If intelligent life is out there, it probably resides in a solar system with many planets. The more planets a star has, a recent study found, the more circular the orbits tend to be. Because planets on circular orbits do not move toward or away from their star, their climates may be stable enough to foster advanced life. Our own solar system fits that pattern. The sun has eight or nine planets (depending on how you count), and most of them have fairly circular paths. Earth's orbit, for example, has an eccentricity of just 1.7 percent. (Eccentricity ranges from 0 percent for a perfect circle to nearly 100 percent for extreme ellipses.) Mercury and Pluto pursue oval-shaped orbits, with eccentricities of 21 and 25 percent, respectively, but even Pluto—whose planetary status is controversial—seems tame when compared with many of the planets orbiting other stars, where eccentricities can exceed 60, 70, even 80 percent. As far as we know, such wild worlds exist only in solar systems with one or two planets, say astronomers Mary Anne Limbach and Edwin L. Turner of Princeton University, who conducted the study. In contrast, solar systems with four or more planets feature moderately round orbits. The conclusions, based on 403 planets with previously measured orbital eccentricities in hundreds of solar systems, appeared in January in the Proceedings of the National Academy of Sciences USA. More analysis via Scientific American.
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