TRAPPIST-1 Family of Planets: Rocky Through and Through? Precise measurements reveal that the exoplanets have remarkably similar densities, which provides clues to their composition Measuring the mass and diameter of a planet reveals its density, which can give scientists clues about its composition. Scientists now know the density of the seven TRAPPIST-1 planets with a higher precision than any other planets in the universe, other than those in our own solar system. Credit: NASA/JPL-Caltech. The more precisely scientists can measure a planet’s density, the more they know about what it’s made of. The TRAPPIST-1 planetary system—home to the largest batch of roughly Earth-sized planets ever found—has offered astronomers a glimpse at the possible diversity among rocky worlds in the universe, including the Earth.