GJ 3090 b, located 73 light-years from Earth, was discovered in 2022, but its unusual orbit was determined through new measurements. The sub-Neptune, with a mass approximately 4.5 times that of Earth, moves opposite to the direction of its star’s rotation; the angle between the orbital and rotational axes is 136 degrees. This retrograde orbit was detected using the NIRPS and HARPS instruments on the 3.6-meter ESO telescope in Chile by examining changes in starlight during planetary transits. GJ 3090 b became the first red dwarf planet with a retrograde orbit detected with NIRPS. There is no large planet or star in the system that could have caused the change in direction. The University of Geneva team is considering that the planet may have formed in a second gas and dust disk moving in the opposite direction.
Why it matters
This measurement provides a concrete example showing that the direction of an orbit in the formation of planetary systems may not always align with the rotation of the host star. The absence of a large planet or star in the system that could have later reversed the orbit points the explanation toward the formation process rather than known gravitational interactions. For this reason, the University of Geneva team’s second gas and dust disk scenario stands out as a possibility consistent with the current data; however, it is not a confirmed result. Determining such an orbit around a red dwarf with NIRPS also expands the scope of the observation method for studying similar systems. The open question is whether the retrograde motion truly originates from the planet’s formation.