Giant gas ball four times the mass of Jupiter is in a three-star system. It swings around one, but sees sunrises and sunsets from all three. Take that Tatooine

A gaseous planet with three suns and a mass four times that of Jupiter has been spotted by astronomers.

Located 320 light years away in the constellation of Centaurus the planet, known as HD 131399Ab, goes one better than Luke Skywalker’s home planet of Tatooine in the film Star Wars, which famously boasted two sunrises.

But the triple-sun system is far from simple – HD 131399Ab orbits around only the brightest of the three stars which is thought to be around 80% more massive than our own sun. The other two stars, it is thought, are rapidly spinning around each other, with all three stars in orbit around the system’s centre of mass.

The upshot is that an observer on the planet would experience up to three sunrises at certain times of year, while at other times they would be bathed in constant daylight.

“You could see sunsets and sunrises of three stars, or could see single stars rising or setting,” said Daniel Apai, a co-author of the research from the University of Arizona. “The stars have different temperatures which gives them different colours so there would be a bluer, a yellower and a redder star,” he added.

While a handful of planets are currently known to exist in three-star systems, the 16-million-year-old HD 131399Ab is one of the youngest planets to be discovered outside our solar system.

What’s more its proposed orbit is the widest ever seen for an exoplanet in a multi-star system, taking an estimated 550 Earth years for the planet to travel around the brightest star, with a radius around 80 times the distance of the Earth from the sun.

Facebook Twitter Pinterest An illustration of the orbit of the gas giant planet in the HD 131399Ab system (red line) and the orbits of the three stars (blue lines). Photograph: ESO

“What makes this system special is that the orbit of the planet is so long that [it] also feels the influence of the other stars,” said Apai. “If it would be a little bit longer, then these other two stars could wrestle the planet away.”

Quite how it ended up in such an orbit is something of a puzzle. “It is possible that it formed pretty much where it is now, but it is not something our models would predict,” said Apai. Other possibilities, he says, are that it formed close to the brightest star and was kicked into its current orbit by a bigger planet, or that it formed near the other two stars and was later ejected, ending up on its current path. “We should be able to figure this out soon and in either case the results will be exciting,” said Apai.

Published in the journal Science by an international team of astronomers, the study reveals how HD 131399Ab was discovered from ground-based observations using the European Southern Observatory’s Very Large Telescope in Chile with a device known as the Spectro-Polarimetric High-Contrast Exoplanet Research Instrument.

The set-up allows astronomers to take a direct image of exoplanets, provided they orbit far enough away from their star and are hot enough to allow detection by thermal imaging.

The approach, says Apai, offers advantages over instruments such Nasa’s Kepler Space Observatory which looks for a dip in the light intensity of a star as an indirect clue that a planet is passing in front of it. “This system, in contrast, is one where we have actually taken an image and we see three stars and we can see the planet itself,” he said.



But the technique had seldom been used to explore multiple star systems as it was previously thought unlikely that a planet would be found at the necessary distance.

“We were only looking around single stars because we had made the assumption that the planets had formed where we found them,” said Zoë Leinhardt, a computational astrophysicist at the University of Bristol who was not involved in the study. “What [the authors] are suggesting is that the planet didn’t form where they found it but that it moved there,” she added.

The research also provides intriguing insights into the nature of the newly discovered planet. A gas giant, it is primarily composed of hydrogen and helium, with an upper atmosphere that contains a mix of water and methane. Below that, the researchers predict, are clouds of tiny silicate grains, with the deeper regions of the atmosphere boasting clouds of liquid iron.

Thought to have a mass four times that of Jupiter, the planet has temperature of about 850K (577 degrees celsius) which is far cooler than other exoplanets detected using the same technique.

While the team are still working out how the planet ended up in its current orbit, its discovery, they say, suggests that the range of planetary systems present in the universe is far wider than we thought.

Leinhardt agrees. “This is another example of a system that was ignored because we assumed we understood something,” she said.