A planet could theoretically become a star by accumulating enough gas, specifically hydrogen, to reach the conditions necessary for nuclear fusion. The minimum mass required for spontaneous fusion is approximately 10^28 kg, which is around 20 times the mass of Jupiter. If a planet collects sufficient hydrogen and achieves the necessary temperature and pressure, fusion could ignite in its core. This process hinges on both mass and composition, as a hydrogen-rich environment is essential. Ultimately, while the concept is intriguing, the practicalities of such a transformation are complex and unlikely.