© Universität Bern / University of Bern, Illustration: Thibaut Roger
Artist's composition of the two newly discovered brown dwarfs.
Pluto is not a planet, according to the vast majority of astronomers. While it orbits the Sun and is mostly round, it does not orbit alone, instead traversing the solar system accompanied by several moons, including a companion almost half its size. This is the main reason for its demotion in 2006.
A few holdouts continue to debate this definition, but they may have a new epistemic challenge to contend with: What makes a star? When a distant object is too small and too faint to be a star, but also too big to be an exoplanet, and is not solitary, how can you be sure what it is?
Astronomers recently found a most mystifying example of such in-between objects: a pair of planetlike orbs, some 450 light-years away, that aren't bound to any host star and travel the void together. They are brown dwarfs, which are dim not-quite-stars that never grew large enough to fuse hydrogen. But they are tiny, even by brown dwarf standards, and they look more like planets than anything stellar, according to Clémence Fontanive of the University of Bern in Switzerland, the astronomer who discovered them. The larger brown dwarf of the pair sits along the boundary astronomers use to differentiate stars from planets, around 13 times the mass of Jupiter. The smaller one weighs in at only eight times the size of Jupiter.
"According to that definition, it should be a planet. But if you define that a planet should form around a star, then it's not really a planet, either," Fontanive said. She calls them "planetary mass brown dwarfs."
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