The largest planet in the Solar System is Jupiter. And it's not just a little larger than everything else, Jupiter is 2.5 times bigger than all the other planets in the Solar System combined.
Let's take a look at just how big Jupiter is. Jupiter is one of the four gas giants; that is, it is not primarily composed of solid matter. It is the largest planet in the Solar System, having a diameter of 142,984 km at its equator. Jupiter's density, 1.326 g/cm�, is the second highest of the gas giant planets, but lower than any of the four terrestrial planets. You could line up 11 Earths side by side, and they wouldn't quite equal the width of Jupiter.
But perhaps the most amazing number is Jupiter's volume. If Jupiter was just a hollow shell, you could fill it up with 1321 Earths, and still have a little room over. Jupiter is pretty much as big as a planet can get. If you put more mass into Jupiter, it would actually compress down further. Even if Jupiter had four times as much mass, it wouldn't be any larger than it is today.
The planet was known by astronomers of ancient times and was associated with the mythology and religious beliefs of many cultures. The Romans named the planet after theRoman god Jupiter.[14] When viewed from Earth, Jupiter can reach an apparent magnitude of -2.94, making it on average the third-brightest object in the night sky after the Moon andVenus. (Mars can briefly match Jupiter's brightness at certain points in its orbit.)
Jupiter is primarily composed of hydrogen with a quarter of its mass being helium; it may also have a rocky core of heavier elements. Because of its rapid rotation, Jupiter's shape is that of an oblate spheroid (it possesses a slight but noticeable bulge around the equator). The outer atmosphere is visibly segregated into several bands at different latitudes, resulting in turbulence and storms along their interacting boundaries. A prominent result is the Great Red Spot, a giant storm that is known to have existed since at least the 17th century when it was first seen by telescope. Surrounding the planet is a faint planetary ring system and a powerfulmagnetosphere. There are also at least 63 moons, including the four large moons called theGalilean moons that were first discovered by Galileo Galilei in 1610. Ganymede, the largest of these moons, has a diameter greater than that of the planet Mercury.

Jupiter has been explored on several occasions by robotic spacecraft, most notably during the early Pioneer and Voyager flyby missions and later by the Galileo orbiter. The most recent probe to visit Jupiter was the Pluto-bound New Horizons spacecraft in late February 2007. The probe used the gravity from Jupiter to increase its speed. Future targets for exploration in the Jovian system include the possible ice-covered liquid ocean on the moon Europa.
Don't worry about Jupiter getting any bigger; it's actually slow shrinking on its own. Each year, Jupiter compresses down about 2 cm per year. This gravitational compression generates a tremendous amount of heat. So Jupiter actually gives off more heat than it receives from the Sun.