Located 73 light-years from Earth, GJ 3090 b follows a highly tilted, retrograde orbit that challenges existing theories of planetary formation. Researchers are investigating how the unusual system came into existence.
Astronomers have identified an unusual exoplanet that appears to move against the rotation of its host star, earning it the nickname the “rebel planet”.
Known as GJ 3090 b, the planet orbits a red dwarf that is smaller and cooler than the Sun. Unlike planets that generally follow the direction of their host star’s rotation, GJ 3090 b travels along a sharply misaligned orbit in the opposite direction.
The discovery was made by researchers at the University of Geneva, who used the NIRPS spectrograph to determine the planet’s orbital configuration. The instrument, partly developed in Geneva, is installed at the La Silla Observatory in Chile’s Atacama Desert.
“To our great surprise, GJ 3090 b not only has a strongly misaligned orbit, but also moves retrograde, in the opposite direction to its star’s rotation,” said Yann Carteret, a researcher at the University of Geneva’s Department of Astronomy and lead author of the study published in Astronomy & Astrophysics.
The finding raises questions about how the planet formed and what forces could have pushed it into such an unusual trajectory.
An unexplained planetary system
First detected in 2022 by NASA’s Transiting Exoplanet Survey Satellite (TESS), GJ 3090 b lies approximately 73 light-years from Earth. It has a radius around 2.2 times that of Earth and a mass approximately 4.5 times greater, placing it in the category of sub-Neptune planets.
So far, astronomers have found no evidence of another body in the system that could readily explain its unusual orbit.
One possibility is that a massive companion, such as a gas giant, a brown dwarf or another star, exerted enough gravitational influence to alter the planet’s trajectory. However, observations have yet to reveal such an object.
The discovery also challenges expectations about how planetary systems develop.
Planets generally form from discs of gas and dust surrounding young stars. Because the material in these discs rotates in the same direction as the central star, planets are expected to inherit a broadly aligned orbital configuration.
GJ 3090 b appears to violate this pattern, with an orbit that is not only strongly tilted but also runs in the opposite direction to its star’s rotation.
A rare orbital configuration
To describe the relationship between a planet’s orbit and its host star’s rotation, astronomers measure an angle known as Psi. The parameter helps researchers understand how planetary systems evolve and whether gravitational interactions may have disrupted their original configuration.
In our Solar System, all eight planets orbit the Sun in the same general direction as its rotation, although their orbital planes are slightly tilted relative to the Sun’s equator. These small deviations are thought to reflect gravitational disturbances during planetary formation and the early evolution of the Solar System.
The angle Psi for planets in our own system ranges from approximately three to seven degrees, with Earth’s value slightly above seven degrees.
For GJ 3090 b, however, researchers measured an angle of 136 degrees using the NIRPS instrument. Such a large value indicates a strongly retrograde orbit.
The system is among only five known planetary systems with a Psi angle exceeding 70 degrees, highlighting just how unusual its configuration is.
Scientists consider alternative explanations
The absence of a massive companion capable of explaining the planet’s trajectory has prompted researchers to consider other possibilities.
Vincent Bourrier, a co-author of the study at the University of Geneva, said the team would need to investigate alternative explanations for the system’s unusual architecture.
One hypothesis is that the host star may have acquired a second disc of gas and dust that was misaligned with its original rotation and moved in the opposite direction. Planets could then have formed within this secondary disc, potentially explaining GJ 3090 b’s retrograde orbit.
For now, the mechanism behind the planet’s unusual motion remains unresolved. Further observations and theoretical work will be needed to determine whether the system formed under exceptional conditions or evolved through processes that current models do not fully explain.
The discovery could ultimately help astronomers refine their understanding of how planetary systems form and why some develop configurations that differ so dramatically from our own.
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