A big planet with a bulging waistline stands to the upper right of the Moon this evening. Jupiter looks like a brilliant star, outshining all the other planets and stars in the sky at that time. And if you look before the sky gets dark, you may also spot the little planet Mercury well to their lower right, just above the horizon.
Jupiter is the biggest planet in the solar system. It’s about 88,000 miles in diameter at the equator — roughly 11 times the size of Earth. But it’s several thousand miles smaller through the poles. That makes the planet look like a squashed beachball that bulges in the middle.
The difference is caused by Jupiter’s composition and by its rotation on its axis.
The planet probably has a small, rocky core, but most of its great bulk consists of hydrogen and helium, the two lightest chemical elements. They form an outer envelope of gas and liquid that’s thousands of miles thick. Below that envelope, the hydrogen is squeezed so tightly that it probably forms a thick liquid metal.
And although Jupiter is huge, it spins quickly. It completes one turn on its axis every 10 hours. At that speed, a spot on its equator moves at about 28,000 miles per hour, compared to about 1,000 miles an hour at Earth’s equator.
The combination of that fast rotation and Jupiter’s mostly gas and liquid composition pushes material outward at the equator — making the solar system’s largest planet look like a big beachball.
Script by Damond Benningfield, Copyright 2014
For more skywatching tips, astronomy news, and much more, read StarDate magazine.