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successful planets [Q&A]

Vinay

Rakshit of Vidyodaya Public School asks, what are failed stars?

I am not familiar with the term, but my guess is that it refers to objects that are just below the mass threshold necessary to compress the core sufficiently to set off nuclear fusion. It is the onset of nuclear fusion that marks an object as being a star. Without that, the only source of self-luminance is gravitational contraction. The mass threshold is around 8% of the mass of the Sun, or 90 times the mass of Jupiter, roughly 2 1032 gm. Such sub-stellar objects are usually called brown dwarfs. Perhaps instead of calling them failed stars, we can call them successful planets.