New answers tagged fundamental-astronomy
All matter radiates (except if it's at absolute zero temperature), regardless of its composition (you got that of Mercury badly wrong). The most important form of radiation is the black-body radiation which only depends on the temperature of the material, but line emission and absorption may also be important (but depends on the composition and ionisation ...
Stars' light is produced by thermonuclear fusion of hydrogen, helium and other elements at their nucleus. This processes are impossible in planets since you need high pressures and temperatures >10,000 k
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