| Astrophysics (Index) | About |
Super-Earth (or superearth) is a common term for extra-solar planets more massive than the Earth but substantially less massive than Uranus and Neptune, basically 2-10 Earth masses though there is no consensus on exact limits and some would use it for masses up to 20 Earth masses. Often, a planet's radius is known but not its mass (e.g., from the transit method), and the term super-Earth might be applied to a planet because it has a somewhat larger radius than Earth, presumably a radius consistent with the above mass range.
Usage of the term super-Earth regarding the planet's constituents and structure varies. Sometimes the term is only meant to indicate the mass, even if the planet has a large atmosphere in the manner of a gas giant. In other instances, it is meant to be Earth-like, i.e., a rocky planet with on the order of 1% of its mass as atmosphere which is considered the most likely atmosphere percentage in the super-Earth mass range. In the latter case, other terms have been used for gas planets of such a mass, such as mini-Neptune, sub-Neptune, gas dwarf, mini gas giant, or transitional planet for planets with large atmospheres in the manner of Neptune, etc. The term super-puff means essentially a larger-than-Earth (super-Earth) rocky core, but with a large, non-Earth-like atmosphere. Super-Earth statistics show two peaks in radius (1.3 versus 2.4 Earth radii), the radius-range between termed the Fulton gap or evaporation valley. The latter term refers to the theory that photoevaporation of the atmosphere due to being near the host star happens so rapidly that generally a large atmosphere is either essentially intact or is essentially gone.
Current statistics and analysis suggest super-Earth's are very common, perhaps the most common type of planet, in late 2018 being roughly 1000 out of the 4000 exoplanets known. Current thought is about a third of systems have a super-Earth, compared to perhaps a sixth with gas giants, in contrast to the solar system that has gas giant Jupiter but no super-Earth. The super-Earth ubiquity needs explanation. Some models suggest the larger ones would naturally experience runaway accretion and become gas giants, and the fact that they didn't suggests the gas supply was cut off, i.e., the disk (transitional disk) dispersed after a short lifetime. Another possibility is that some have undergone substantial atmospheric escape, e.g., from being very close to the host star.