Neptune-Sized Planet Found Orbiting Its Star in Reverse
Astronomers have identified GJ 3090 b, a Neptune-sized planet orbiting an M dwarf in the opposite direction to the star’s rotation. Using the NIRPS spectrograph, the team measured an orbital obliquity of about 136 degrees, confirming the first known retrograde planet around this type of star. Researchers are examining whether the planet formed from a second, tilted disk of material acquired early in the system’s history.
GJ 3090 b is a Neptune-sized world circling a small, cool M dwarf. Its orbit runs opposite the star's spin, making it the first confirmed retrograde planet around such a star. NIRPS near-infrared measurements yielded an obliquity near 136 degrees, a three-dimensional angle linking stellar rotation and planetary path.
Researchers considered whether an additional disk that arrived misaligned early could have shaped the planet's backward motion. They found no massive outer companion or stellar partner that easily explains the tilt. This is also the smallest M-dwarf planet with a measured 3D obliquity, reported in Astronomy & Astrophysics Letters.
This result could shape how astronomers and educators discuss planetary formation, especially around the galaxy's most common stars. It may spark public curiosity about exoplanets and inspire students considering science careers. For instrument teams and mission planners, it could highlight the value of near-infrared spectrographs for measuring orbital geometry. Its broader societal effect is likely indirect, filtered through science communication, education, and future research rather than immediate daily life.