Brown dwarfs are smallish objects sitting somewhere between stars and planets, making them notoriously hard to find. But a recent citizen science project aimed at finding the elusive Planet 9 has instead revealed a treasure trove of these oddities, right next door.
Brown dwarfs are weird. They’re too small to ignite nuclear fusion of hydrogen in their cores, so they can’t be stars. But they’re much bigger than planets and form more like stars do. Indeed, they’re big enough to (temporarily) fuse deuterium in their cores right after they’re born.
Since they represent such an important bridge between planets and stars – a cosmic “missing link”, if you will – astronomers are very curious about where they are, what masses they have, and how many exist in the universe.
Too bad they’re really hard to find. They’re small (less than a tenth the mass of the sun) and don’t generate heat on their own. Instead the only way they emit light is through the release of heat from their formation, making them incredibly, inscrutably dim. Needless to say, astronomers don’t know a lot about brown dwarfs.
The good news is that a citizen science project accidentally captured over a hundred of them in the solar neighborhood. The project, Backyard Worlds: Planet 9, is intended to enlist amateur astronomical sleuths in the hunt for the hypothetical ninth planet in the outer edges of the solar system.
The citizen scientists haven’t found that planet, but that doesn’t mean their work has been fruitless. After identifying the candidate brown dwarfs, follow-up observations with the W. M. Keck Observatory on Maunakea in Hawaii, NASA’s Spitzer Space Telescope, Mont Megantic Observatory, and Las Campanas Observatory studied them in more detail.
Those follow-ups revealed that some of the brown dwarfs are among the coolest known (explaining why we haven’t spotted them before). Some of them even have temperatures comparable to our own planet, despite being thousands of times more massive.