Peculiar exoplanet intrigues scientists by orbiting its star in the wrong direction.
An international team of astronomers led by the University of Geneva identified a rare retrograde and sharply tilted orbit for the exoplanet GJ 3090 b, located in a system that lacks any massive companion to explain the gravitational disruption. The findings were formally released on Monday, September 21, 2026, alongside publication in the scientific journal Astronomy & Astrophysics.
“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,” stated Yann Carteret, lead author of the study and doctoral researcher at the Department of Astronomy of the University of Geneva.
Infrared instruments track orbital tilt at La Silla
Researchers calculated the geometry of the system using the Near-InfraRed Planet Searcher spectrograph, mounted on the 3.6-meter telescope at the European Southern Observatory La Silla site in Chile, alongside data from the High Accuracy Radial velocity Planet Searcher. The instrumentation allowed scientists to examine six separate planetary transits with high-resolution spectroscopy to determine the alignment between the orbit and the star.
The three-dimensional orbital obliquity, known as the Psi angle, reached 136 degrees relative to the equatorial plane of the host star. In comparison, Earth maintains a Psi angle of 7 degrees relative to the equator of the Sun. Across the history of exoplanetary research, only five multiplanetary systems have shown a Psi angle above 70 degrees, making the extreme misalignment of this world an exceptional find for observational astronomy.
“It is the performance of NIRPS in the infrared that made it possible to achieve the precision needed to measure the angle between the planet’s orbital plane and the star’s equatorial plane for such a small planet,” stated Léna Parc, co-author of the study and doctoral researcher at the University of Geneva.
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Physical properties of the distant sub-Neptune
The planet completes one full trip around its host star in 2.85 to 2.9 days. Classified as a sub-Neptune, it presents key physical metrics that differentiate it from the gas giants commonly linked to extreme orbital shifts:
- Radius measured at 2.2 times the radius of Earth.
- Mass calculated at 4.5 times the mass of Earth.
- Orbital period of 2.85 to 2.9 days around the host star.
- Orbital obliquity measured at a Psi angle of 136 degrees.
- Distance from Earth estimated at 73 light-years.
- Six transits recorded and analyzed with high-resolution spectroscopy.
Missing massive companions force new hypotheses
Standard models of planetary formation establish that planets and their host stars rotate in the same general direction because both condense out of the same rotating disk of gas and dust. In systems where extreme misalignments or backward orbits occur, researchers usually find a massive nearby body, such as a gas giant planet or a binary companion star, whose strong gravitational pull dragged the smaller world off course over time.
Detailed analysis of GJ 3090 revealed no massive stellar companions or outer gas giants. While the NIRPS instrument confirmed two additional smaller planets within the system, none possess the mass or orbital distance required to cause such an intense tilt. The absence of an external gravitational disturber prompted researchers to investigate whether the star itself captured an external, misaligned reservoir of matter during its infancy.
“The absence of a massive companion to explain this unusual orbit will lead us to explore other hypotheses. In the case of GJ 3090, the star could have accreted a misaligned, retrograde secondary disc in which the planets in the system then formed,” explained Vincent Bourrier, co-author and researcher at the University of Geneva.
“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,” stated Andrew Winter, co-author and astrophysicist at Queen Mary University of London.
Milestone for red dwarf observations since 2022 discovery
Initial detection of GJ 3090 b occurred on January 1, 2022, through photometric data gathered by the Transiting Exoplanet Survey Satellite. Follow-up observations have now confirmed that it is the first retrograde exoplanet ever identified around a red dwarf star, also known as an M-class dwarf. It also stands as the smallest planet around this star class to have its three-dimensional obliquity directly calculated.
Whether the planetary architecture arose strictly from the capture of a secondary gas disk or resulted from an alternative dynamical mechanism remains unconfirmed, leaving the final origin open until further complementary astronomical observations are conducted.
