How small are the smallest celestial objects that form like stars, but don’t produce their own light?

How common are they compared to full-fledged stars? How about “rogue planets,” which formed around stars before being tossed into interstellar space? When NASA’s James Webb Space Telescope launches in 2021, it will shed light on these questions.

Answering them will set a boundary between objects that form like stars, which are born out of gravitationally collapsing clouds of gas and dust, and those that form like planets, which are created when gas and dust clump together in a disk around a young star. It will also distinguish among competing ideas about the origins of brown dwarfs, objects with masses between 1% and 8% of the Sun that cannot sustain hydrogen fusion at their cores.



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