Scientists discover planet with three suns far, far away

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

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 per cent 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 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 this it is one of the coldest exoplanets to be imaged directly.