Scientists have discovered mini-Neptune-type exoplanets around four red dwarfs that are located close to their parent stars and are likely in unusual eccentric orbits, writes The Astronomical Journal.
Mini-Neptunes are planets that are intermediate in size between Earth and Neptune and are relatively common outside the Solar System. They represent promising objects for studying exoplanetary atmospheres with the James Webb Space Telescope. The new mini-Neptunes have a radius roughly 2-3 times that of Earth and an orbital period around the stars of less than eight days.
The observations were made using a number of ground-based telescopes equipped with MuSCAT astronomical cameras to study the atmospheres of transiting exoplanets. The radial velocities of the host stars were measured using the IRD (InfraRed Doppler) spectrometer on the Subaru telescope. Measurements show that the upper limit of the masses of these planets is less than 20 times the mass of Earth.
The relationship between the measured radii and masses of the mini-Neptunes shows that they are not rocky planets like Earth. Their interior probably contains volatile substances such as water ice crystals. At least three planets — TOI-782 b, TOI-2120 b and TOI-2406 b — are located in eccentric orbits, which is unusual for planets orbiting red dwarfs. Ordinarily, the orbits of short-period planets should be circular due to the influence of tidal forces.
One explanation for this phenomenon is that their interior, which resembles the interior of Neptune, is not subject to tidal influences. This allows the planets to maintain a non-zero eccentricity for billions of years.