Astronomers have discovered GJ 3090 b, a small Neptune-sized exoplanet orbiting a cool red dwarf star in a completely retrograde path. The finding challenges conventional planet formation theory by lacking the massive companion typically required to tilt such worlds.
In our own Solar System, planets orbit the Sun in an orderly fashion, aligned closely with the solar equator and moving in the direction of the Sun’s rotation. Yet exoplanetary systems across the galaxy routinely reveal architectures that completely upend those tidy rules. The latest and most puzzling example centers on GJ 3090 b, an exoplanet roughly 2.18 to 2.2 times the radius of Earth and about 4.5 to 4.52 times its mass. Located approximately 72 light-years away, the sub-Neptune circles its small host star in just about 2.9 days, completing an orbit once every 2.85 to 2.9 days.
The discovery was detailed in an international study published in Astronomy & Astrophysics under the title Upside down: GJ 3090 b the first retrograde exoplanet around an M dwarf detected with NIRPS,
led by researchers at the University of Geneva. Astronomers determined that the planet travels along a sharply tilted path running directly opposite to the rotational spin of its parent star. That backward movement represents a first for planetary systems built around cool, dim M dwarfs.
Uncovering the Retrograde Orbit via High-Precision Infrared Instruments
Planets typically inherit their angular momentum from the same rotating cloud of gas and dust that gives birth to their host star. Under standard formation models, a collapsing stellar nursery flattens into a protoplanetary disk where newborn worlds naturally travel in the same direction that the star turns. GJ 3090 b ignores that standard expectation entirely.
To decode the unusual geometry, researchers used the Near-InfraRed Planet Searcher (NIRPS), an infrared spectrograph installed on the European Southern Observatory’s 3.6-meter telescope at the La Silla Observatory in Chile, alongside data from the High Accuracy Radial velocity Planet Searcher (HARPS).
The detection relies on how a planet alters starlight as it crosses a rotating stellar disk. One side of a spinning star moves toward an observer while the other moves away, creating distinct shifts in spectral lines. For GJ 3090 b, the sequence was completely reversed, proving it crosses the red hemisphere first.
“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.”
Yann Carteret, University of Geneva
Combining the NIRPS measurements with stellar axis orientations, the team calculated a true three-dimensional orbital obliquity of approximately 136 degrees. Dr. Andrew Winter, a lead author from Queen Mary University of London, noted the extreme nature of the configuration: This is a remarkable planetary system because the planet is not simply tilted relative to its star — it is orbiting in the opposite direction. That immediately raises the question of how such an unusual orbit could have formed.
The Missing Bully and the Puzzle of Planetary Dynamics
Finding a backwards exoplanet is not entirely unprecedented, but previous instances usually came packaged with an obvious culprit. In the rare multi-planet systems where worlds are heavily tilted, astronomers can typically point to a massive outer companion whose heavy gravitational tug slowly dragged its neighbor out of alignment over millions of years.
At GJ 3090, however, that familiar dynamic appears broken. The research team searched thoroughly for a wide stellar companion or a massive outer planet capable of forcing such an extreme orbital tilt, but turned up empty-handed.
“We looked for the kind of massive companion that could have forced the planet into such an extreme orbit, but we don’t find evidence for one. That suggests we may need to think differently about how this system acquired its unusual architecture.”
Yann Carteret, University of Geneva
Yet the absence of a heavy gravitational disturber leaves classical dynamic models struggling to account for the planet’s trajectory.
Primordial Disk Flips and Rogue Protoplanetary Material
With violent gravitational scattering ruled out by the lack of a companion, scientists are turning their attention to events that occurred much earlier in the stellar system’s infancy. One leading hypothesis suggests the explanation lies in how the red dwarf gathered its surrounding material.
Senior lecturer and researcher Vincent Bourrier from the University of Geneva proposed that the young star may have later acquired fresh gas and dust from the surrounding interstellar medium. If that incoming stream arrived from a different direction, it could have settled into a second protoplanetary disk spinning backward relative to the star itself. Planets born inside that rogue disk would inherit a retrograde orientation naturally, bypassing any need for a later violent encounter.
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If GJ 3090 b formed from a misaligned secondary accumulation of gas, its backwards motion serves as a preserved fossil of chaotic birth conditions rather than the result of a later collision.
“The idea that a planetary system could be rebuilt from a second, differently oriented disk is particularly exciting. It suggests that the environment around a young star can play a much bigger role in determining the architecture of its planets than we might have expected.”
Vincent Bourrier, University of Geneva
As astronomers continue to catalog M dwarfs—the most common stars in the Milky Way—GJ 3090 b establishes a vital technical milestone as the smallest planet around such a star to have its three-dimensional orbital obliquity measured. Further observations across similar multi-planet systems will determine whether rogue secondary disks represent a common pathway in planetary evolution or remain an extreme galactic anomaly.
