Astronomers discovered a sub-Neptune exoplanet orbiting its red dwarf star backward on a sharply tilted path, challenging standard theories of planet formation. Researchers found no massive companion star or giant planet nearby to explain the retrograde motion, leaving the unusual orbit of GJ 3090 b a total mystery.
Planets are supposed to follow a predictable script. When a massive cloud of gas and dust collapses under gravity to form a young star, the leftover material flattens into a rotating disk. Worlds born within that spinning disk normally inherit its angular momentum, orbiting their host star in the same direction and close to its equatorial plane. Even our own Solar System bends this rule slightly, with Earth tilted just over seven degrees relative to the solar equator.
That tidy picture breaks down completely when looking at GJ 3090 b. Located approximately 72 light-years from Earth, the sub-Neptune exoplanet has a radius about 2.18 times that of Earth and a mass roughly 4.52 times greater. It completes an orbit around its small red dwarf host every 2.85 to 2.9 days. Its defining feature, however, is not its size or speed, but the bizarre orientation of its path.
NIRPS and HARPS Observations Reveal a 136-Degree Tilt
The planet was already known because it regularly passes in front of the star from Earth’s viewpoint, creating tiny, measurable dips in starlight. To figure out how that orbit was tilted, an international research team observed six of these transits using the Near-InfraRed Planet Searcher, an infrared spectrograph installed on the European Southern Observatory’s 3.6-meter telescope at the La Silla Observatory in Chile. Researchers also incorporated observations from the High Accuracy Radial velocity Planet Searcher.
Tracking the planet as it blocked different portions of the rotating stellar surface allowed scientists to measure shifts in spectral lines. One side of the star moves toward Earth while the other moves away, creating a Doppler signature that exposes the orbital geometry. The team collected 142 spectra with the near-infrared spectrograph and 51 HARPS spectra during about 23 hours of observing time. The combined measurements yielded a projected spin-orbit angle near 208 degrees, which translated to a true three-dimensional obliquity of approximately 136 degrees.
Measuring the angle Psi (the small angle between their orbital planes and the star’s equatorial plane) provides clues about the formation and evolution of planetary systems.
Yann Carteret, doctoral student in the Department of Astronomy at the University of Geneva’s Faculty of Science
An angle greater than 90 degrees means the object travels against the rotation of its host. That makes GJ 3090 b the first known retrograde planet around an M dwarf, as well as the smallest planet around such a cool, common star to have a three-dimensional obliquity measurement.
Missing Culprits Leave Astronomers Searching for Answers
When astronomers discover an exoplanet on an extreme orbit, they usually look for a gravitational bully nearby. In the few known multi-planet systems where a world is tilted more than 70 degrees from its star’s equator, a massive companion star or a giant outer planet is typically found lurking in the system. Over millions of years, that heavy object’s gravity can tug a neighbor completely out of alignment.
At the GJ 3090 system, however, that standard detective story hits a dead end. Although the system contains at least one other confirmed planet, the research team found no evidence of a massive outer planet or companion star capable of explaining the extreme tilt.
Could a Second Disk Explain the Retrograde Orbit?
With gravitational disruption ruled out as the primary cause, the researchers are investigating whether the unusual setup dates back to the system’s birth. One leading hypothesis suggests that the young star gathered fresh gas from its surroundings long after its initial formation. If that incoming gas arrived from a different direction, it could have settled into a second, misaligned disk of material spinning backward compared to the star itself.
Planets born inside that rogue second disk would have inherited its backwards orientation straight from the start, requiring no dramatic collision or gravitational tug-of-war later in life. While the idea remains a hypothesis, it points to a much more chaotic environment around young stars than traditional models assume.
What Unusual Orbits Preserve from the Past
Unusual orbits act almost like geological fossils, preserving traces of the specific events and conditions that shaped a system billions of years ago. As instruments like NIRPS continue pushing the limits of infrared astronomy, researchers expect to find more orbital oddities hiding among dim, common stars across the Milky Way. Further observations will be required to test whether a second disk truly birthed this inverted world.
