They have discovered many extra-solar planets out there way bigger than Jupiter, so as Paul said, mass is only part of the equation.
A spark or flame on Jupiter would not cause it to turn into a sun, even if the chemistry of the planet was similar to a sun. What is needed is a nuclear detonator (for want of a better word). Something to start the fusion of the hydrogen atoms into heavier elements and thus cause the release of energy due to the mass of the new elements being less of the mass of the elements that contributed to it creation. Ahh E=mc^2.
Something like that anyway!!! I'm no nuclear fisisist
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