Return of the King

That brilliant starlike object low in the southern sky is the planet Jupiter. It will grace the nighttime sky through the rest of summer and most of the fall.

Like that other gleaming planet, Venus, it appears so bright because what we see is the Sun's light reflected from Jupiter's thick cloud cover. It appears almost as bright as Venus because, though it is much further away, its disk is a many times bigger.

But, it's that big only because itıs puffed up with hot air. From a rocky core only about twice as big as the Earth, Jupiter's enormous atmosphere towers over fifty thousand miles -- as compared with ten miles for the Earth.

So great is the pressure of this tremendous tower of gas over ever square inch of its surface that the surface temperature of the planet, a planet over five times further from the sun than we are, is many thousands of degrees. At its surface, Jupiter is the hottest planet in the solar system, even though it is so far from the sun.

Where did it get this tremendous atmosphere? Actually, a better question is: why isn't ours like itıs? The solar system was full of hydrogen gas - like the atmosphere of Jupiter - when all the planets were formed, four and a half billion years ago. But when the Sun erupted into flame , its vigorous stellar wind -- actual particles leaving it -- dispersed this gas from the inner solar system. Far from the Sun, though, Jupiter, Saturn, Uranus and Neptune captured most of the remaining gas.

Will Jupiter ever lose its atmosphere? Yes, in billions of years. In its death throes, the atmosphere of the sun will ignite and the Sun will expand like Betelgeuse and Antares. It will consume its inner children, and boil off the atmosphere of the giant Jupiter. Then the rocky core of Jupiter, hidden since its formation, will be laid bare.

(08/28/09)