Super-Neptune

Super-Neptunes, also known as sub-Saturns,^[1] are a rare population of exoplanets that have properties between that of Neptune and Saturn (20-100 M_🜨, 4-8 R_🜨, 0.687-1.64 g/cm³). According to the core-accretion model of planet formation, most planets that reach a ~20 M_🜨 threshold are expected to rapidly expand to gas giant sizes (≥100 M_🜨) in a mechanism known as runaway gas accretion^[2][3]. 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.^[2] 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.^[2] 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.

Conversely, the delayed onset hypothesis^[4][5] argues that the true mass threshold for runaway accretion is actually ~100 M_🜨, meaning that Super-Neptunes –and even Saturn– never actually underwent runaway accretion according to this theory.