Jupiter is by far the largest planet in the Solar System, which is fitting for a body named for the mighty king of the Roman gods.
Jupiter is nearly 143,000 km (90,000 miles) in diameter at the equator. This is more than eleven times the diameter of Earth, but that doesn't really give you a sense of Jupiter's size. If we could recycle Jupiter, we would have enough material to make all the rest of the planets in the Solar System, plus satellites, dwarf planets and asteroids. In fact, we could do this twice and still have material left over.
The planet has been known since ancient times. Although it's five times farther away from the Sun than Earth is, its great size reflects a lot of sunlight, making it a very bright object in the sky.
Through a telescope Jupiter is even more impressive. What we see is not a solid surface but colored bands of cloud running parallel to the equator. This NASA image shows them colored white, red, brown and orange. Over the centuries of observing Jupiter, the widths of the bands and the intensity of the color have varied. Nonetheless the general pattern of these zones (the lighter regions) and belts (the darker regions) has been been remarkably stable during this time.
However the most famous of Jupiter's features is the Great Red Spot, a giant storm with winds whirling around at 360 km/h (225 mph). It's so big you could line up three Earths across it. It may be that English scientist Robert Hooke saw it as long ago as 1664. However the first recorded sighting was by Heinrich Schwabe in 1831, which means it has been present for 180 years – but probably longer.
Beneath the cloud layer Jupiter's atmosphere is mostly hydrogen. It gets thicker and the pressure increases with depth. Farther down there is a deep layer of metallic hydrogen mixed with some helium. In the metallic hydrogen state the atoms are so squashed together that hydrogen behaves like a liquid metal – it even conducts electricity. Finally, in the center of the planet, there is probably a dense rocky core with a mass of perhaps 10-15 or more Earths. This sounds big, but it's less than five percent of Jupiter's mass.
The gravitational strength of a planet is related to its mass, but decreases rapidly with the distance away from the center of the planet. Although Jupiter is much more massive than Earth, it also has a much greater diameter and is less dense, so at the cloud tops it has just two and a half times the gravitational strength of Earth.
Jupiter has long years, as it takes nearly twelve Earth years to go around the Sun. However it doesn't have much in the way of seasons, because its axis is only slightly tilted. This means that during a year the amount of sunlight reaching any given latitude would change very little. Contrast this with Earth's seasonal changes.
The years may be long on Jupiter, but the days are short. Jupiter turns so quickly on its axis that its day is just under ten hours. When a planet spins on its axis, the equator bulges and the poles are flattened. This is true of Earth, but Jupiter's rapid spin has a particularly noticeable effect on its shape.
Since Jupiter is so much farther from the Sun than Earth is, it's not surprising that the cloud top temperature is a chilly -140 degrees Celsius (-230 degrees Fahrenheit). However the core temperature may be around 24,000 degrees Celsius (43,000 degrees Fahrenheit). The surface of the Sun is only about a quarter of that temperature. In fact, Jupiter gives out almost as much heat energy as it receives from the Sun, a feature it shares with Saturn and Neptune.
When the planets formed, they were extremely hot, because they were made through numerous collisions. The energy of these collisions heated them up, but also so did the contraction that followed when gravity squeezed them. Although they have cooled in the billions of years since they formed, Jupiter continues to have considerable internal heat as it's still slowly contracting.
Interestingly, all four of the giant planets have ring systems, though the others aren't as spectacular as Saturn's. Jupiter has three main rings, which are quite faint and made mostly of dust. They were discovered in 1979 by the Voyager spacecraft.
Jupiter is rich in moons. Galileo discovered the four largest ones in 1610. Four hundred years later (as of December 2012) there were 67 confirmed moons. However 49 of them are less than 10 km (6 miles) in diameter and were discovered only in the last 35 years or so.
Some asteroids have become tiny moonlets through the pull of Jupiter's gravity. But others have a different fate, for we know that Jupiter's gravity can break up comets and other small bodies that get too close. The most famous one was comet Shoemaker-Levy 9. In 1994, having been pulled into Jupiter's gravitational field, it broke up and was sent plunging into the planet. Smaller impacts by other objects were also detected in 2009 and 2010. The 2010 impacts were independently observed by an amateur astronomer in Australia and another in the Philippines.
(1) Phillips, Tony. “Jupiter Impact: Mystery of the Missing Debris”
(2) "Great Red Spot" A Dictionary of Astronomy, Ian Ridpath, Oxford University Press, 2007. Oxford Reference Online. Oxford University Press. Ian Ridpath. [accessed 09.22.10]