Super-Earth
Evolution
### Evolution

New research suggests that the rocky centres of super\-Earths are unlikely to evolve into terrestrial rocky planets like the inner planets of the Solar System because they appear to hold on to their large atmospheres. Rather than evolving into a planet composed mainly of rock with a thin atmosphere, the small rocky core remains engulfed by its large hydrogen\-rich envelope.

Theoretical models show that Hot Jupiters and Hot Neptunes can evolve by hydrodynamic loss of their atmospheres to Mini\-Neptunes (as it could be the Super\-Earth [GJ 1214 b](/wiki/GJ_1214_b "GJ 1214 b")), or even to rocky planets known as [chthonian planets](/wiki/Chthonian_planet "Chthonian planet") (after migrating towards the proximity of their parent star). The amount of the outermost layers that is lost depends on the size and the material of the planet and the distance from the star. In a typical system, a gas giant orbiting 0\.02 AU around its parent star loses 5–7% of its mass during its lifetime, but orbiting closer than 0\.015 AU can mean evaporation of the whole planet except for its core.

The low densities inferred from observations imply that a fraction of the super\-Earth population has substantial H/He envelopes, which may have been even more massive soon after formation. Therefore, contrary to the terrestrial planets of the solar system, these super\-Earths must have formed during the gas\-phase of their progenitor [protoplanetary disk](/wiki/Protoplanetary_disk "Protoplanetary disk").


How much mass is lost from gas giants as they orbit their parent star?