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.
References
[edit]- ^ "'Sub-Saturns' May Force Scientists to Revise Idea of How Planets Form". Space.com. 12 January 2019. Retrieved 20 October 2020.
- ^ a b c Pollack, James B.; Hubickyj, Olenka; Bodenheimer, Peter; Lissauer, Jack J.; Podolak, Morris; Greenzweig, Yuval (1996). "Formation of the Giant Planets by Concurrent Accretion of Solids and Gas". Icarus. 124 (1): 62–85. doi:10.1006/icar.1996.0190.
- ^ Ida, S.; Lin, D. N. C. (2004-03-20). "Toward a Deterministic Model of Planetary Formation. I. A Desert in the Mass and Semimajor Axis Distributions of Extrasolar Planets". The Astrophysical Journal. 604 (1): 388–413. doi:10.1086/381724. ISSN 0004-637X.
- ^ Alibert, Yann; Venturini, Julia; Helled, Ravit; Ataiee, Sareh; Burn, Remo; Senecal, Luc; Benz, Willy; Mayer, Lucio; Mordasini, Christoph; Quanz, Sascha P.; Schönbächler, Maria (2018-08-27). "The formation of Jupiter by hybrid pebble–planetesimal accretion". Nature Astronomy. 2 (11): 873–877. doi:10.1038/s41550-018-0557-2. ISSN 2397-3366.
- ^ Helled, Ravit (2023). "The mass of gas giant planets: Is Saturn a failed gas giant?". Astronomy & Astrophysics. 675: L8. doi:10.1051/0004-6361/202346850. ISSN 0004-6361.
