Artist’s depiction of the TRAPPIST-1 star and its seven worlds.NASA/JPL-Caltech/R. Hurt (IPAC) The TRAPPIST-1 star system is home to the largest batch of roughly Earth-size planets ever found outside our solar system. Discovered in 2016 some 40 light-years away, these seven rocky siblings offer a glimpse at the tremendous variety of planetary systems that likely fill the universe. A study accepted by the Planetary Science Journal shows that the planets share similar densities. That could mean they all contain roughly the same ratio of materials thought to be common to rocky planets, such as iron, oxygen, magnesium and silicon. If so, then while the TRAPPIST-1 planets might be similar to each other, they appear to differ notably from Earth: They’re about 8% less dense than they would be if they had the same chemical composition as our planet.