There is a peculiar system 1,600 light-years from our solar system. It is composed of two brown dwarfs, massive objects too big to be planets and too small to fuse hydrogen and become stars. But that is not the strange part: Scientists have now discovered a Venus-sized planet around the smaller brown dwarf.
A rocky planet orbiting a brown dwarf has never been observed before. This system is therefore thought to be either a scaled-up version of a moon going around a planet or a scaled-down version of a planet going around a star. The ratio between the mass of the host brown dwarf and that of the planet is the same as the ratio between the Sun and Uranus and between Jupiter and Callisto (its second largest moon). The astronomers argue that this suggests the same mechanisms formed Uranus, Callisto and the new planet.
Planets and moons form from an accretion disk, a rotating ring-like structure surrounding a larger gravitational body made of dust and gas. Over time, these materials coalesce into large objects that we recognize as planets and moons. The latest discovery indicates that the formation of objects from accretion disks are similar for planets, brown dwarfs and stars.
OGLE-2013-BLG-0723B, the name of the object in question, orbits its host in about 400 days and at a distance of 50 million kilometers (31 million miles). It is 30 percent less massive than the Earth and only slightly less massive than Venus. The discovery and its importance were published in a paper in the Astrophysical Journal.
The discovery was possible thanks to a technique called microlensing. If two massive objects – in this case a brown dwarf and a star – align from our perspective in the sky, the gravity of the brown dwarf acts like a lens and magnifies the light we receive from the more distant star. From our point of view, the star gets brighter and brighter, reaching a maximum when both objects are perfectly aligned and then becoming progressively dimmer until it returns to its original luminosity.
When a planet orbits the brown dwarf, there are jumps in the luminosity of the star, suddenly becoming brighter than expected. Most exoplanets are discovered because they pass in front of their stars and we can see a tiny eclipse; this technique doesn't work well for brown dwarfs, though, because they don't emit enough visible light. Microlensing, therefore, is the only way to discover rocky planets orbiting brown dwarfs.