GJ 3090 b, a sub-Neptunian planet around a red dwarf, orbits its star in a retrograde orbit at an angle of about 136 degrees. The researchers found no massive body in the system that could easily explain how it arrived at its unusual orbit.
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Researchers from Queen Mary University of London helped identify the planet's reverse orbit, a finding that could provide new clues about the formation and evolution of planetary systems.
When a star and its planets form from the same spinning disk of gas and dust, you would expect the planets to orbit it in the same direction as the star itself. But GJ 3090 b, a planet with a radius about 2.2 times that of Earth and a mass of about 4.5 Earth masses, does exactly the opposite.
An international team led by the University of Geneva measured the three-dimensional structure of the orbit and found that the angle between the star’s rotational axis and the planet’s orbital axis is about 136 degrees. This means that it is a retrograde orbit — the planet moves around the star in the opposite direction to its rotation.
The system is located around GJ 3090, an M-type red dwarf, one of the most common small, cool stars in the galaxy. According to the researchers, this is the first retrograde planet to be confirmed around such a dwarf.
Measurement using NIRPS and HARPS
The researchers used the NIRPS spectrograph, which operates in the near-infrared range and is mounted on ESO's 3.6-metre telescope at the La Silla Observatory in Chile. HARPS was also used for some of the observations. By observing the planet as it passed in front of its star, the researchers were able to measure the direction of the orbit relative to the star's rotation.
"To our great surprise, GJ 3090 b is not only in a highly misaligned orbit, but also orbits in the opposite direction to its star's rotation," said Jan Carteret, the study's first author and a doctoral student in the Department of Astronomy at the University of Geneva.
"This is an amazing planetary system, because the planet is not only tilted relative to its star — it orbits it in the opposite direction. This immediately raises the question of how such an unusual orbit could have formed," said Dr. Andrew Winter of Queen Mary University of London, one of the leaders of the study.
How was such a route created?
In a normal planetary system, the star and its planets form from the collapse of the same cloud of gas and dust. The material organizes into a spinning disk, so the planets that form in it should inherit its direction of motion. In the solar system, for example, all eight planets orbit the sun in the same direction.
Highly inclined orbits, especially retrograde orbits, can be caused by gravitational interactions with a distant, massive planet or companion star. However, in GJ 3090, the researchers searched for such a body and found no evidence of a distant companion star or an exoplanet massive enough to explain the extreme inclination.
"We were looking for the kind of massive companion that could have pushed the planet into such an extreme orbit, but we find no evidence of one. This suggests that we will need to change our thinking about how this system got its unusual structure," said doctoral student Jan Carteret.
"The absence of a massive companion to explain the unusual trajectory will lead us to examine other hypotheses," said Vincent Bourrier of the University of Geneva.
Maybe the system was rebuilt from a second disk.
One possibility the researchers raise is that the young star early on accreted additional material from its surroundings. If this material had an angular momentum in a different direction than the original disk, it could have created a second protoplanetary disk, tilted or even retrograde with respect to the star's rotation.
In such a scenario, the planets formed from the second disk would have inherited the unusual direction of motion already at the time of their formation. That is, a later violent event would not have been necessary to "flip" the orbit.
This possibility is particularly interesting because it expands the possible role of the environment in which the star was born. Rather than seeing the architecture of a planetary system as an almost exclusive result of the original disk, it is possible that material added later could reconstruct part of the system.
Bourrier added: "The idea that a planetary system can be rebuilt from a second disk, whose orientation is different, is particularly exciting. It suggests that the environment around a young star can play a much larger role in determining the structure of its planetary system than we would expect."
A small system with a surprising architecture
GJ 3090 b is also the smallest planet around an M-dwarf for which the 3D orbital inclination has been measured. The measurement demonstrates the importance of near-infrared observations for studying orbits around small, cool stars. Further observations will be needed to test the second disk scenario and to determine how common such extreme structures are in other systems.
The system provides researchers with a new test for models of planetary system formation. If it turns out that this is indeed a system built from material that later accreted in a different direction, it would suggest that a young star's environment can influence the final structure of its planetary system much more than has been previously assumed.
Questions and Answers
What is special about GJ 3090 b?
This is a sub-Neptunian planet orbiting a red dwarf in the opposite direction to the star's rotation. According to the researchers, this is the first confirmed case around an M-type dwarf.
What is the angle of the track?
The angle between the star's rotational axis and the planet's orbital axis is approximately 136 degrees, so the orbit is considered retrograde.
Why aren't researchers satisfied with an explanation of gravitational perturbation?
Because no evidence of a distant companion star or massive exoplanet was found in the system that could easily explain the unusual inclination.
What alternative explanation is offered?
It is possible that early in its life the star accumulated an additional disk of gas and dust in a different direction from the original disk, and the planets that formed from it inherited the unusual orbit.
The scientific article
Yann Carteret and others, Upside down: GJ 3090 b, the first retrograde exoplanet around an M dwarf detected with NIRPS,Astronomy & Astrophysics, September 2026. University of Geneva announcementTranslation source: SciTechDaily, according to Queen Mary University of LondonTranslation: Nachum Shereshevsky.
More on the subject on the science website
- This shouldn't exist: Astronomers discover planet orbiting "in the opposite direction"
- New method reveals: Some hot Jupiter-type planets migrated “quietly” into the disk
- A solar system in the making? Two planets observed forming in a disk around a young star
- The discovery of the planet TIDYE-1, the youngest so far, shatters the theories about the formation of planets
- The small star that hosts a giant planet and stretches the laws of astronomy
