The seven planets orbiting the ultracool dwarf star TRAPPIST-1 are mostly rocky, with some potentially holding more liquid water than Earth. New research reveals the density of the worlds within this crowded system to a greater precision than ever before. The findings reveal that some of the planets could have up to 5 percent of their mass in liquid water form, about 250 times as much water as found in Earth's oceans. "All the TRAPPIST-1 planets are very Earth-like — they have a solid core, surrounded by an atmosphere," In addition to narrowing down the composition of the exoplanets, the...