A “Backward” Planet Has Astronomers Rethinking How Some Worlds Form

Astronomers have discovered an unusual planet that travels around its star in the opposite direction to the star’s own rotation.

The planet, known as GJ 3090 b, is the first known retrograde planet found around an M-dwarf star, according to researchers from the University of Geneva and their international collaborators.

A Planet Moving the Other Way

GJ 3090 b is a sub-Neptune located about 73 light-years from Earth. It is approximately 2.2 times the radius of Earth and has about 4.5 times Earth’s mass.

The planet completes an orbit around its red dwarf star in roughly 2.9 days. Researchers measured the angle between the planet’s orbital axis and the star’s rotation axis at about 136 degrees, indicating that the planet travels in a retrograde direction.

An Unusual Planetary System

Planets normally form from a rotating disk of gas and dust surrounding a young star. Because they form from the same material, their orbits generally follow the direction of the star’s rotation.

GJ 3090 b does not follow that pattern. Even more unusually, researchers have not found a massive companion star or large outer planet that could have dramatically altered its orbit after formation.

How Scientists Found the Unusual Orbit

The research team used the NIRPS near-infrared spectrograph on the European Southern Observatory’s 3.6-meter telescope at La Silla Observatory in Chile.

The team studied six transits of GJ 3090 b, with four of those observations also examined using the HARPS spectrograph. These observations allowed researchers to reconstruct the three-dimensional relationship between the planet’s orbit and the rotation of its star.

A Possible Explanation

Researchers propose that the unusual orbit may have been established very early in the system’s history.

One possibility is that the star acquired additional gas and dust from its surroundings. If that material formed a second disk with a different orientation, planets born from that disk could have inherited an unusual orbital direction.

This explanation remains a proposed formation scenario rather than a confirmed account of exactly how GJ 3090 b acquired its orbit.

A New Clue About Planet Formation

The discovery gives astronomers another example of how different planetary systems can be from our own Solar System.

Studying unusual worlds such as GJ 3090 b could help scientists understand the different ways planets form and how their orbits evolve over time.

Sources

• University of Geneva — “Une exoplanète rebelle tourne à rebours de son étoile” — September 21, 2026.
• INAF — “GJ 3090 b, l’esopianeta al contrario” — September 30, 2026.
• NASA Exoplanet Archive — GJ 3090 planetary system data.
• Live Science — “Astronomers discover ‘backward’ planet orbiting its star in the wrong direction” — September 30, 2026.