GJ 3090 b orbits its red dwarf backwards and leaves theorists without a giant companion to blame
An international team led from the University of Geneva reports in Astronomy & Astrophysics (Volume 713, September 2026) that the sub-Neptune GJ 3090 b travels on a retrograde orbit around its M-dwarf host — the first such confirmed case around a red dwarf, and the first strongly misaligned multi-planet system without a known massive companion. The preprint posted to arXiv on 21 September 2026 (arXiv:2609.24870) and an Instituto de Astrofísica de Canarias outreach note the same day summarise six transit observations with the NIRPS infrared spectrograph and HARPS. The measured three-dimensional obliquity is ψ = 136° with asymmetric uncertainties of +24°/−18°.
How the tilt was measured
GJ 3090 is an M2 red dwarf. GJ 3090 b is the inner world of a confirmed multi-planet system; transit photometry first flagged it, while NIRPS on ESO’s 3.6-metre telescope at La Silla helped confirm the planet, weigh it and map its unusual geometry. The IAC note puts the planet’s radius near 2.2 Earth radii and its mass near 4.5 Earth masses — a sub-Neptune in the common Galactic census. High-resolution, high-cadence spectra captured the Rossiter-McLaughlin effect: the anomaly in the star’s rotationally broadened lines as the planet blocks approaching or receding limbs during transit. Through the RM-revolutions technique applied in the near infrared, the team recovers a retrograde, highly misaligned orbit.
That geometry matters because spin–orbit angle is a fossil of formation and migration. In multi-planet systems it helps separate competing migration stories. The authors find no evidence for a massive outer planet or a wide stellar binary that could have tilted the inner orbit through gravitational scattering. That disfavours a late kick from a heavy body and points instead toward a primordial misalignment of the protoplanetary disk.
Hypothesis versus conclusion
The paper’s preferred reading is late secondary disk accretion that need not share the stellar spin axis, followed by disk-driven migration into the observed architecture. That remains a proposed mechanism, not a filmed history. What is measured is the obliquity and the absence of a detected massive companion in the data in hand. What is not proven is that no unseen body exists at all, or that every M-dwarf multi-planet system formed the same way. The work also showcases NIRPS’s reach: near-infrared RM observations can now probe orbital architectures of smaller planets around M dwarfs that optical campaigns largely missed.
Obliquity studies have long leaned on hot Jupiters and brighter stars because the RM signal scales with planet size and stellar rotation. Pushing the method onto a sub-Neptune around a cool M dwarf with near-infrared stability is why NIRPS matters here: redder wavelengths suit red-dwarf spectra, and high cadence catches the transit anomaly before it washes out. The six-transit data set is the empirical core; everything about secondary disk accretion is interpretation layered on that core. Readers should keep the split visible. A rebellious orbit is a measurement. A formation pathway is a ranked hypothesis the authors themselves mark as most likely given the absence of a detected heavy companion.
IAC’s outreach adds institutional colour without changing the numbers: Spanish participation in NIRPS, La Silla’s 3.6-metre mount, and UNIGE leadership on the A&A letter. The DOI and volume stamp place the result in the September 2026 issue rather than in rumour. Until someone finds a hidden massive body or remeasures a lower obliquity, GJ 3090 b stands as the first documented retrograde planet on an M dwarf and a warning that multi-planet systems can be born tilted without a late dynamical bully in the file.
Sources
