Astronomers have detected the first ever exoplanet to traverse a wide orbit in a chaotic triple star system. The seemingly 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 resulting from the interaction between the three stellar bodies.
Multi-star systems are thought to be as common as single-star analogues like our own Solar System, however the discovery of an exoplanet in a relatively stable orbit around such a stellar configuration is a rare find indeed. Back in September 2011, the Kepler Space Telescope identified the exoplanet Kepler-16b in 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 by astronomers from the University of Arizona have one upped Kepler-16b with the discovery of an exoplanet holding an apparently stable orbit in a triple star system. The astronomers were able to pick out the heat signature of the exoplanet thanks to the direct imaging capabilities of the Spectro-Polarimetric High-contrast Exoplanet REsearch instrument (SPHERE) equipment mounted on the European Southern Observatory's Very Large Telescope (VLT), located at the Paranal Observatory, Chile.
Situated in the constellation of Centaurus some 320 light-years from Earth, HD 131399Ab represents the first exoplanet to be discovered by SPHERE. The instrument grants the VLT outstanding 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 parent star.
ESO graphic displaying the likely orbital paths of HD 131399Ab and its three parent stars based on computer simulations
The newly discovered exoplanet would be interesting enough in its own right even if it had not been for the triple star system in which it was discovered. Based on HD 131399Ab's heat signature, the planet is believed to be a gas giant similar to the planet Jupiter, yet four times its size. The exoplanet is thought to be only 16 million years old, making it one of the youngest exoplanets ever discovered, and with a temperature of only 580º C (1,076º F) it is also one of the coldest exoplanets to be directly imaged.
While the team has emphasized that further observations will be needed to nail down the orbit of the 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, is estimated to be almost twice the mass of our Sun, while the remaining two stars that make up the system, HD 131399B and HD 131399C, are thought to be significantly smaller.
HD 131399Ab orbits the largest of the three stars at a distance of around 80 Astronomical Units (AU), while the less massive stars spin around one another with a separation of roughly 10 AU, or double the distance between the Sun and Saturn, as they orbit A at a distance of 300 AU.
"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," states Kevin Wagner, a PhD student at the University of Arizona who discovered HD 131399Ab.
Based on the gravitational properties of the triple star system, it is likely that the stability of the exoplanet's orbit rests on a knife edge. Should its trajectory alter for any reason causing it to traverse any closer or further away from HD131399A, the unpredictable influence of the stars could lead to the exoplanet's orbit becoming very unstable very quickly.
The team plans to undertake further observations designed to constrain the orbit of the planet as well as the nature of its three parent stars. By analyzing a body such as HD 131399Ab, astronomers could gain insights into how planetary formation may have occurred in so extreme and unusual an environment.
Scroll down to view a video rendering of artists impression of HD 131399Ab orbiting within the triple star system