Using observations obtained with the Very Large Telescope (VLT) of ESO, a team of astronomers led by David Bennett from the University of Notre Dame, USA, report the discovery of a new substellar system. The observations were made using gravitational microlensing, a novel and increasingly successful technique that uses the gravitational field of massive objects, such as stars or planets, as a natural lens to magnify and distort the light of background source stars.

The new system, called OGLE-2016-BLG-1190Lb, consists of a brown dwarf —a "failed star" not massive enough to sustain nuclear fusion in its core— and an even fainter companion that is either another brown dwarf or a massive planet. The companion is one of the lowest-mass objects ever found using microlensing, opening the possibility that microlensing surveys could become powerful tools to detect and study such low-mass objects in the Galaxy.

The discovery was made using the Optical Gravitational Lensing Experiment (OGLE), which has discovered more than 100 planets and brown dwarfs using gravitational microlensing since 1992. "This detection demonstrates that OGLE can be used to discover low-mass companions that orbit either brown dwarfs or binary stars," says Bennett. "The faintness of both the source star and the lens system made this discovery challenging, but we were able to determine that the lens is likely either a very low-mass brown dwarf or a giant planet orbiting a more massive brown dwarf or a distant star," adds Przemek Mróz, a co-author from Warsaw University Observatory, Poland.