Inside-out dense iron planets probably the result of massive collision

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How violent are the early histories of solar systems? Planets are built by the collisions of smaller bodies, so a certain amount of violence is probably unavoidable. Our own Earth-Moon system seems to have been formed by a smash-up of two planets, while Uranus seems to have been flipped on its side by a collision, and Mercury seems to have lost a lot of its material early in its history. Is this sort of history common as planets form?
Answering these questions requires a detailed understanding of the planets themselves, knowledge difficult to attain for any solar system but our own. But now, following up on observations made with the Kepler space telescope, researchers are suggesting they've found evidence of a smash-up in an exosolar system about 1,750 light years from Earth.
That's dense
Kepler-107 has a Sun-like star orbited by at least four planets. The planets are tightly packed around the star, with orbital periods ranging from three to 14 days. The lengths of the orbits of neighboring planets can be expressed as simple ratios of integers (5:2, 3:1, and so on). This creates what are called 'resonant orbits,' where the periodic alignment of the bodies helps stabilize and reinforce the orbits. Generally, this is thought to occur when planets that form farther from the star are migrating inward toward it; the resonances help balance things out and keep the planets from continuing on into the star.
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