super-Neptune

thumb|Artist's interpretation of a super-Neptune Super-Neptunes, also known as a sub-Saturns, are a rare population of exoplanets that have properties between that of Neptune and Saturn (, , 0.687-1.64 g/cm3). According to the core-accretion model of planet formation, most planets that reach a threshold are expected to rapidly expand to gas giant sizes (≥) in a mechanism known as runaway gas accretion. Despite this, Super-Neptunes sit between this bimodal distribution of sub-Neptunes and gas giants, failing to either begin or fully complete runaway accretion.

Planet formation occurs over a period of ~106 years, with runaway accretion occurring only in the last 105 years. The average lifespan of a protoplanetary disk, which contains the material that becomes a planet, is between 105-107 years before stellar activity quickly disperses the disk. Therefore, one theory is that Super-Neptunes are failed gas giants that were quenched of material during their runaway phase before they could reach larger sizes.