Smaller than stars, but bigger than giant planets, brown dwarfs are too small to sustain the hydrogen fusion process that fuels stars and allows them to remain hot and bright for a long period of time. So after formation, brown dwarfs slowly cool down and contract over time. The contraction usually ends after a few hundred million years, although the cooling is continuous. But sometimes, what is thought to be a brown dwarf is actually a planet -- or at least planet-like. A team at the Carnegie Institution for Science in Washington DC has discovered that what astronomers had previously thought was one of the closest brown dwarfs to our own Sun is in reality a Planetary Mass Object.