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The western evening sky is hosting a family reunion this month. Everyone is invited -- and no one has to bring the potato salad.
The family members are three planets of the solar system -- Venus, Mars, and Saturn. They fade into view as the sky darkens. Venus is the brightest of the three -- it's the brilliant "evening star." Mars and Saturn form a tight pairing to Venus's upper left, with Saturn slightly higher in the sky.
The planets are siblings -- they were born at the same time, from the same disk of material encircling the newborn Sun. But their location in the disk caused these siblings to turn out quite differently.
Venus and Mars were born close to the Sun, just as Earth was. Energy from the Sun vaporized small particles of ice, and blew away much of the hydrogen and helium gas left over from the Sun's own birth. That left mainly small grains of solid material. These grains were pretty thickly spread, so they ran into each other and merged to form larger and larger bodies -- eventually making planets.
Saturn was born far away from the Sun, where conditions were much colder. So the planet incorporated bits of ice as well as solid rock. And as it grew bigger, its gravity pulled in a lot of hydrogen and helium, so Saturn grew to giant proportions. Today, it's about 10 times wider than Venus, and almost 20 times wider than Mars. So while it's a sibling to those other worlds, there's not much of a family resemblance.
Script by Damond Benningfield, Copyright 2010
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