Neptune-Sized Planet Orbits Backward Around M Dwarf Star, Puzzling Astronomers

Astronomers have long assumed that planets born from the same spinning disk of gas and dust that forms a star should orbit in the same direction the star rotates. It is the natural expectation, the baseline assumption written into how we understand planetary birth. That assumption just broke.

A team led by researchers at Queen Mary University of London has discovered GJ 3090 b, a Neptune-sized planet that orbits its host star in the opposite direction—the first planet of its kind ever found circling an M dwarf, one of the small, cool stars that dominate our galaxy. Using the NIRPS near-infrared spectrograph, an instrument that splits light into its component wavelengths to measure orbital geometry in three dimensions, the team calculated that the planet's orbit tilts at approximately 136 degrees relative to the star's spin axis. The planet is not simply tilted. It is going backward.

"This is a remarkable planetary system because the planet is not simply tilted relative to its star—it is orbiting in the opposite direction," said Dr. Andrew Winter, a lead author of the study. "That immediately raises the question of how such an unusual orbit could have formed." The discovery, published in Astronomy & Astrophysics Letters in September 2026, forces astronomers to reconsider how planetary systems take shape and evolve.

The most intriguing explanation does not require a violent collision or gravitational catastrophe. Instead, the researchers propose that the star may have acquired a second disk of gas and dust early in its history—material tilted relative to the original disk from which the star itself formed. If planets formed from this newly acquired, misaligned material, they would naturally inherit its orientation. GJ 3090 b's backward orbit would then be a fossil record of that early encounter, preserved in the planet's path without needing a later dramatic event to explain it. "The idea that a planetary system could be rebuilt from a second, differently oriented disk is particularly exciting," said Assistant Professor Vincent Bourrier of the University of Geneva. "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."

The team did not stop at speculation. They searched the GJ 3090 system for the kind of massive companion—either a distant planet or a stellar partner—that could have gravitationally wrenched the planet's orbit into such an extreme configuration. Such interactions are the conventional explanation for retrograde planets in other systems. They found no evidence of such a companion. "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," said PhD student Yann Carteret of the University of Geneva. "That suggests we may need to think differently about how this system acquired its unusual architecture."

The measurement itself represents a technical milestone. GJ 3090 b is the smallest planet around an M dwarf for which astronomers have successfully measured a three-dimensional orbital obliquity, demonstrating that near-infrared observations can probe the orbital geometry of planets around these common stars with precision previously thought difficult. The discovery opens a new window onto how planetary systems form and evolve around the most abundant stars in the galaxy.

What remains unknown is whether GJ 3090 b's backward orbit is a singular oddity or the first confirmed example of a pattern. Further observations will be needed to test whether the system truly formed from a second, misaligned disk and to determine whether other planetary systems around M dwarfs harbor similarly extreme orbital arrangements. The answer could reshape how astronomers understand the diversity of planetary architectures across the cosmos.