A: Jupiter is the most massive planet in our solar system, weighing more than twice as much as all the other planets combined. But it still falls far short of the heft needed to ignite nuclear fusion and become a star.
Jupiter’s mass is about 4.17 x 1027 pounds (1.89 x 1027 kilograms); the Sun’s mass is about 4.27 x 1030 pounds (1.98 x 1030 kg), or about 1,048 times the mass of Jupiter. An alternative way of expressing this is Jupiter weighs less than 0.1 percent the Sun’s mass. According to surveys, stars with about one-quarter — or 25 percent — the Sun’s mass are the most common. That mass is still nearly 262 times Jupiter’s.
Smaller stars do exist: Based on the heat and pressure required for nuclear fusion in a star’s core, astronomers believe the cutoff for the smallest stars may be around 0.08 times the mass of the Sun. That’s still roughly 83 to 85 times Jupiter’s mass. In fact, the smallest star discovered to date, EBLM J0555-57Ab, weighs in at about 85 times the mass of Jupiter.
So, Jupiter would need another 83 to 85 times its mass before it could start fusing hydrogen into helium. However, if you piled just 13 or so more Jupiters onto the gas giant, its new mass might be enough to ignite deuterium fusion. (Deuterium is an isotope of hydrogen.) This wouldn’t make Jupiter a star, but it would make it a brown dwarf. These substellar objects fuse deuterium into hydrogen-3, another isotope of hydrogen. Brown dwarfs are considered neither stars nor planets, and instead occupy a gray area between the two.