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This animation shows the "family feud" that researchers now believe left the Upsilon Andromedae planetary system in its current, somewhat disturbed state. In the beginning, we see three huge Jupiter-like planets orbiting the star in roughly circular orbits. (Not shown here is a fourth planet that orbits very, very close to the star.) But for some reason--because of friction with interplanetary dust, for example - the outer planet has wandered in too close to its sibling in the middle. The result is a kind of rhythmic shoving match, as the two planets' mutual gravitational attraction starts to knock their orbits askew with each close encounter. Matters escalate - and in fairly short order, the offending planet is ejected from the planetary system completely. The middle planet now takes its place the outer planet, and settles into an extended, elliptical orbit much different from the one it started with. In the millennia to come, its gravitational influence will pull the third planet into an elliptical orbit, as well.
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Credit: Trent Schindler/National Science Foundation