been studied far less often, but are potentially just as numerous as planets in single-star systems."

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