Space Facts

Planet Jupiter

Largest of the Solar System Worlds

This true-color simulated view of Jupiter is composed of 4 images taken by NASA's Cassini spacecraft on December 7, 2000. To illustrate what Jupiter would have looked like if the cameras had a field-of-view large enough to capture the entire planet, the cylindrical map was projected onto a globe. Jupiter's moon Europa is casting the shadow on the planet. Image Credit: NASA/JPL/University of Arizona

Jupiter is planet number five of the Solar system. The planet is named for the chief Roman god, or king of the gods. Jupiter, in Roman mythology, was also god of sky and thunder.

The planet Jupiter is big, all right. Ten Earths wide! In fact, its atmosphere is far thicker than the diameter of our entire home planet. The big red spot on the cloud surface of Jupiter is as large as Earth. That's one big hurricane. This king of planets commands a retinue of four large moons (the "Galilean" moons), only one of which is smaller than our own moon. Besides its own faint ring structure, Jupiter also has more than fifty smaller moons, which range from 1 to 250 kilometers across. Most of these were discovered in the last eight years.

One day at the cloud-tops of Jupiter is 9 hours 50 minutes long. The Jovian year is 11.86 Earth years long. Jupiter is 317.89 times as massive as Earth, but can hold 1,319 times Earth's volume. In fact, the density of Jupiter is not much more than that of water. Accordingly, the surface gravity is not as much as it might have been if its density more closely matched that of our own planet — 2.643 times that at Earth's surface. That would mean any object that weighs 100 kilograms would weigh 264.3 kilograms at the stationary cloud tops. At the equator, the cloud tops are anything but stationary. The daily spin of the planet alone would result in a 9% drop in weight from the centrifugal force, bringing our hypothetical object down to 240.5 kilos.

Size comparison of gas giant planets with Earth (left to right): Earth, Jupiter, Saturn, Uranus, and Neptune.		The last four planets of the Solar system are called "gas giants" because the bulk of each such planet is gaseous. The other type of planet is called "terrestrials" because of their rough similarity to Earth. The Solar System Facts table gives more comparisons between the planets.

Follow Jupiter's Galilean Moons and Red Spot JupSat Pro provides an indispensible aid to your telescope observations of Io, Europa, Ganymede and Callisto. Get your copy of JupSat Pro, today. Features include: positions, transit times, eclipses, and much more. Click picture now to find out more! [Windows software]

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Could Jupiter Ever Sustain Human Life?

No way, José! The atmosphere on Jupiter is thousands of kilometers deep. At the bottom of that atmosphere is a bone crushing pressure where no Earthly life form could conceivably live. Jupiter doesn't have a solid surface, but more like a thick, high-pressure soup — and hot! This gas giant is still giving off excess heat from the original compression of formation 4.5 billion years ago. Ten Jupiters might be enough to form a star, albeit weak and sputtering — perhaps similar to the flare star, Proxima Centauri, our closest neighboring star.

Each of the Galilean moons of Jupiter — Io, Europa, Ganymede and Callisto — are too small and cold to be habitable. None of them could be terraformed, largely because of their size and distance from the sun. Also, the radiation belt of Jupiter poses a hazard. The Jovian dynamo produces a far stronger magnetic field than does that of Earth, and the radiation belt is thus more lethal.

Another factor against the four Galilean moons is their period of rotation. For each of these moons, rotation matches the period of revolution. Like Earth's moon, each of the Galilean moons shows the same face to its parent world. Why is this important to habitability? "Rotation" means "length of day." Their days are 42, 85, 172 and 400 hours long (Io, Europa, Ganymede and Callisto, respectively). Their afternoons may not get very warm, but their early morning hours, before sunrise, would be unbelievably frigid. Io's day is perhaps only marginally acceptable, but the moon is so close to Jupiter it suffers another problem. The tidal friction focused on Io makes it the most volcanically active body in the Solar system.

Jupiter might one day be an interesting place to visit, but not a fun place to live.

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