sub-Neptune

{{Short description|Planet with a radius or mass smaller than Neptune}}{{Distinguish|Mini-Neptune}}

The term sub-Neptune can refer to two different types of planets. It may describe a planet that has a smaller radius than Neptune, even if that planet actually has a larger mass.[https://arxiv.org/abs/1812.09242 Detection and characterization of an ultra-dense sub-Neptune planet orbiting the Sun-like star HD 119130], R. Luque et al, 21 Dec 2018 Alternatively, it can describe a planet with a smaller mass than Neptune, even if that planet has a larger radius (like a super-puff planet). Both definitions are sometimes used within the same scientific publication.[https://arxiv.org/abs/1910.06794 Tidally-Induced Radius Inflation of Sub-Neptunes], Sarah Millholland, 17 Oct 2019

Neptune-like planets are considerably rarer than sub-Neptune sized planets, despite being only slightly bigger.{{cite web | url=https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/ | title=Why are there so many sub-Neptune exoplanets? | date=17 December 2019 }}[https://arxiv.org/abs/1912.02701 Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis], Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019 This "radius cliff" separates sub-Neptunes (radii < 3 Earth radii) from Neptunes (radii > 3 Earth radii). This radius cliff is thought to arise because during formation when gas is accreting, the atmospheres of planets that size reach the pressures required to force the hydrogen into the magma ocean stalling radius growth. Then, once the magma ocean saturates, radius growth can continue. However, planets that have enough gas to reach saturation are much rarer, because they require much more gas.

On 29 November 2023, astronomers reported the discovery of six sub-Neptune exoplanets orbiting the star HD 110067, with radii ranging from 1.94{{nbsp}}{{Earth radius}} to 2.85{{nbsp}}{{Earth radius}}.{{cite journal |author=Lugue, R. |display-authors=et al. |title=A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067 |url=https://www.nature.com/articles/s41586-023-06692-3 |date=29 November 2023 |journal=Nature |volume=623 |pages=932-937 |doi=10.1038/s41586-023-06692-3 |url-status=live |archiveurl=https://archive.today/20231129185254/https://www.nature.com/articles/s41586-023-06692-3 |archivedate=29 November 2023 |accessdate=29 November 2023 |arxiv=2311.17775 }}{{cite journal |last=Clery |first=Daniel |title=Astronomers stunned by six-planet system frozen in time - Undisturbed family of “sub-Neptunes” in rhythmic orbits could hold clues to planet formation |url=https://www.science.org/content/article/astronomers-stunned-six-planet-system-frozen-time |date=19 November 2023 |journal=Science |doi=10.1126/science.ze93fui |url-status=live |archiveurl=https://archive.today/20231129211552/https://www.science.org/content/article/astronomers-stunned-six-planet-system-frozen-time |archivedate=29 November 2023 |accessdate=19 November 2023 |url-access=subscription }}{{cite news |last=Ghosh |first=Pallab |title='Perfect solar system' found in search for alien life |url=https://www.bbc.com/news/science-environment-67488931 |date=29 November 2023 |work=BBC |url-status=live |archiveurl=https://archive.today/20231129212406/https://www.bbc.com/news/science-environment-67488931 |archivedate=29 November 2023 |accessdate=29 November 2023 }}