Failed Stars & Super-Jupiters

The strange celestial objects that don’t make the cut as either planets or stars

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The brown dwarf is seen as a stellar failure, a dropout from the school of star formation. These gigantic objects, with their puffy, gaseous outer layers, are the universe’s students that didn’t quite make the grade. In brown dwarfs, nuclear fusion – the process that gives stars their power – has given up the ghost, leaving them relatively cold, with some no hotter than the human body. Neither planet nor star, brown dwarfs fall into the grey area between the most massive gas giant planets like Jupiter – hence why they’re known as ‘super-Jupiters’, because of their massive, gaseous nature – and the smallest stars. Their existence blurs the lines between what is a planet and what is a star, forcing us to question the differences between how planets and stars form.

Stars form when clouds of molecular gas collapse under gravity and condense until the pressure and temperature at the center of the cloud is so great that nuclear fusion reactions – which turn nuclei of the element hydrogen into heavier helium nuclei – ignite. This kind of top-down formation is one of the key differences between how stars and planets form. Meanwhile, the worlds of our Solar System and many others that astronomers have been studying over the past 25 years form through a bottom-up process, where a core gradually builds up, becoming bigger and bigger. For the most massive planets, the core has enough gravity to begin stealing gas from the proto-stellar nebula around it, and this is where gas giants such as Jupiter and Saturn got their hefty atmospheres.