Mega-Earth

A mega-Earth or massive solid planet is a type of terrestrial exoplanet that is very massive and dense — more massive than super-Earths. The term "mega-Earth" was coined in 2014, though it remained an informal category until a quantitative definition was proposed for it in 2026. Based on the measured radii and densities of known exoplanets as of March 2026, mega-Earths appear to be a distinct category of exoplanets that are between 2.1 and 5.0 Earth radii (R_🜨) and have densities higher than Earth's (5.5 g/cm).

Based on their observed high densities, mega-Earths are inferred to be largely made of solid material, such as rock, metal, and ice. If the mega-Earth is rich in volatiles like water, it may harbor a supercritical ocean under a thin atmosphere of hydrogen, helium, and other gases. The existence of mega-Earths challenges conventional theories for planetary formation, as massive planets should accrete large amounts of gas in addition to solid material from the host star's protoplanetary disk. It has been hypothesized that mega-Earths may either be remnant cores of evaporated gas giants or white dwarfs or products of consecutive collisions between super-Earths. Mega-Earths as evaporated gas giants have been theorized to occur frequently around luminous massive stars and supermassive black holes.