Call it sibling rivalry.
The planets in young solar systems constantly push and pull each other. This shoving match alters the orbits of the planets, deciding where each planet ends up -- and even which planets survive.
This concept is borne out by the discovery of hundreds of planets in other star systems. Many of the planets are giants, but they're in tight orbits around their parent stars. Under current models of how planets form, there's just no way for such a planet to take shape so close to its star. That means the planet must have been born farther out, but moved inward, toward the star.
One way to do that is through interactions with other planets -- and especially other giant planets. The gravity of such worlds is strong, so it effects the orbits of other objects around it.
In our own solar system, the gravity of Jupiter probably threw billions of "leftovers" from the birth of the planets completely out of the solar system. These interactions pushed Jupiter itself closer to the Sun.
Similar interactions are common in other star systems, too. But so are interactions between the planets themselves. These interactions push the planets into orbits that are lopsided or tilted. They can even push a planet so close to its parent star that it gets torn apart or gobbled up.
Eventually, the surviving planets reach a balance, so they form a stable system -- a family of siblings that have learned how to get along.
Script by Damond Benningfield, Copyright 2010
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