Astronomers said Thursday they had found a planetary system with three super-Earths orbiting a bright, dwarf star — one of them likely a volcanic world of molten rock. The four-planet system had been hiding out in the M-shaped, northern hemisphere constellation Cassiopeia, “just” 21 light years from Earth, a team reported in the journal Astronomy & Astrophysics. Super-Earths have a mass higher than Earth’s but are lighter than gas giants like Neptune, Saturn or Jupiter. They can be made of gas, rock, or both.