Scientists have discovered mini-Neptune type exoplanets around four red dwarfs that are located close to their parent stars and are likely on unusual eccentric orbits. This is reported in an article published in The Astronomical Journal.
Mini-Neptunes are planets that fall in size between Earth and Neptune and are relatively common outside the Solar System. They are promising objects for studying exoplanetary atmospheres with the James Webb Space Telescope. The new mini-Neptunes have a radius about 2-3 times larger than Earth and orbit their stars in less than eight days.
Observations were conducted using a series of ground-based telescopes equipped with MuSCAT astronomical cameras to study the atmospheres of transit exoplanets. The radial velocities of the parent stars were measured using the InfraRed Doppler spectrometer (IRD) on the Subaru Telescope. The measurements showed that the upper limit of the mass of these planets is less than 20 times the mass of Earth.
The ratio between the measured radii and masses of the mini-Neptunes indicates that they are not rocky planets like Earth. Their interiors likely contain volatile substances, such as water ice crystals. At least three planets – TOI-782 b, TOI-2120 b, and TOI-2406 b – are located on eccentric orbits, which is unusual for planets orbiting red dwarfs. Typically, short-period planet orbits should be circular due to tidal forces.
One explanation for this phenomenon is that their Neptune-like interiors are not subject to tidal effects. This allowed the planets to maintain a non-zero eccentricity for billions of years.