GJ 3090 b becomes the first retrograde planet found around a red dwarf

Researchers observed six transits of GJ 3090 b with the NIRPS and HARPS spectrographs and measured the Rossiter-McLaughlin effect—the temporary distortion in a star's spectrum as a planet crosses its rotating surface. Their analysis gives the planet a three-dimensional obliquity of 136 degrees, with an uncertainty of plus 24 or minus 18 degrees, placing its orbit firmly in the retrograde regime.

The peer-reviewed result makes GJ 3090 b the first retrograde planet detected around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive outer planet or wide stellar companion. That absence disfavors, but does not eliminate, later gravitational disturbance by a massive body. The authors favor a protoplanetary disk that began misaligned—possibly after late secondary gas accretion—followed by disk-driven migration; that history is an interpretation of the measured architecture, not a directly observed event.

Why it matters

Orbital alignment preserves clues about planetary-system formation, and extending these measurements to small planets around common red dwarfs tests whether their histories differ from those of hot Jupiters.

What the source establishes

Peer-reviewed spectroscopy and orbital geometry. The retrograde orientation is measured; the proposed disk history is a favored explanation rather than a confirmed reconstruction.