Jupiter's Mass & Size
Jupiter's size can be described in four different ways: by mass, diameter, volume and surface area. Any one of this would still make Jupiter the largest planet in the Solar System. At 1.43 x 1015 km3 (3.43 x 1014 mi3), its volume is 1321 times that of the Earth. Its surface area 6.14 x 1010 km2 or 2.37 x 1010 mi2 is 120.4 times larger than the Earth and its diameter at the equator (142,984 km or 88,846 mi) is 11 times wider than the Earth; in fact, the Great Red Spot alone is three times the diameter of the Earth. Jupiter is also the heaviest planet in the Solar System with a mass of 1.898 x 1027 kg, which is about 318 times greater than the Earth. If the masses of all the other planets in the Solar System were added up, Jupiter would still be 2.5 times heavier.
Because of its composition (mainly hydrogen), Jupiter would be considered a star, not a planet, if it were 80 times larger than it is now. That's because it would need to be that much more massive to be able to compress and produce enough heat to ignite hydrogen fusion, which is the nuclear fusion process that generates a star's energy. Compared to the Sun, the Solar System's star, Jupiter is only one-tenth its diameter and 0.001 times its mass. It is, however, only 30% smaller than the smallest red dwarf, which is a kind of star that has a very low mass. Because of these similarities, Jupiter can actually be considered as either a very large planet or a star that failed to form.
Despite its massive size, Jupiter is actually shrinking by 2 cm (0.78 in) each year. In fact, its diameter is currently half what it was when it first formed. Jupiter generates a lot of internal heat that is believed to come from residual heat left by the collapse of the primordial nebula when the Solar System formed. The planet radiates 1.6 times more energy than it receives from the Sun. Although the amount of interior heat it generates is equal to the amount of heat it receives from the Sun, additional heat is believed to be radiated by a process known as the Kelvin–Helmholtz mechanism. This mechanism explains what occurs when the surface of a planet or a star cools. As the temperature goes down, the pressure also drops, which causes the planet to contract as a result. This compression, in turn, causes the planet's core to heat up. Jupiter and Saturn are two planets that exhibit the Kelvin–Helmholtz mechanism, but it is believed that Jupiter radiates more heat through this mechanism than it receives from the Sun, whereas the same may not be true of Saturn. Brown dwarfs, which are astronomical objects whose masses are too small to be a star and whose core temperatures are not high enough to sustain hydrogen fusion, also exhibit the Kelvin–Helmholtz mechanism.
Jupiter's size is believed to be as big as it can possibly be in terms of diameter based on its composition and the history of its formation. If the planet were heavier than it is now, it would shrink considerably. This is because the additional mass would result in an increase in gravitational force that would cause the interior to become much more compressed. This compression would result in a decrease in the planet's volume, even with the additional mass.
Jupiter is not the largest planet known to exist. That distinction goes to WASP-17b, which can be found orbiting the star WASP-17 in the constellation Scorpius 1,000 light years from Earth. The discovery of this extrasolar planet (meaning it lies outside the Solar System) was announced in 2009. Although its radius is about twice as wide, its mass is only half that of Jupiter.