Astronomers have discovered GJ 3090 b, a sub-Neptune exoplanet orbiting backward around its parent red dwarf star 73 light-years away from Earth. Reported by researchers led by Yann Carteret at the University of Geneva, this retrograde orbit challenges traditional planet formation models because the system lacks the massive gravitational companions usually required to tilt planetary paths.
Mapping the Retrograde Orbit of GJ 3090 b
Planets typically form within the spinning protoplanetary disk left behind by a collapsing cloud of gas, inheriting a clean, prograde orbit aligned with the host star’s rotation. Our solar system exemplifies this orderly architecture, with all eight planets orbiting the Sun within a mere 7 degrees of its equatorial plane. Yet, exoplanet systems frequently defy this baseline, and none more starkly than GJ 3090 b.
Located in the southern constellation Phoenix, GJ 3090 b measures roughly 2.2 times the radius of Earth with 4.5 times its mass, completing a tight transit cycle every 2.9 days. To uncover the mechanical reality of this orbit, an international team turned to the European Southern Observatory’s 3.6-meter telescope in Chile. Using the Near-InfraRed Planet Searcher (NIRPS) infrared spectrograph and the High Accuracy Radial velocity Planet Searcher (HARPS), researchers tracked six planetary transits across the face of the host star.
During a transit, astronomers look for the Rossiter-McLaughlin effect. As a star rotates on its axis, the hemisphere spinning toward the observer appears blueshifted, while the hemisphere turning away registers as redshifted. A standard planet crossing the stellar disk obscures the blue side first, creating a temporary red shift in the composite light curve before shifting back. For GJ 3090 b, the team observed the exact opposite sequence across five separate passes: it crossed the redshifted hemisphere first, followed by the blueshifted side.
Calculations derived from these spectral shifts place the planet’s orbital obliquity at approximately 136 degrees relative to the stellar spin axis. Any orbital tilt exceeding 90 degrees denotes a retrograde trajectory, meaning the sub-Neptune moves entirely against the rotational direction of its parent red dwarf.
The Missing Bully Problem in Stellar Dynamics
Backward orbits are not entirely unprecedented in exoplanetology, but they generally come with a visible perpetrator. In the handful of multi-planet systems where objects exhibit orbital tilts greater than 70 degrees, astrophysical models rely on gravitational drama to explain the misalignment. Massive outer planets or stellar companions typically exert long-term secular perturbations, gradually tugging a neighboring world out of its primordial plane over millions of years.
At the GJ 3090 system, however, that familiar narrative falls apart. While observations indicate the presence of another planet in the system—which researchers suspect may also share a retrograde path, pending further data—the system completely lacks a massive companion heavy enough to have altered GJ 3090 b’s orbital inclination through gravitational scattering.
“Measuring the angle Psi (the small angle between their orbital planes and the star’s equatorial plane) provides clues about the formation and evolution of planetary systems. It is therefore natural for us to try to measure it for other planetary systems,” explains Yann Carteret, a doctoral student in the Department of Astronomy at the University of Geneva’s Faculty of Science and lead author of the study. “To our great surprise, not only is the planet GJ 3090 b on a highly misaligned orbit, but it also orbits retrogradely, in the opposite direction to the rotation of its star.”
Primordial Disk Flips and Peter Pan Disks
With classical dynamical scattering models ruled out by the absence of a heavy outer perturber, researchers are forced to look at scenarios originating before planet formation even concluded. Co-author Vincent Bourrier, a senior lecturer and researcher at UNIGE, points to a mechanism predating the planetary bodies themselves: the primordial disk flip.
Under this hypothesis, the young red dwarf initially gathered its primary protoplanetary disk, which subsequently dissipated. A fresh stream of gas and dust from the surrounding interstellar medium later fed the system. While a portion of this infalling material fed directly into the star, a fraction established a secondary protoplanetary disk. If this rogue material entered at a sharp angle, it would retain an angular momentum vector completely opposite to the stellar rotation. Planets born inside this counter-spinning disk would naturally inherit a retrograde trajectory from birth, eliminating the need for a later gravitational shove.
Astronomers have found supporting circumstantial evidence for prolonged disk life cycles in observations of “Peter Pan disks”—circumstellar disks that persist tens of millions of years longer than standard stellar evolution models predict. GJ 3090 b may represent the physical outcome of a Peter Pan disk that matured late into a planetary system under reversed rotational conditions.
A Technical Milestone for M Dwarf Observations
Beyond shaking up planetary formation theory, the detection of GJ 3090 b establishes a critical technical milestone. It marks the smallest exoplanet to date to have its orbital obliquity mapped around an M-dwarf star. As high-precision infrared spectrograph instruments like NIRPS continue to survey low-mass stellar populations, the discovery of GJ 3090 b opens a new observational window into the diverse, chaotic architectures hidden across the galaxy.
The findings have been published in Astronomy & Astrophysics, expanding the catalog of systems that challenge our assumptions about how orderly planetary systems are born.
