Astronomers Found a Planet Orbiting Backwards Around a Red Dwarf and the Usual Culprit Is Missing

A small world 4.5 times Earth's mass is circling its star in the wrong direction. Normally there's a bully nearby to blame. This time, there isn't.

Planets usually keep the direction they were given at birth. A star forms from a collapsing cloud of gas, and the leftover material settles into a spinning disk. Planets grow in that disk and generally orbit in the same direction their star rotates.

GJ 3090 b has other ideas. The planet circles a small red dwarf approximately 72 light-years from Earth on a sharply tilted path, traveling opposite to its star’s rotation. Astronomers have seen planets do this before. What makes this one puzzling is that they cannot find the massive companion that might have pushed it onto such an orbit.

“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. It is therefore natural for us to try to measure it for other planetary systems,” said Yann Carteret, a doctoral student in the Department of Astronomy at the University of Geneva’s Faculty of Science and first author of the study.

“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.”

GJ 3090 b is a sub-Neptune exoplanet, approximately 2.2 times Earth’s radius and 4.5 times its mass. It completes a trip around its star every 2.85 days. The planet was already known since it regularly passes in front of the star from Earth’s viewpoint, briefly dimming its light. Those crossings gave researchers a way to investigate its orbit.

The international team observed six of the crossings using the Near-InfraRed Planet Searcher (NIRPS), an infrared spectrograph installed on the European Southern Observatory’s 3.6-meter telescope at the La Silla Observatoryon a telescope in Chile, along with observations from the European Space Agency’s 3.6-meter telescope High Accuracy Radial velocity Planet Searcher (HARPS).

As the planet passed across the rotating star, it blocked light from different parts of its surface. By tracking slight changes in that light, the team determined how the planet’s orbit is oriented relative to the star’s spin.

The answer was an angle of about 136 degrees between the star’s spin axis and the planet’s orbital axis. An angle greater than 90 degrees means the planet is on a retrograde orbit: it travels against the direction of the star’s rotation.

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“To our great surprise,” Carteret said, “it also orbits retrogradely, in the opposite direction to the rotation of its star.”

According to the researchers, GJ 3090 b is the first planet found on a retrograde orbit around a red dwarf. Their findings were published in Astronomy & Astrophysics.

Where’s the bully?

Backwards planets aren’t unheard of. But in the few cases known, there’s usually a suspect: a heavy planet or companion star whose gravity, over millions of years, has tugged a neighbor’s orbit out of line.

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Only five other multi-planet systems are known to host a planet tilted more than 70 degrees from its star’s equator. In those cases, astronomers can point to a massive object nearby. At GJ 3090, they’ve found no such thing.

So the team is looking at other possibilities. One idea, suggested by co-author Vincent Bourrier, a senior lecturer and researcher at UNIGE, is that the problem started before the planets existed. The young star may have gathered up a second disk of material, one that was tilted and spinning backwards compared to the star itself. If the planets formed inside that rogue disk, they’d have been born on backwards orbits, with no need for a later gravitational shove.

It’s a neat idea, but it’s still a hypothesis. It also hints at something unsettling: our tidy picture of star-and-disk formation may be too simple, especially around small stars like GJ 3090.

A gravitational shove from another large object can explain some severely tilted planetary orbits. GJ 3090 has at least one other confirmed planet, but the team found no evidence of a massive outer planet or companion star capable of explaining GJ 3090 b’s path. Their observations narrow the possibilities considerably, although they cannot rule out every unseen object.

“The absence of a massive companion to explain this unusual orbit will lead us to explore other hypotheses,” Bourrier said.

One possibility is that the star acquired fresh gas after its initial formation. If that gas arrived from a different direction, it could have formed a second disk spinning out of step with the star. Planets born in that disk would have inherited its unusual orientation.

The second disk remains a proposed explanation, not something the researchers observed. The study establishes GJ 3090 b’s unusual present-day orbit, while leaving the events that produced it open to further investigation. Measurements of the other planets’ orbital orientations could help test whether they, too, formed from a misaligned disk.

For now, this small planet poses an important question: Was its orbit overturned after it formed, or was the material that made it already moving the other way?