Three hundred and forty light years from Earth, in the constellation Centaurus, there burns a highly unlikely triple star system called HD 131399. It consists of a big, bright star — star A — and a binary pair of stars, B+C. Orbiting star A, there’s a big exoplanet: a gas giant with the working title of HD 131399Ab, in an exotic orbit we thought couldn’t be stable. Now, for the first time, researchers have directly imaged the exoplanet, and found it to be a gas giant, some four times the size of Jupiter.
While the title image is an artist’s conception of the exoplanet, straight from the ESO, the image above is a composite of directly captured visible-spectrum exposures. Where Kepler looks for exoplanets by watching their host star for periodic dimming, these photographic images come from the ground-based Very Large Telescope in the Atacama, Chile, which was pointed straight at the planet itself. The researchers used the VLT’s SPHERE instrument for its ability to do coronography, originally developed to let scientists block out the light from our sun so they could study its corona. Being able to mask out the ternary stars let the scientists directly image HD 131399Ab using adaptive optics, which use mirrors to correct for atmospheric distortion. That produces a photographic image not unlike what you might see with the naked eye. Furthermore, from spectrographic data, we know there’s both water and methane in the exoplanet’s atmosphere. Not only can we see what it looks like to the eye, we know what it’s made of and what its surface weather would be like — cold, slushy and inhospitable to life as we understand it.
Conditions on HD 131399Ab are a result of its place in space. The exoplanet orbits star A at a distance of about 80 AU, which is about twice as far away from its star as Pluto is from our Sun. Because it’s so far away, the orbital dynamics of the system mean that even though star A is 80% bigger than our sun, in the exoplanet’s sky it would be much, much dimmer than our sun — but still hundreds of times brighter than our full moon. Kevin Wagner, the paper’s first author, elaborates: “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 is only orbiting one star (Star A); we were surprised to find it where it is. As long as the planet stays close to Star A it should be safe – but if it ends up further out, it could be pulled away by Stars B+C,” said Daniel Apai, coauthor of the report and assistant professor of Astronomy and Planetary Sciences at the University of Arizona. Apai and Wagner are also part of NASA’s Earths in Other Solar Systems team (EOS for short). “Our computer simulations showed that this type of orbit can be stable, but if you change things around just a little bit, it can become unstable very quickly.”
The next step is to keep the telescopes watching this system, to better constrain its orbital dynamics. “Since the planet hasn’t moved significantly yet in its ~550 yr orbit, we haven’t been able to determine its trajectory yet,” said Wagner.
Planets in unstable orbits are usually ejected within a few thousand to a few hundred thousand years. This planet, though, is some 16 million years old, and it’s still with its parent system, so the researchers believe its orbit is stable. Once they have more data, they’ll be able to say conclusively whether the young planet will stay with its trio of stars or be flung away into space.
No matter what its trajectory ultimately turns out to be, this planet is among the first of its kind that we’ve ever directly checked out, and it’s breaking new ground in our understanding of multiple-star systems. “We can’t say yet what this means for our broader understanding of the types of planetary systems out there,” Wagner said, “but it shows there is more variety out there than many would have deemed possible.”
You can read more at the authors’ AMA on /r/science, or take a look at the ESO’s own work on the ternary system.