A planet circling its star backwards sits 136 degrees off, and nobody can explain it

The planet GJ 3090 b circles its red dwarf star in the opposite direction from the way the star spins, on an orbit tilted 136 degrees from the stellar equator. A team led by Yann Carteret of the University of Geneva measured the angle with a near-infrared spectrograph on a 3.6-metre telescope in Chile, and found nothing in the system big enough to have pushed the planet there.

The result is the first retrograde planet around an M dwarf, and the paper that reports it carries a blunt conclusion: the usual explanations for a flipped orbit do not apply here.

A tilt of 136 degrees, with error bars

The measurement appears in the Astronomy & Astrophysics paper titled “Upside down: GJ 3090 b the first retrograde exoplanet around an M dwarf detected with NIRPS.” It reports a three-dimensional orbital obliquity of 136 degrees, with an uncertainty of plus 24 and minus 18 degrees. Anything above 90 degrees means the planet moves against the star’s rotation, so even the lower edge of the error bar, 118 degrees, keeps the orbit retrograde. The upper edge reaches 160 degrees, close to a perfectly head-on, upside-down orbit.

GJ 3090 b is a sub-Neptune of about 2.18 Earth radii and 4.52 Earth masses that completes an orbit every 2.9 days. According to the paper, it is the smallest planet around an M dwarf with a three-dimensional obliquity measurement, which makes the detection a technical result as well as an odd one.

NIRPS on the La Silla 3.6-metre telescope

The observations came from NIRPS, a near-infrared spectrograph on ESO’s 3.6-metre telescope, which ESO’s telescope page lists beside the HARPS planet hunter at La Silla Observatory in Chile. Live Science reports that the team analysed about 23 hours of observations covering 10 planetary transits. The method is the Rossiter-McLaughlin effect: as the planet crosses the star, it blocks light from the side rotating toward the observer, then the side rotating away, and the star’s spectrum shifts in a pattern that reveals which way the planet is moving relative to the spin.

ESO’s NIRPS page explains why the instrument suits the job. It was built by an international team led by the Université de Montréal’s Institute for Research on Exoplanets, saw first light in 2022, and covers wavelengths from 0.95 to 1.8 microns, tuned to M-type red dwarfs with 10 to 50 percent of the Sun’s mass. Those stars are cool and dim in visible light, so a spectrograph working in the infrared can follow the small shifts in their spectra that a planet the size of GJ 3090 b produces. NIRPS works alongside HARPS, the older spectrograph on the same telescope, so the two cover complementary wavelengths.

No giant neighbor to blame

In the five other known systems with strongly misaligned multi-planet architectures, the paper notes, a massive outside perturber or a companion star explains the tilt. Carteret and colleagues searched for exactly that here and came up empty. Carteret said the team looked for the kind of massive companion that could have forced the planet into such an extreme orbit, and concluded that the system’s unusual architecture may require different thinking. The Instituto de Astrofísica de Canarias, which contributed to NIRPS and had five researchers among the authors, quotes him as saying the team was greatly surprised that the planet is not only highly misaligned but also retrograde.

The system holds two, or possibly three, planets smaller than Neptune, according to Live Science, so the data show no hot Jupiter, no eccentric giant on a wide orbit or distant brown dwarf to point at. The paper also reports no wide binary stellar companion, the other usual suspect in misaligned systems.

A proposal, not an answer

The authors do offer a hypothesis. They suggest that the star accreted a second, misaligned disc of material late, a disc spinning in a different plane from the original, so that planets formed in it and migrated inward while keeping their tilt. Vincent Bourrier, a co-author, is quoted by the IAC saying the star could have accreted a misaligned, retrograde secondary disc.

That idea has not been tested against other systems, and it is the only candidate on the table. The paper runs as a short letter, page L15 of volume 713. The error bar of plus 24 and minus 18 degrees is the most obvious thing additional transits could narrow, and a tighter angle would bear directly on how a second disc must have been oriented if the hypothesis holds. Until a second retrograde planet turns up around a quiet red dwarf, or the team finds an unseen companion after all, the 136-degree figure stands as an orbit with no demonstrated cause.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

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