Team discovers amazing planet in three-star system

Using the European Space Organization’s Very Large Telescope, astronomers have identified an incredibly unique planet orbiting a star inside a three-star system.

The discovery, reported in the journal Science, was particularly unexpected because the arrangement was thought to be too unstable to exist. It suggests that these types of systems could be more common than previously thought.

Discovering a Unique World

The planet, HD 131399Ab, is not like any other known world, the study team said. Its orbit around the brightest of the three stars is by far the widest known within a multi-star system. These kinds of orbits tend to be unstable, due to the shifting gravitational forces from the other two stars in the system, and planets in steady orbits were considered to be very improbable.

Situated around 320 light-years from Earth in the constellation of Centaurus, HD 131399Ab is approximately 16 million years old, which makes it one of the youngest exoplanets found to date, and it’s also a rare example of an exoplanet imaged directly. With a temperature close to 580 degrees C and an estimated mass of four Jupiter masses, the planet is also one of the most frigid and smallest directly-imaged exoplanets.

“For about half of the planet’s orbit, which lasts 550 Earth-years, three stars are visible in the sky; the fainter two are always much closer together, and change in apparent separation from the brightest star throughout the year,” Kevin Wagner, a graduate researcher at the University of Arizona, said in a news release.

In a likely scenario modeled by the study team, planet HD 131399Ab orbits around ‘star A’ with a radius of approximately 80 au, around twice as wide as Pluto’s, and brings the planet to approximately one third of the distance between star A and the pair of stars B and C. The authors said a range of orbital scenarios is likely, and a determination on the long-term stability of the system will be based on follow-up observations.

“If the planet was further away from the most massive star in the system, it would be kicked out of the system,” explained study author Daniel Apai, an astronomer from the University of Arizona. “Our computer simulations have shown that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly.”

“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,” Wagner added. “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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Image credit: ESA

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