Free Astronomy Magazine September-October 2016

SPACE CHRONICLES bell, separated by a distance rough- ly equal to that between the Sun and Saturn (10 au). In this scenar- io, planet HD 131399Ab travels around the star A in an orbit with a radius of about 80 au, about twice as large as Pluto’s in the Solar System, and brings the plan- et to about one third of the sepa- ration between star A and the B/C star pair. The authors point out that a range of orbital scenarios is possible, and the verdict on the long-term stabil- ity of the system will have to wait for planned follow-up observations that will better constrain the plan- et’s orbit. “If the planet was further away from the most massive star in the system, it would be kicked out of the system,” Apai explained. “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 unsta- ble very quickly.” Planets in multi-star systems are of special interest to astronomers and planetary scientists because they provide an example of how the mechanism of planetary formation T his annotated composite image shows the newly discovered exoplanet HD 131399Ab in the triple-star system HD 131399. The image of the planet was obtained with the SPHERE imager on the ESO Very Large Telescope in Chile. This is the first exoplanet to be discovered by SPHERE and one of very few directly-imaged planets. With a temperature of around 580 degrees Cel- sius and an estimated mass of four Jupiter masses, it is also one of the coldest and least massive directly-imaged exoplanets. This picture was created from two separate SPHERE observations: one to image the three stars and one to detect the faint planet. The planet appears vastly brighter in this image than in would in reality in comparison to the stars. [ESO/K. Wagner et al.] T his wide-field image shows a piece of the constellation of Centaurus (The Centaur) centred on the position of the triple star HD 131399. It was created from im- ages from the Digitized Sky Sur- vey 2. HD 131399 itself appears as a star of moderate brightness exact- ly at the centre of the picture. [ESO/ Digitized Sky Survey 2] functions in these more extreme scenarios. While multi-star systems seem exotic to us in our orbit around our solitary star, multi-star systems are in fact just as common as single stars. “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 plan- etary systems, but it shows that there is more variety out there than many would have deemed possi- ble,” concludes Kevin Wagner. “What we do know is that planets in multi-star systems have been stud- ied far less often, but are potential- ly just as numerous as planets in single-star systems.” n