Jupiter Diameter=142,800 km or 11.3 times that of Earth.

Jupiter, often called the King of the Planets, is the largest planet in our solar system and fifth from the sun. Like the rest of the Gaseous planets, Jupiter is composed primarily of the gases hydrogen and helium and has no solid surface. Jupiter is unique, however, in the fact that it gives off more energy than it receives from the Sun. 1,000 km beneath Jupiter's cloud tops, the enormous pressure turns the hydrogen gas into liquid hydrogen. Jupiter spins so fast (its day is the shortest of all the planets, at ten hours) that its liquid mass bulges outward at the equator. The temperature at Jupiter's cloud tops is -148 C. Jupiter is also known for its storms; the Great Red Spot is the center of a storm system that was first seen more than 300 years ago. Viewed from Earth, Jupiter is the fourth brightest object in the sky after the Sun, the Moon, and Venus. Jupiter's year is 11 years and 314.96 days long and with it on its huge orbit it drags at least 16 moons. The largest four are Europa, Io, Callisto, and Ganymede, and only Europa is smaller than our Moon.

Saturn Diameter=120,700 km or 9.4 times that of Earth.

Saturn is the sixth planet from the Sun, the second of the gaseous planets, and also the second largest planet in our solar system. Like Jupiter, it is composed mostly of hydrogen and helium and here too, at 1,000 km below its clouds, the hydrogen gas turns into liquid. Saturn is more loosely packed than Jupiter and its spin, only slightly slower, gives it the greatest equatorial bulge of all the planets. Saturn's day is 10 hours and 41 minutes long, and its year 29 years and 167.25 days. It is 1,429,370,000 km from the Sun and the temperature at Saturn's cloud tops is -178 C. Saturn is probably the most beautiful planet in our solar system, best known for its bright and complex system of rings. These measure about 274,000 km across but are only about 15 meters thick. The rings are not solid, but made up of billions of ice particles ranging in size from that of a sugar grain to that of a house. Saturn has at least 17 moons, the largest of which are Titan, Rhea, Iapetus, Dione, and Tethys.

Uranus Diameter=51,700 km or 4 times that of Earth.

Little was known about Uranus until Voyager 2 was sent there in 1986, because of its great distance from us. Uranus shows little cloud activity because it generates so little internal heat and also because of a layer of methane-ice crystals, like an ice fog, that further acts to block what lies beneath. The atmosphere on Uranus is 85% hydrogen and 15% helium. The planet rotates once every 16.7 hours and completes an orbit around the sun in 84 years and 3.65 days; the temperature here is -213 C. Uranus is the only planet to have a tipped rotation axis, rotating almost on its side. The planet seems to roll around the Sun rather than spin, resulting in each pole experiencing 21 years of sunlight and 21 years of darkness! This makes Uranus the planet with the longest winter. Uranus also has a system of nine skinny rings, which are nearly invisible from Earth but were discovered by accident in 1977 when Uranus passed in front of a star and blocked out some of its light. Uranus has 15 satellites, the largest being Titania, Oberon, Umbriel, Ariel, and Miranda.

Neptune Diameter=48,600 km or 3.8 times that of Earth

Neptune is the eighth planet from the Sun and the last of the giant ones. It is so remote that no specific atmospheric feature had ever been seen from Earth until its encounter with Voyager 2 in 1989, when the Great Dark Spot was discovered. This feature is remarkably similar to Jupiter's Great Red Spot; it has the same proportions and lies at the same latitude south of the equator. Below the visible surface of Neptune is a bottomless atmosphere of hydrogen and helium that, thousands of miles down, eventually merges with a hot slush of hydrogen, helium, and water ice sloshing around inside the planet. This core region rotates once every 16 hours, but the outer atmosphere spins more slowly and at different rates at different latitudes. The differences in the rotation creates friction which produces heat inside the planet. This in turn causes weather, such as the Great Dark Spot. The temperature at Neptune's cloud tops is about -216 C; its year about 165 Earth years. Like the other giants, Neptune also has rings; there are five of them, 3 thread-thin ones, and 2 so fuzzy that Voyager 2 could barely detect them. Neptune has 8 satellites; the largest three are Triton, 1989N1, and Neried.

Pluto Diameter=2,280 km or 18% that of Earth

Pluto is so small and so far away that it was not even discovered until 1930. It is the last planet in the solar system that we know of (it would take 10 years to travel there by spacecraft), and it is also the smallest, only about two-thirds the size of our Moon. Pluto also has a very strange orbit. It takes almost 249 years for the planet to go around the Sun and at times it crosses the orbit of Neptune, making Neptune the farthest planet in the solar system. Pluto's orbit is tilted 17 degrees (see diagram below), the largest tilt of all the planets, and this keeps Pluto and Neptune from colliding. From 1979 until 1999 Pluto is the eighth planet from the Sun and Neptune the ninth. Pluto's atmosphere is extremely thin but it does contain methane and the warmest temperature there is about -223 C. Pluto's day is 6 Earth days, 9 hours, and 17 minutes, spinning in the opposite direction of Earth. One satellite, Charon, orbits Pluto and it is about half the size of the planet.

Pluto's orbit, shown in green, is tilted 17 degrees, causing it to come inside the path of Neptune (blue) at times, making Pluto the eighth planet for several years at a time.

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