Diagram of discovered orbits of exoplanets. The orbits of exoplanets close to their parent stars tend to become circular over time, but three of the newly discovered exoplanets, except the lower left one, have retained elliptical orbits despite being in orbit for more than a billion years be old. Credit: Astrobiology Center
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Diagram of discovered orbits of exoplanets. The orbits of exoplanets close to their parent stars tend to become circular over time, but three of the newly discovered exoplanets, except the lower left one, have retained elliptical orbits despite being in orbit for more than a billion years be old. Credit: Astrobiology Center
Researchers discovered mini-Neptunes around four red dwarfs using observations from a global network of ground-based telescopes and the TESS space telescope. These four mini-Neptunes are close to their host stars, and all three are likely in eccentric orbits.
The research was published in The astronomical magazine.
Planets the size of Earth and Uranus/Neptune, known as mini-Neptunes, do not exist in our solar system. However, mini-Neptunes are relatively common outside the Solar System and are promising targets for atmospheric characterization by the James Webb Space Telescope. What do mini Neptunes look like?
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 using MuSCATs (an array of multicolored simultaneous cameras for studying the atmospheres of passing exoplanets). These mini-Neptunes have a radius about 2 to 3 times that of Earth and an orbital period of less than eight days.
Furthermore, these radial velocity measurements of their parent stars, obtained with the Subaru Telescope’s IRD (InfraRed Doppler), indicate that the upper limit for the masses of these four planets is less than twenty times the mass of 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 likely contain volatiles 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 retained non-zero eccentricities for billions of years.
A possible interpretation of this is that their interior is not sensitive to tidal effects. The mass-radius relationship of these four mini-Neptunes suggests that they are not rocky planets. The interior of these mysterious mini-Neptunes may therefore be similar to that of Neptune.
Short-period mini-Neptunes are promising targets for atmospheric observations with the James Webb Space Telescope. Further detailed follow-up observations are expected to improve our understanding of the internal compositions and atmospheres of short-period mini-Neptunes.
More information:
Yasunori Hori et al., The discovery and follow-up of four transiting short-period sub-Neptunes orbiting M dwarfs, The astronomical magazine (2024). DOI: 10.3847/1538-3881/ad4115
Magazine information:
Astronomical magazine