Evidence item

We present six transit observations of the sub-Neptune GJ 3090 b obtained with NIRPS and HARPS. GJ 3090 b is the inner planet of a confirmed multi-planet system orbiting an M dwarf (M2). Using high spectral resolution and high temporal cadence spectroscopic observations, we analyzed the Rossiter-McLaughlin effect induced by GJ 3090 b to determine its orbital obliquity. Through the RM revolutions technique, we find that the planet is on a retrograde orbit with a derived 3D obliquity of ψ=136^{+24}_{-18} deg. We find no evidence of massive outer planetary or wide stellar binary companions, which disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk. Our results establish GJ 3090 b as the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion.