A study published in The Astronomical Journal has discovered mini-Neptunes around four red dwarfs using a global network of ground-based telescopes and the TESS space observatory.
These four mini-Neptunes are close to their parent stars, and all three are likely in eccentric orbits.
Planets between the size of Earth and Uranus/Neptune, known as mini-Neptunes, are not found in our solar system. However, mini-Neptunes are relatively common outside the solar system.
This study discovered four transiting short-period mini-Neptunes (TOI-782 b, TOI-1448 b, TOI-2120 b and TOI-2406 b) orbiting red dwarfs through follow-up observations with ground-based telescopes equipped with MuSCAT (a series of simultaneous multicolor cameras to study atmospheres of transiting exoplanets). These mini-Neptunes have radii about 2 to 3 times that of Earth and orbital periods of less than eight days.
Furthermore, these radial velocity measurements of their parent stars, obtained with the IRD (InfraRed Doppler) at the Subaru Telescope, indicate that the upper limit of the masses of these four planets is less than 20 times the mass of the Earth. The relationship between the measured radii and the upper mass limits of these mini-Neptunes suggests that they are not rocky planets like Earth. Their interiors are likely to contain volatile substances such as icy materials such as H2O and atmospheres.
The team also found that at least three (TOI-782 b, TOI-2120 b, TOI-2406 b) of these four mini-Neptunes are likely in eccentric orbits. In general, the orbit of a short-period planet around a red dwarf should be circular due to tidal dissipation. However, three short-period mini-Neptunes around red dwarfs have maintained non-zero eccentricities for billions of years.
One possible interpretation of this is that its interiors are not susceptible to the effects of the tides. The mass-radius ratio of these four mini-Neptunes suggests that they are not rocky planets. Therefore, the interiors of these mysterious mini-Neptunes may be similar to those of Neptune.
Short-period mini-Neptunes are promising targets for atmospheric observations with the James Webb Space Telescope. More detailed follow-up observations are expected to improve our understanding of the internal compositions and atmospheres of short-period mini-Neptunes.
