The 2006 International Astronomical Union General Assembly defined a planet as an object that "(a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighbourhood around its orbit." This definition is broad enough to include the four terrestrial and four jovian planets in spite of the vast difference in size and composition, but Pluto was excluded. Pluto now belongs to a new class of objects known as dwarf planets.
The most striking difference between terrestrial and jovian planets is size. Earth, the largest terrestrial planet, measures 7,926 miles (12,756 km) in diameter. Jupiter dwarfs Earth, with a diameter of 88,700 miles (142,700 km). More than 1,000 Earths could fit inside Jupiter. Even Neptune, the smallest jovian planet, has a diameter almost four times that of the Earth. The smallest terrestrial planet, Mercury measures a mere 3,032 miles (4879 km) in diameter. Jovian planets are also called gas giants.
The dense, terrestrial planets have solid surfaces composed of rock and metal, and have relatively thin atmospheres. Jovian planets are primarily hydrogen and helium, with thick, dense atmospheres and surfaces of compressed liquid hydrogen. In spite of their size, jovian worlds rotate much faster than terrestrial planets. A day on Jupiter lasts just 9 hours, 55 minutes. Saturn is best known for its rings, but rings encircle all four jovian planets. No terrestrial planets have rings.
Running between Mars and Jupiter, the asteroid belt forms the boundary between the inner and outer solar system. The four terrestrial worlds are inner planets. They circle close to the sun and to each other, in tight, round orbits. The jovian outer planets are much farther away from the sun and more spread out, and their orbits are more elongated and elliptical. From Neptune, the daytime sun would appear as a distant gleam against a black, star-filled sky.