Sub-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.[1] 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.[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 R⊕ to 2.85 R⊕.[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. ↑ Detection and characterization of an ultra-dense sub-Neptune planet orbiting the Sun-like star HD 119130, R. Luque et al, 21 Dec 2018
2. ↑ Tidally-Induced Radius Inflation of Sub-Neptunes, Sarah Millholland, 17 Oct 2019
3. ↑ 3.0 3.1 3.2 "Why are there so many sub-Neptune exoplanets?". 17 December 2019. https://astrobites.org/2019/12/17/why-are-there-so-many-sub-neptune-exoplanets/.
4. ↑ Superabundance of Exoplanet Sub-Neptunes Explained by Fugacity Crisis, Edwin S. Kite, Bruce Fegley Jr., Laura Schaefer, Eric B. Ford, 5 Dec 2019
5. ↑ Lugue, R. (29 November 2023). "A resonant sextuplet of sub-Neptunes transiting the bright star HD 110067". Nature 623: 932-937. doi:10.1038/s41586-023-06692-3. Archived from the original on 29 November 2023. https://archive.today/20231129185254/https://www.nature.com/articles/s41586-023-06692-3. Retrieved 29 November 2023.
6. ↑ Clery, Daniel (19 November 2023). "Astronomers stunned by six-planet system frozen in time - Undisturbed family of “sub-Neptunes” in rhythmic orbits could hold clues to planet formation". Science. doi:10.1126/science.ze93fui. Archived from the original on 29 November 2023. https://archive.today/20231129211552/https://www.science.org/content/article/astronomers-stunned-six-planet-system-frozen-time. Retrieved 19 November 2023.
7. ↑ Ghosh, Pallab (29 November 2023). "'Perfect solar system' found in search for alien life". BBC. Archived from the original on 29 November 2023. https://archive.today/20231129212406/https://www.bbc.com/news/science-environment-67488931. Retrieved 29 November 2023.

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