Most massive alien worlds are gas giants like Jupiter. But now, astronomers say they’ve found a new type of exoplanet: a rocky world much larger than Earth that may boast only a thin sheath of an atmosphere. The orb in question (in the foreground of artist’s concept), dubbed Kepler-10c, circles its 11-billion-year-old, sunlike star once every 45 days. Previously estimated to have a diameter about 2.3 times that of Earth (giving it a volume slightly more than 12 times our planet’s), new observations with ground-based sensors suggest that Kepler-10c is 17 times as hefty as Earth, the researchers report today in Boston at a meeting of the American Astronomical Society and in a forthcoming issue of The Astrophysical Journal. Those figures for mass and volume—which suggest a dense, rocky composition and seemingly discount a large, thick atmosphere—peg the planet as the first rocky “mega-Earth” to be found. Kepler-10c is surprising, the researchers say: Previously, astronomers surmised that any planet that massive would have gravitationally slurped up gases in its neighborhood as it formed, eventually growing to become a gas giant like those in the outer reaches of our solar system. The existence of large rocky worlds like Kepler-10c may boost the chances of potentially habitable worlds throughout the cosmos, the researchers contend.