A Distant Planet Orbiting Completely Backward Is Baffling Scientists

A Planet That Defies the Rules

About 73 light-years from Earth lies an exoplanet that has it all backward. While planets generally orbit in the same direction as their host star, this one travels around its star in the opposite direction to the star's rotation — leaving astronomers bewildered. The discovery, published on September 21 in the journal Astronomy & Astrophysics, is already forcing scientists to rethink some of the most fundamental assumptions about how planetary systems form.

GJ 3090 is a red dwarf — a star that is cooler and smaller than our Sun. The planet GJ 3090 b, which orbits it, was first detected by the TESS space telescope, which scans the sky for transits — small, periodic variations in a star's brightness caused by a planet passing in front of it. The planet has a radius 2.2 times that of Earth and a mass 4.5 times that of Earth, making it a sub-Neptune, the most abundant class of exoplanets in our galaxy.

Just How Backward Is It?

The orbital tilt angle — known as Psi — for GJ 3090 b is approximately 136 degrees. To put that in perspective, a perfectly aligned planet, like those in our own solar system, would have an angle of zero. An angle above 90 degrees means the planet is not just tilted — it's orbiting in full reverse. As team member Yann Carteret of the Université de Genève put it: "To our great surprise, not only is the planet GJ 3090 b on a highly misaligned orbit, but it also orbits retrogradely, in the opposite direction to the rotation of its star."

A star and its planets typically form when a cloud of gas and dust collapses in on itself somewhere in space. That cloud forms a disk around the nascent star, within which the planets will form. This protostar and its disk then rotate in the same direction. In this ideal scenario, the planets formed in the disk will then rotate in the same direction as their star, in the same plane as the stellar equator. GJ 3090 b throws all of that out the window.

No Obvious Culprit — and That's the Real Mystery

At first, researchers assumed the backward orbit might be the result of a dramatic gravitational interaction with either a sufficiently massive planet nearby or a companion star, which could disturb a planet's orbit and tilt it dramatically over time. However, they found no evidence of a stellar companion or a sufficiently massive outer planet. That absence is what makes this case so unusual.

This planetary system is one of only five ever seen with a Psi greater than 70 degrees — but what is really different here is that those other systems possess a massive object that clearly seems to have caused the extremely misaligned orbits. GJ 3090 b is the first planet on a retrograde orbit discovered around an M dwarf and the first highly misaligned confirmed multi-planet system without a known massive companion.

A Strange Birth Story

The answer, researchers realized, may have to do with the planetary system's formation itself. They propose that the star may have acquired a second, misaligned disk of gas and dust from its surrounding environment. Planets formed from this material would have taken on an unusual orientation, as GJ 3090 b has.

A Swiss-built instrument called NIRPS — a spectrograph installed at the La Silla Observatory — was used to detect two other planets in the system, confirm the presence of GJ 3090 b, measure its mass, and determine its unusual orbital configuration. "It is the performance of NIRPS in the infrared that made it possible to achieve the precision needed to measure the angle between the planet's orbital plane and the star's equatorial plane for such a small planet," said co-author Léna Parc. The detection of a retrograde orbit in such a small planet around such a dim star was itself considered a technical feat. As scientists continue probing distant planetary systems, GJ 3090 b stands as a reminder that the universe builds worlds in ways we are only beginning to understand.