Imagine a planetary system where the star spins one way, but one of its planets goes around it in the opposite direction.
Astronomers have now found exactly such a system.
Editorial Insight
Key Highlights
Important points readers should notice.
Issue/Event: Astronomers identify GJ 3090 b as a retrograde exoplanet orbiting opposite to the rotation of its host star.
Location: GJ 3090 planetary system, about 73 light-years from Earth.
Authority/Organisation: International astronomy team led by the University of Geneva using the NIRPS spectrograph.
Action Taken: Researchers measured the three-dimensional spin-orbit geometry and determined an orbital angle of approximately 136 degrees.
Impact: The discovery raises new questions about how planets around small, cool stars form and acquire unusual orbital paths.
The planet is GJ 3090 b, a sub-Neptune orbiting a small, cool red dwarf known as GJ 3090. New observations show that the planet's orbital path is strongly misaligned with the star's rotation and, most unusually, moves in the opposite direction.
This is known as a retrograde orbit.
The discovery was made using the NIRPS infrared spectrograph at the La Silla Observatory in Chile. Researchers were able to measure the three-dimensional relationship between the planet's orbit and the rotation of its host star.
Editorial Analysis
Why This Matters
GJ 3090 b challenges the simple assumption that planets should always inherit the same rotational direction as their stars. The unusual orbit could provide clues about the early environments in which planetary systems form, including the possibility that stars can acquire material from different directions or that planetary systems can develop through more complicated processes than previously assumed. Because M-dwarf stars are so common, understanding unusual planetary systems around them could also help astronomers better understand the diversity of planetary architectures across the galaxy.
The measured angle, known as the spin-orbit angle or obliquity, is approximately 136 degrees.
That number is important because an angle above 90 degrees indicates that the planet is moving retrograde relative to the star's rotation.
So, Is the Planet Literally Spinning Backwards?
Not necessarily.
This is where the discovery can easily be misunderstood.
“Backwards” describes the direction in which the planet travels around its star, compared with the direction in which the star rotates.
It does not tell scientists which way the planet itself rotates on its own axis.
In other words, its yearly journey around the star is the unusual part—not necessarily the length or direction of its day.
A Small Planet With a Very Strange Orbit
GJ 3090 b is classified as a sub-Neptune.
It has a radius roughly 2.2 times that of Earth and a mass of about 4.5 Earth masses. It completes one orbit around its star in only about 2.9 days.
The planet was originally detected through observations of its transit, when it passes in front of its star from our viewpoint.
The latest observations provided something much more unusual: information about the geometry of its orbit.
Why Should Planets Normally Follow the Star?
Planetary systems generally form from rotating clouds of gas and dust.
As a young star develops, surrounding material forms a rotating disk. Planets are expected to form within that disk, meaning their orbital directions should normally be broadly aligned with the star's rotation.
Our own Solar System follows this general pattern, with the planets orbiting the Sun in broadly similar directions.
GJ 3090 b does not fit that simple picture.
The Biggest Mystery: Where Did the Backwards Orbit Come From?
Astronomers often look for a massive neighbouring object when a planet has an extremely unusual orbit.
A sufficiently massive planet or another star could gravitationally disturb a planet and alter its orbital orientation over time.
But researchers have not found evidence for a massive companion around GJ 3090 that could easily explain the planet's configuration.
That makes GJ 3090 b particularly interesting.
One possibility proposed by the researchers is that the star may have accumulated a secondary disk of material that was itself misaligned and rotating in the opposite direction. Planets could then have formed from that unusual disk.
That remains a hypothesis, not a confirmed explanation.
Why This Discovery Is Special
Retrograde exoplanets are already known.
What makes GJ 3090 b different is the type of star it orbits.
Researchers describe it as the first known retrograde exoplanet identified around an M-dwarf, a category of small, cool stars that is extremely common in the Milky Way.
The discovery therefore gives astronomers another unusual laboratory for studying how planetary systems form and evolve.
