Can a planet become a star?

[JV]

Would it be possible for a planet by collecting gas to become a star. I mean if it were to collect enough hydrogen, the temperature and the pressure at the surface of the planet could get high enough to start fusion ?

[Chronos]

The minimum mass required for spontaneous fusion is about 10^28 Kg [around 20x the mass of Jupiter]. Assuming the composition is amenable to fusion [lots of hydrogen], the core would ignite once the former planet accumulated the necessary mass.