More Moon and Jupiter

StarDate: December 21, 2009

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Winter arrives in the northern hemisphere today. It's the winter solstice -- the shortest day of the year. It's also the longest night of the year, which provides a lot of time for skywatching.

And there's a view worth appreciating in the southwest this evening: the Moon and the planet Jupiter. Jupiter looks like a brilliant star below the Moon. They set by around 10 o'clock.

Jupiter is the largest planet in the solar system. It probably formed as small clumps of rock and ice known as planetesimals rammed together to build an ever-bigger body. As it grew, Jupiter swept up hydrogen and helium gas left over from the birth of the Sun, swelling to giant proportions.

But scientists aren't sure just where Jupiter formed. It may have formed farther from the Sun and migrated inward to its present orbit. Under this scenario, gas and dust around the Sun acted as a brake, slowing Jupiter and causing it to spiral closer to the Sun. Jupiter also gave up some energy when its gravity batted billions of planetesimals out of the solar system.

One bit of evidence that supports this idea is the plethora of Jupiter-sized planets orbiting other stars. Many of these planets are a tiny fraction of Jupiter's distance from the Sun. Astronomers say there's no way for gas-giant planets to form so close to a star. They must have formed farther out and spiraled inward -- a process that may have happened in our own solar system with giant Jupiter.

Script by Damond Benningfield, Copyright 2009

For more skywatching tips, astronomy news, and much more, read StarDate magazine.

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