Using numerical simulations to identify planets stable for millions of years, a team of researchers concluded that six of the seven roughly Earth-sized planets in the TRAPPIST-1 planetary system are consistent with an Earth-like composition. The exception is TRAPPIST-1f, which has a mass of 25% water. This artist’s impression displays TRAPPIST-1 and its planets reflected in a surface. Image credit: NASA / R. Hurt / T. Pyle. TRAPPIST-1 is an ultracool...