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This artist’s impression shows several of the planets orbiting the ultra-cool red dwarf star TRAPPIST-1. New observations, when combined with very sophisticated analysis, have now yielded good estimates of the densities of all seven of the Earth-sized planets and suggest that they are rich in volatile materials, probably water. Credit: ESO/M. Kornmesser

A new study has found that planets orbiting the star TRAPPIST-1 are made mostly of rock, and some could hold more H2O than Earth. The planets’ densities suggest that some of them could have up to 5% of their mass in the form of water. The hotter planets closest to their parent star are likely to have dense steamy atmospheres and the more distant ones probably have icy surfaces...