GRE Reading Comprehension: OG/PP2-GRE阅读OG/PP2 - 984R8WT1IC6V4PTQL

The nearly circular orbits of planets in our solar system led scientists to expect that planets around other stars would also reside in circular orbits. However, most known extrasolar planets reside in highly elongated, not circular, orbits. Why? The best clue comes from comets in our solar system. Comets formed in circular orbits but were gravitationally flung into their present-day elliptical orbits when they ventured too close to planets. Astronomers suspect that pairs of planets also engage in this slingshot activity, leaving them in disturbed, elliptical orbits. If two planets form in close orbits, one will be scattered inward (toward its star) , the other outward. They will likely then travel close enough to neighboring planets to disturb their orbits also.