King of the Planets

He's back. Jupiter, the greatest of the planets. Rising tonight about 10:30, and a half hour earlier each week, he will dominate the evening sky all summer. Brighter than any nighttime star, Venus is the only planet that outshines him, and Venus is many times closer, up to ten times closer. Besides, Venus is now lost in the glare of the Sun, not to emerge as an evening "star" in mid-July. Now Jupiter is the undisputed king of the evening sky.

All planets shine by the reflected light of the sun, and Jupiter is a tremendous mirror -- ten times bigger than Venus (and the Earth.) Jupiter is one of the giant planets: the greatest. The planets far from the sun -- not only Jupiter, but also Saturn, Uranus, and Neptune, are giant planets -- the gas giants. Most of their giant size comes from their tremendous atmospheres of hydrogen gas. Jupiter's atmosphere is forty thousand miles deep, compared with less than a hundred miles for the Earth. The rocky cores of the giant planets are bigger than the Earth, but not that much bigger. They are all still shrouded in their first atmospheres, atmospheres that not only they, but also the inner planets: Mercury, Venus, Mars, and even the earth, had at their formation.

That's what the planets looked like four and a half billion years ago -- eight rocks, some bigger than the Earth, some smaller, but none really huge, covered by tremendous atmospheres thousands of miles deep. The outer planets, in the depth of interplanetary space, far from the Sun, kept their enormous atmospheres. The planets, close to the Sun, lost these shrouds, blown away by the vigorous solar wind - actual particles boiled off the surface of the young Sun.

But it's not really cold on Jupiter. Oh, the cloud tops are cold, hundreds of degrees below zero. But then, so is the top of Earth's atmosphere. At ground level, the mighty atmosphere of Jupiter presses down so much that the temperature is over thirty thousand degrees. The surface of Jupiter is the hottest solid ground in the solar system.

Of course, not the hottest place. That distinction goes to the sun, whose core blazes at over ten million degrees. But if Jupiter were larger, about a hundred times larger, the atmospheric pressure would have raised the surface temperature to ten million degrees, and nuclear fusion would have begun. We would have another sun in our sky. Jupiter is a failed sun, but a mighty planet.

(06/11/08)