The largest planet in our solar system by far is Jupiter, which beats out all the other planets in both mass and volume. Despite its size, it only has a density of 1.326 g/cm3, which is less than one fourth of Earth's density. The low density is one way scientists are able to determine that it is made mostly of gases and probably does not have a solid core. Jupiter is not the largest possible planet in the Universe. TrES-4 is about 1.4 times the size of Jupiter. Scientist theorize that a gas gain could get 15 times the size of Jupiter before it began deuterium fusion, making it a brown dwarf star.