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Exoplanets are planets outside the solar system. Mini-Neptunes (planets with a radius ranging from 1.8 to 4 the radius of the Earth) are one of their most populated clusters, even though they do not exist in our Solar System. This makes them become a key to planetary formation understanding, representing a fascinating and quickly growing area of exoplanetary science and astrobiology.
The radius of mini-Neptunes can be obtained using the transit method by measuring the changes in the flux received from a star when a planet transits; this is, its orbital movement makes it pass between its host star and an observer.
The mass of transiting sub-Neptunes is determined using the radial velocity method, based on the Doppler shift of the light received from a star that moves because of the gravitational interaction with a planet that orbits it.
The combination of these two methods leads to the determination of densities of sub-Neptunes. Some authors [1] state that different densities of these planets lead to different compositions: super-Earths, rocky planets with extended H/He atmospheres or water worlds, planets with a high (up to 50% or more) water composition.
However, others [2] find degeneracies and contradictions by considering different evolutionary models, concluding that density is not enough to characterize mini-Neptunes and there is more information needed, for instance, by studying their atmospheres.
Future developments are expected to revolutionize exoplanetary science and break these degeneracies to understand how mini-Neptunes are and their formation mechanisms.
[1] Luque, R., Pallé, E. (2022). "Density, not radius, separates rocky and water-rich small planets orbiting M dwarf stars". Science 377, 1211-1214. DOI: 10.1126/science.abl7164.
[2] Rogers, J. G., Schlichting, H. E., Owen, J. E. (2023). "Conclusive Evidence for a Population of Water Worlds around M Dwarfs Remains Elusive". The Astrophysical Journal Letters 947(1), L19. DOI: 10.3847/2041-8213/acc86f.
