A Venus-Mass Planet Orbiting a Brown Dwarf

Brown dwarfs are objects with masses that fill the gap between stars and planets. Like stars, brown dwarfs are also known to host planets around them. Most of these planets are gas giants with more than 1/10th the mass of their host brown dwarfs. These systems tend to resemble scaled down versions of binary star systems rather than normal planetary systems around stars.

Nevertheless, an object identified as OGLE-2012-BLG-0358Lb is known to be a planetary-mass object (i.e. secondary object) with 1.9 ± 0.2 times the mass of Jupiter. It orbits a brown dwarf (i.e. primary object) with 0.022 times the Sun’s mass at a projection separation of roughly 0.87 AU. This system has a low secondary-to-primary mass ratio of roughly 0.08, making it more analogous to normal planetary systems around stars than scaled down versions of binary systems.

Udalski et al. (2015) present the discovery of a system consisting of a brown dwarf orbited by a Venus-mass planet, with both objects orbiting a very low-mass star. This system was discovered via a technique known as gravitational microlensing and the event is dubbed OGLE-2013-BLG-0723. Gravitational microlensing occurs when a foreground object crosses the line of sight to a background star and the gravitational field of the foreground object acts like a lens, magnifying the brightness of the background star. In this case, the foreground object is the newly discovered system.

The three objects in the system are identified as OGLE-2013-BLG-0723Bb for the Venus-mass planet, OGLE-2013-BLG-0723B for the brown dwarf and OGLE-2013-BLG-0723A for the very low-mass star. The Venus-mass planet has 0.69 ± 0.06 the mass of Earth. It orbits at a projected separation of 0.34 ± 0.03 AU from a brown dwarf with 0.031 ± 0.003 times the Sun’s mass.

The brown dwarf itself forms a binary system with a very low-mass star with 0.097±0.009 times the Sun’s mass. Both the brown dwarf and the very low-mass star have a projected separation of 1.74 ± 0.15 AU. The Venus-mass planet and brown dwarf in the OGLE-2013-BLG-0723 system can be considered either as a scaled down version of a planet plus star system or as a scaled up version of a moon plus planet system.

Light curve of the microlensing event OGLE-2013-BLG-0723, including models with and without the planet. Left inset shows the planetary anomaly, which includes not just the obvious spike, but also a more extended low level depression. Udalski at al. (2015).

Reference:
Udalski at al. (2015), “A Venus-Mass Planet Orbiting a Brown Dwarf: Missing Link between Planets and Moons”, arXiv:1507.02388 [astro-ph.EP]