July 25, 2016

Astronomers discover a planet surviving in a strange system with three suns

by Megan Cartwright

At just four times’ the mass of Jupiter and a mere 580°C—hot enough to melt lead—a recently-discovered exoplanet is one of the smallest and coldest planets found outside our solar system. But what makes it really exceptional is that this planet is a survivor. Against the odds, HD 13199Ab is following a wide, stable orbit inside a complicated system of three stars—a tangle of competing gravitational fields that astronomers had long thought would fling planets into deep space. While this isn’t the first planet detected inside a multi-star system, its discovery is helping scientists understand how planets form in extreme environments, and understand more about how planets like Earth formed in calmer one-star systems.

To study the unusual system, the study’s first author Kevin Wagner had to travel from his PhD program at the University of Arizona to the Very Large Telescope in Chile last year. With the telescope’s high-tech SPHERE instrument, Wagner able to observe the brightest and most massive of the system’s three stars, which are nearly 320 light years away in the Centaurus constellation. The star was so overwhelming that Wagner had to block out its radiation before he could detect the telltale signature of infrared heat coming off an exoplanet.

Follow-up observations told him more about the planet’s complicated home life. The system’s two smaller stars twirl about each other, like a couple ballroom dancing. Together, these smaller stars orbit about the massive star at distance nearly 300 times greater than the space between Earth and the Sun. Caught between these twirling stars and the massive one, the young planet sweeps a wide orbit twice as far from the massive star as Pluto is from our sun. “If the planet was further away from the most massive star,” says study co-author Daniel Apai in a press release, “it would be kicked out of the system.”

By running simulations with data collected from the SPHERE instrument, the scientists were able to figure out what this planet would be like to an observer from Earth. Even though it’s relatively small, its surface gravity is more than six times that of Earth. Anyone who could stand upright against that powerful force that would get a spectacular view of triple sunrises and sunsets, interspersed with long seasons of constant daylight. Very long seasons: it takes HD 13199Ab almost 550 Earth-years to make one circuit around the massive star, thanks to its strangely wide orbit.

An artist’s impression of the planet (lower-left) as it faces the three stars in its system. (Credit: ESO/L. Calçada/M. Kornmesser)

“It is not clear how this planet ended up on its wide orbit in this extreme system,” says Wagner in the same press release, “and we can’t say yet what this means for our broader understanding of the types of planetary systems [possible].” Even though the planet has survived so far, it could nonetheless have a longer-term unstable orbit that will eject it into deep space. However, Wagner and his fellow astronomers will have plenty of chances to see if more multi-star systems harbor other unexpected survivors. The (relatively) small, cold HD 131399Ab and its complicated home are just one of the first systems the astronomers have studied in their survey of roughly 100 star systems. In the meantime, Wagner says that their study of this unusual planet “shows that there is more variety out there than many would have deemed possible.”

Megan Cartwright

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Megan Cartwright is a freelance science/medical writer near Seattle. Before becoming a full-time writer, she worked as a scientist studying infectious diseases and vaccines, and earned her Ph.D. in Toxicology from the University of Washington. Megan has written for Slate and Bitesize Bio, and helps write blog posts and learning modules on chemistry for Visionlearning.