Astronomers have detected the firstever exoplanet to traverse a wide orbit in a chaotic triple star system. Theseemingly stable orbit of the exoplanet known as HD 131399Ab came as a surprise, as it was previously assumed that such a body would have its orbit significantly disturbed by the tumultuous gravitational influence resultingfrom the interaction between the three stellar bodies.
Multi-star systems are thought to be ascommon as single-star analogues like our own Solar System, howeverthe discovery of an exoplanet in a relatively stable orbit aroundsuch a stellar configuration is a rare find indeed. Back in September2011, the Kepler Space Telescope identified the exoplanet Kepler-16bin orbit around a binary star system. A number of these planets have been subsequently discovered and enthusiastically dubbed "Tatooine" worlds by media outlets across the globe.
A team of researchers led byastronomers from the University of Arizona have one upped Kepler-16bwith the discovery of an exoplanet holding an apparently stable orbit in atriple star system. The astronomers were able to pick out the heatsignature of the exoplanet thanks to the direct imaging capabilitiesof the Spectro-Polarimetric High-contrast Exoplanet REsearchinstrument (SPHERE) equipment mountedon the European Southern Observatory's Very Large Telescope (VLT), located at the Paranal Observatory, Chile.
Situated in the constellation ofCentaurus some 320 light-years from Earth, HD 131399Ab represents thefirst exoplanet to be discovered by SPHERE. The instrument grants the VLToutstanding infrared capabilities augmented by advanced systems designed to mitigate the disturbances created by Earth's atmosphere, and to reduce the glare created by an exoplanet's parentstar.
The newly discovered exoplanet would beinteresting enough in its own right even if it had not been for thetriple star system in which it was discovered. Based on HD 131399Ab'sheat signature, the planet is believed to be a gas giantsimilar to the planet Jupiter, yet four times its size. The exoplanetis thought to be only 16 million years old, making it one of theyoungest exoplanets ever discovered, and with a temperature of only580º C (1,076º F) it is also one of thecoldest exoplanets to be directly imaged.
While the team has emphasized thatfurther observations will be needed to nail down the orbit ofthe planet, computer modeling has come up with a likely scenario that outlines the composition and disposition of the unusual solar system.
According to this preliminary scenario, the largest star, HD131399A, isestimated to be almost twice the mass of our Sun, while the remainingtwo stars that make up the system, HD 131399B and HD 131399C, arethought to be significantly smaller.
HD 131399Ab orbits the largest of thethree stars at a distance of around 80 Astronomical Units (AU), while the less massive stars spin around one another with aseparation of roughly 10 AU, or double the distance between the Sunand Saturn, as they orbit A at a distance of 300 AU.
"For about half of the planet'sorbit, which lasts 550 Earth-years, three stars are visible in thesky; the fainter two are always much closer together, and change inapparent separation from the brightest star throughout the year,"states Kevin Wagner, a PhD student at the University of Arizona who discovered HD 131399Ab.
Based on the gravitational propertiesof the triple star system, it is likely that the stability of the exoplanet's orbit rests ona knife edge. Should its trajectory alter for any reason causing itto traverse any closer or further away from HD131399A, theunpredictable influence of the stars could lead to the exoplanet's orbit becoming very unstable very quickly.
The team plans to undertake furtherobservations designed to constrain the orbit of the planet as well as thenature of its three parent stars. By analyzing a body such as HD131399Ab, astronomers could gain insights into how planetaryformation may have occurred in so extreme and unusual anenvironment.
Scroll down to view a video rendering of artists impression of HD 131399Ab orbiting within the triple star system
Source: ESO