First ‘backwards’ planet discovered around a small star challenges ideas about how planets form

Astronomers have discovered the first known planet orbiting a small, cool star on a retrograde orbit, travelling around its star in the opposite direction to the star's rotation. 

New observations of GJ 3090 b, a small Neptune-sized planet, provide a rare glimpse into the complex processes that can shape planetary systems and raise new questions about how planets acquire their orbits. 

The research, involving researchers at Queen Mary University of London, is published in Astronomy & Astrophysics Letters. 

Planets are generally expected to orbit in the same direction as their host star. This is because stars and their planets form from the same rotating disk of gas and dust, meaning planets should initially inherit the direction of that disk. 

But GJ 3090 b appears to have taken a very different path. 

Using high-resolution observations from the NIRPS near-infrared spectrograph, the researchers measured the three-dimensional orientation of the planet's orbit. They found an orbital obliquity of approximately 136 degrees, confirming that the planet is moving in a retrograde direction. 

Dr. Andrew Winter, a lead author from Queen Mary University of London, said: 

“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.” 

Retrograde planets are particularly interesting because their unusual orbits are often thought to result from dramatic gravitational interactions. A sufficiently massive planet or a companion star could, for example, disturb a planet's orbit and tilt it dramatically over time. 

The researchers therefore searched for evidence of another massive object in the GJ 3090 system. 

However, their observations found no evidence for a wide stellar companion or a sufficiently massive outer planet that could readily explain the planet's extreme orbital misalignment. 

This makes the system particularly intriguing. 

“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.” says PhD student Yann Carteret from the University of Geneva. 

The researchers propose that the explanation may instead lie much earlier in the system's history. 

One possibility is that the star acquired a second, misaligned disk of gas and dust from its surrounding environment. Planets could then have formed from this material, inheriting its unusual orientation. 

If correct, this would mean that GJ 3090 b's backwards orbit is not necessarily the result of a later violent encounter. Instead, it could preserve evidence of the unusual conditions under which its planetary system formed. 

“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.” Assistant Professor Vincent Bourrier from the University of Geneva adds. 

The study also demonstrates the growing importance of near-infrared observations for studying planets around M dwarfs — the small, cool stars that make up the majority of stars in our Galaxy. 

GJ 3090 b is the smallest planet around an M dwarf for which a three-dimensional orbital obliquity has been measured, providing an important new target for understanding the diversity of planetary systems. 

The researchers say that further observations will be needed to test the proposed formation scenario and establish whether other planetary systems contain similarly extreme orbital configurations. 

The discovery highlights how studying planets with unusual orbits can provide more than just evidence of exotic worlds. Their movements can act as fossils of planetary formation, preserving clues about the events and environments that shaped their systems billions of years ago.