9. Jupiter: a giant primitive planet


Jupiterís orbital radius is 5.2 times the radius of the Earthís orbit, so the planetís distance from Earth changes relatively little in the course of a year. As a consequence, its apparent size and brightness are fairly constant, unlike the behavior of Mars and Venus. When these nearby planets are on the same side of the Sun as the Earth, they appear much bigger and brighter than when they move to the opposite side of the Sun. Jupiter is a true monarch of the planets, the largest planet in the solar system with a radius of about 11 times that of the Earth. The giant is so large that it could contain more than 1,300 Earth-sized planets inside its volume. Yet, Jupiter is only 318 times as massive as our planet. So Jupiter must be composed of something lighter than the rock and iron that constitute the Earth.

If we divide the mass by the volume, we find a mean density of 1,330 kilograms per cubic meter, only about one quarter the mean mass density of the Earth. In fact, the mass density of Jupiter is only slightly greater than that of water, at 1,000 kilograms per cubic meter, and this implies that Jupiter, like the Sun, is composed primarily of hydrogen. No other element is light enough to account for the low density of the planet.

Physical properties of Jupitera

Mass1.8992 x 1027 kilograms = 317.894 ME
Equatorial radius at one bar7.1492 x 107 meters = 11.19 RE
Polar radius at one bar6.6854 x 107 meters
Mean mass density1,330 kilograms per cubic meter
Rotation period9.9249 hours = 9 hours 55 minutes 29.7 seconds
Orbital period11.86 Earth years
Mean distance from Sun7.7833 x 1011 meters = 5.203 AU
Age4.6 x 109 years
Atmosphere86.4 percent molecular hydrogen, 13.6 percent helium
Energy balance1.67 Ī 0.08
Effective temperature124.4 degrees kelvin
Temperature at one bar level165 degrees kelvin
Central temperature17,000 degrees kelvin
Magnetic dipole moment 20,000 DE
Equatorial magnetic field strength4.28 x 10-9 tesla or 14.03 BE
a The symbols ME, RE, DE and BE denote respectively the mass, radius, magnetic dipole moment, and magnetic field strength of the Earth.

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Copyright 2010, Professor Kenneth R. Lang, Tufts University