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Massive planet found within a triple-star system

It defies what researchers thought was possible

ESO/L. Calçada

More than 340 light years away, a giant planet is trapped in a dance with three stars. It’s a unique world, where suns rise and set three times a day, and where one year lasts more than half a millennium. It’s so bizarre that the existence of such a place was thought to be impossible — but astronomers have captured a rare, direct picture of this exotic domain.

Using the Very Large Telescope in northern Chile, researchers have directly imaged this planet — named HD 131399Ab — within a triple-star system, according to a new study published today in Science. It’s a feat that’s incredibly difficult to do. Multi-star systems are super bright, so much so that they often drown out any planets orbiting nearby, making them hard to spot.

"One of the hardest observations you can do in modern astronomy."

"It’s definitely one of the hardest observations you can do in modern astronomy," study author Kevin Wagner, a first-year PhD student at the University of Arizona and the person who discovered HD 131399Ab, told The Verge. But by using a combination of advanced optical techniques, the astronomers were able to observe this planet in the near-infrared. That makes it just one of a handful of exoplanets — or planets located outside our Solar System — to be imaged directly.

That imaging paints a picture of a fairly unique world. HD 131399Ab is thought to be a gas giant, about four times the mass of Jupiter, with water and methane in its atmosphere. And its orbit is astonishing to the researchers. It travels around one of the stars in the system at a distance that’s about twice that of the Sun and Pluto, an incredibly wide orbit that lasts 550 Earth-years. For most of that time, the planet has a routine daytime and nighttime, as the three suns rise and set close together. But as the planet moves throughout the system, there comes a time when one sun rises just as another sets. This leads to a period where HD 131399Ab is bathed in constant sunlight — an interval that lasts 140 years.

An artistic rendering of the orbits within the triple-star system. (ESO/L. Calçada/M. Kornmesser)

It’s a scenario that’s never been seen before. Researchers suspected it was nearly impossible for a planet to have that wide of an orbit in a three-star system. It was thought that such a planet would be unstable, and that the gravity from the other stars would throw the planet off course and even eject it from the system entirely. But today’s study reveals that such a set-up can actually exist, opening up the possibility that other similar systems exist elsewhere in the Universe. "That would have seemed like science fiction to us in the past," said Wagner.

It was thought that such a planet would be unstable

Wagner and his team were able to directly image HD 131399Ab using a combination of techniques known as adaptive optics and coronagraphs. With adaptive optics, astronomers use mirrors to correct for all the distortion introduced by Earth’s atmosphere. The result: images that seem as though they’ve been taken from space. The coronagraphs helped to block out the majority of the stars’ light. Originally, coronagraphs were designed to block out light from the Sun, allowing astronomers to study the much fainter outer atmosphere — or corona — that surrounds our star. That same technique was applied to this triple-star system so that HD 131399Ab could be seen.

The unusual positioning of the stars also helped the astronomers get their picture. Two of the stars, situated around 900 million miles apart, orbit tightly around each other, with the third star in the system situated 27 billion miles away from the two other stars. The stellar pair and the third star all orbit around each other. HD 131399Ab orbits around the third loner star, but only gets as close as 18 to 19 billion miles from the star duo. It all worked out just right so that the pair of suns don’t introduce too much light on the exoplanet, giving astronomers the ability to image the alien world directly.

The orbital paths of HD 131399Ab and its three suns. (ESO)

Direct imaging isn’t the typical way that scientists find exoplanets. Often, these objects are found indirectly, by observing how the planets affect their host stars. But when direct imaging is possible, astronomers can learn a lot more about these worlds than through indirect means. Through their observations, Wagner’s team have pinpointed HD 131399Ab’s size and atmospheric composition. They’ve determined the planet to be just 16 million years old, making it relatively young in a Universe that’s thought to be 14 billion years in age. And it’s estimated to have a temperature of about 1,076 degrees Fahrenheit (580 degrees Celsius). That makes it one of the coldest exoplanets to be imaged directly.

This planetary system may not be around for long

But what makes HD131399Ab so unique is its strange orbit, especially since astronomers aren’t sure how it got in that position. It’s possible that the exoplanet formed right where it is or that it somehow migrated into the system. Either way, it’s going to rewrite the rules for what types of systems are conceivable in the Universe, according to Wagner.

It’s also possible that this planetary system won’t be around for long. While imaging shows the planet to be in a stable orbit now, that may not always be the case. On its orbit around the lonely star, the planet comes fairly close to the other two stars in the system. That may be close enough so that eventually HD 131399Ab is wrestled off of its orbit and shot out into the Universe. "It’s pretty astonishing to see a system like this that’s right on the edge of stability," said Bruce Macintosh, an exoplanet expert at Stanford University, who was not involved in the study. "If we came back in another 20 million years, the planet might not even be there."

Earth-sized exoplanet found orbiting nearby star in 2015