Sub-Neptune Explained

The term sub-Neptune can refer to a planet with smaller radius than Neptune even though it may have a larger mass^[1] or to a planet with a smaller mass than Neptune even though it may have a larger radius like a super-puff and both meanings can even be used in the same publication.^[2]

Neptune-like planets are considerably rarer than sub-Neptune sized planets, despite being only slightly bigger.^{[3] [4]} This "radius cliff" separates sub-Neptunes (radii < 3 Earth radii) from Neptunes (radii > 3 Earth radii).^[3] 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.^[3]

On 29 November 2023, astronomers reported the discovery of six sub-Neptune exoplanets orbiting the star HD 110067, with radii ranging from 1.94 to 2.85.^{[5] [6] [7]}

See also

• Super-Earth
• Mini-Neptune
• Mega-Earth

Further reading

• The nature and origins of sub-Neptune size planets, Jacob L. Bean, Sean N. Raymond, James E. Owen, 22 Oct 2020

References

1. https://arxiv.org/abs/1812.09242 Detection and characterization of an ultra-dense sub-Neptune planet orbiting the Sun-like star HD 119130
2. https://arxiv.org/abs/1910.06794 Tidally-Induced Radius Inflation of Sub-Neptunes
3. Web site: Why are there so many sub-Neptune exoplanets? . 17 December 2019 .
4. https://arxiv.org/abs/1912.02701 Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis
5. Lugue, R. . et al. . A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067 . 29 November 2023 . . 623 . 932-937 . 10.1038/s41586-023-06692-3 . live . https://archive.today/20231129185254/https://www.nature.com/articles/s41586-023-06692-3 . 29 November 2023 . 29 November 2023 . 2311.17775 .
6. Clery . Daniel . Astronomers stunned by six-planet system frozen in time - Undisturbed family of “sub-Neptunes” in rhythmic orbits could hold clues to planet formation . 19 November 2023 . . 10.1126/science.ze93fui . live . https://archive.today/20231129211552/https://www.science.org/content/article/astronomers-stunned-six-planet-system-frozen-time . 29 November 2023 . 19 November 2023 .
7. News: Ghosh . Pallab . 'Perfect solar system' found in search for alien life . 29 November 2023 . . live . https://archive.today/20231129212406/https://www.bbc.com/news/science-environment-67488931 . 29 November 2023 . 29 November 2023 .