As of today, more than 5600 exoplanets, planets that are gravitationally bound to stars other than our sun, have been discovered. Within that catalog exists a vast array of planetary classes – small rocky worlds like our own, ocean worlds completely covered in liquid water, and gas giants that dwarf even Jupiter, among many others.
Planets come in a range of sizes and compositions and are differentiated from stars primarily due to their inability to sustain nuclear fusion in their cores. So how big does a planet have to get before it becomes a star? And what are the characteristics of objects that straddle this gray area between planets and stars?
"This mass limit is considered to be about 75 to 80 Jupiter masses. This is basically the minimum mass to sustain hydrogen fusion. Below this mass, the gravitational force inwards is insufficient to generate the temperature needed for core fusion of hydrogen and the ‘failed’ star instead forms a brown dwarf," says Basmah Riaz, a Loyola Marymount University astrophysicist who studies objects called brown dwarfs.
