Jupiter: The Star that Might Have Been

Jupiter is at opposition and visible all night long in our winter skies. Opposition means that Jupiter is opposite the Sun as seen from Earth. This also means that Jupiter is about as large and bright as we ever see it, and therefore a great time to look at the king of planets that might have been a star.

Jupiter is indeed the king of the solar system, containing 77 percent of the total mass of the planets. With a diameter of 88,748 mi. (142.796 km) Jupiter is eleven times the diameter of Earth. If Earth were the size of a dime, Jupiter would be the size of a dinner plate! Jupiter is so big that it ha been said that it is almost a star.

It is quite interesting that the composition of Jupiter is very much like our Sun. Jupiter is 89 percent hydrogen and eleven percent helium. The Sun is about 78 percent hydrogen and 19.8 percent helium (the rest of the Sun is made up of heavy elements.) Jupiter's density is only 1.3 times that of water. Also, like the Sun, Jupiter is a gaseous ball with no real surface. So why isn't Jupiter a star?

Jupiter does radiate about twice as much heat energy as it receives from the Sun, but this heat comes from an internal reservoir left over from the planet's birth some 4.6 billion years ago. The Sun produces it's heat energy through nuclear fusion. The core of the Sun is about 16 million C (29 million F,) so hot that a pinhead would kill a person up to 100 miles away! Jupiter's core is about 33,000 C (54,000 F,) nearly 500 times cooler than the Sun. If Jupiter's composition is so similar to the Sun, then why doesn't it shine like the Sun?

The factor which prevents Jupiter from burning like our Sun is mass. If our king of the planets were about sixty times more massive than it is, it would indeed be a star!. More mass would not make Jupiter grow in size, but rather, cause the planet to collapse from compression under gravity. At this point, thermonuclear reactions would ignite and Jupiter would become a luminous star with a diameter of about 100,000 miles (161,000 km.)

Jupiter is a fascinating world, very different from Earth. Deep down inside Jupiter, there is a core about the size of Earth, and likely made up of similar iron and silicates. But the pressure at the core is 30,000 times that of Earth's atmosphere. If Earth were to be subjected to similar pressures, it would compress our planet to about half it's diameter and increase the density to three or four times that of iron!

Jupiter's core is surrounded by a shell of metallic hydrogen about 25,000 mi. (40,000 km.) thick. Metallic hydrogen is an exotic form of hydrogen not found on Earth. Under the high pressures on Jupiter, hydrogen is compressed from a gas to a metal! Further outward, as the temperatures drop, the hydrogen changes to a liquid state. The liquid hydrogen continues outward until the temperatures and pressures decrease enough for the hydrogen turns into a gas, thus beginning Jupiter's "atmosphere."

Don't feel cheated that Jupiter hadn't become a star though. If our Solar System had had binary stars, it would have been very difficult, if not impossible, for life to have evolved! Look for the "star-that-would-have-been" in the eastern sky after dark.

Copyright © 2002 Kathy A. Miles and Charles F. Peters II