Forbidden planets

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Science  29 Jul 2016:
Vol. 353, Issue 6298, pp. 438-441
DOI: 10.1126/science.353.6298.438

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When astronomers discovered the first exoplanet around a normal star, there was joy—and bewilderment. The planet was half as massive as Jupiter, but impossibly close to the star. Theorists who study planet formation could see no way for a planet that big to grow in such tight confines around a newborn star. Soon, more "hot Jupiters" turned up in planet searches, and they were joined by other oddities. More recently, NASA's Kepler spacecraft found that the most common type of planet in the galaxy is something between the size of Earth and Neptune, which has no parallel in our solar system and was thought to be almost impossible to make. Theorists are trying to catch up—coming up with scenarios for growing previously forbidden kinds of planets, in places once thought off-limits.