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 for the lower left, have retained elliptical orbits despite being more than a billion years 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 for the lower left, have retained elliptical orbits despite being more than a billion years 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 parent stars, and all three are likely in eccentric orbits.
The research is published in The Astronomical Journal.
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 and are promising targets for atmospheric characterization with the James Webb Space Telescope. What do miniNeptunes look like?
This study discovered four transiting short-period mini-Neptunes (TOI-782 b, TOI-1448 b, TOI-2120 b, TOI-2406 b) orbiting red dwarfs through follow-up observations by ground-based telescopes with MuSCAT (and a series of simultaneous multi-color cameras for studying the atmospheres of transiting exoplanet). These mini-Neptunes have radii about 2–3 times that of Earth and orbital periods of less than eight days.
Moreover, these measurements of the radial velocity of their parent stars, obtained with IRD (InfraRed Doppler) on the Subaru Telescope, indicate that the upper mass limit of these four planets is less than 20 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 H2About and atmosphere.
The team also found that at least three (TOI-782 b, TOI-2120 b, TOI-2406 b) of these four mini-Neptunes are likely to be on eccentric orbits. In general, the orbit of a short-period planet around a red dwarf should be circular due to tidal loss. However, three short-period mini-Neptunes around red dwarfs have maintained non-zero eccentricities for billions of years.
One possible interpretation is that their interiors are not susceptible to tidal effects. The mass-radius relationship of these four mini-Neptunes suggests that they are not rocky planets. Thus, the interiors of these mysterious mini-Neptunes may be similar to those of Neptunes.
Short-lived mini-Neptunes are promising targets for observing the atmosphere with the James Webb Space Telescope. Further detailed follow-up observations are expected to improve our understanding of the internal composition and atmospheres of short-period mini-Neptunes.
More information:
Yasunori Hori et al., Discovery and Observation of Four Transiting Sub-Neptunes Orbiting M Dwarfs, The Astronomical Journal (2024). DOI: 10.3847/1538-3881/ad4115
Information from the diary:
Astronomical Journal