New planetary-mass object found in quadruple system

This Picture of the Week shows the unique stellar system HIP 81208, as captured by ESO’s Very Large Telescope (VLT) in Chile. Astronomers thought HIP 81208 was a system consisting of a massive central star (A, the central bright spot), a brown dwarf (B) circling around it, and a low-mass star (C) orbiting further away. However, a new study has revealed a never-before-seen hidden gem: an object (Cb), approximately 15 times more massive than Jupiter, orbiting around the smaller of the two stars (C).

The discovery of Cb means that HIP 81208 is a uniquely intriguing system with two stars and two smaller bodies orbiting each one –– in other words, a hierarchical quadruple system. The mass of the newly found Cb object places it right at the border between planets and brown dwarfs –– failed stars that are not massive and hot enough to fuse hydrogen into helium.

The hidden giant Cb was spotted when a team of astronomers, led by A. Chomez of the Paris Observatory, re-analysed archival data from the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) instrument installed on the VLT. While many other instruments use indirect methods to hunt for far-flung worlds, SPHERE uses a technique known as direct imaging: what we see here is an actual image of the system. Indeed, this is the first hierarchical quadruple system to be found using direct imaging, which will prove invaluable to understanding how complex systems like this one form and evolve.