WISE/IRSA

Odd system features something like a planet orbiting something not quite like a star

Is it a planet orbiting a star, or a moon orbiting a planet? Or something entirely different? Those are the questions astronomers are asking themselves about an unusual group of objects 1600 light-years away. The system includes two brown dwarfs (massive objects not quite large enough to fuse hydrogen and thus qualify as a star), one of which is being circled by another object about 70% the mass of Earth. The objects were detected by researchers monitoring the flickerings of an even-more-distant star whose light is occasionally brought into sharper focus and thus made brighter because of the gravitational effects of the intervening objects. The size, arrangement, and spacing of the individual objects in the group can be inferred from the magnitude and timing of those fluctuations in light. The planet-sized object, whose closest analog in our solar system is Venus, orbits its host at a distance of about 50 million kilometers (about the same distance as Mercury’s closest approach to our sun) once every 18 months or so, the researchers estimate in The Astrophysical Journal. The mass ratio of the brown dwarf–planet pair is about the same as that between our sun and Uranus, and the mass ratio between Jupiter and its second-largest moon Callisto. So, the researchers say, the newly described system provides evidence that the processes by which planets and moons coalesce from disks of gas and dust (as depicted in their early stages around a brown dwarf, shown) can operate similarly at many different scales.