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Our Sun has a mass of 1.989 × 10^30 kilograms (about 33,000 times the mass of Earth) and a surface temperature of nearly 10,000 degrees Fahrenheit. The star TRAPPIST-1 is less than one-tenth the mass of the Sun (slightly larger than Jupiter) and about half as hot. It’s classified as an “ultracool dwarf,” barely a star: not the sort of place astronomers would expect to harbor many planets. And yet, astronomers have found three Earth-like planets orbiting it, the first rocky planets ever found orbiting a star of this type.

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