An international team led by the University of Geneva (opens in new tab) reports that the sub-Neptune exoplanet GJ 3090 b orbits its red-dwarf host roughly 73 light-years away on a highly misaligned path that runs opposite the star’s rotation, according to Space.com (opens in new tab) and a UNIGE news release summarizing results published in Astronomy & Astrophysics.
Using the NIRPS infrared spectrograph at ESO’s La Silla Observatory, with HARPS data also cited in the journal abstract, the researchers measured a three-dimensional stellar-orbital obliquity, Psi, of about 136 degrees. UNIGE doctoral student and lead author Yann Carteret said the team was surprised to find both strong misalignment and a retrograde orbit. Co-author Léna Parc credited NIRPS’s infrared performance for the precision needed on such a small planet.
GJ 3090 b has a radius about 2.2 times Earth’s and a mass about 4.5 Earth masses, Space.com (opens in new tab) and UNIGE said, placing it among common sub-Neptunes. It is the inner world in a confirmed multi-planet system around an M dwarf. UNIGE said only a handful of other multiplanet systems host a planet with Psi greater than 70 degrees, and those typically include a massive companion that can torque orbits—something not found here.
Co-author Vincent Bourrier said the lack of a massive companion pushes the team toward other explanations, including the possibility that the star later accreted a misaligned, retrograde secondary disk in which the planets formed. The A&A paper abstract likewise frames the system as the first retrograde planet reported around an M dwarf and the first highly misaligned confirmed multiplanet system without a known massive companion; those priority labels are the authors’ claims.