Astronomers Find a Planet Orbiting Its Star Backward, With No Culprit in Sight

Astronomers have identified an exoplanet orbiting backward around its star, the first such "retrograde" world ever found circling a red dwarf and one that formation theory cannot easily explain. The planet, designated GJ 3090 b, travels around its host star in the direction opposite to the star's own rotation, according to a paper published this month in Astronomy & Astrophysics by a team led by the University of Geneva.

Most planets orbit in roughly the same plane as their star's equator and in the same direction as its spin, a leftover signature of the flat, rotating disk of gas and dust from which both star and planets condensed. GJ 3090 b does neither. Measurements published by the Geneva-led team put its three-dimensional obliquity, the angle between the star's spin axis and the planet's orbital plane, at 136 degrees, plus 24 or minus 18 degrees. Any angle above 90 degrees means the planet is moving backward relative to its star's rotation. For comparison, the eight planets of the solar system are tilted from the sun's equator by only a few degrees.

What the numbers show

GJ 3090 b is a sub-Neptune roughly 2.18 times the radius of Earth and 4.52 times its mass, completing one orbit of its star in about 2.9 days, a period first established in 2022. It is the innermost of at least three planets known in the system: a second candidate sub-Neptune orbits every 13 days and a confirmed, non-transiting sub-Neptune every 16 days. The host star, GJ 3090, is a cool M-type red dwarf roughly 72 light-years from Earth, considerably smaller and dimmer than the sun.

The team, led by doctoral student Yann Carteret and senior researcher Vincent Bourrier of UNIGE's Department of Astronomy, measured the tilt using the Near-InfraRed Planet Searcher (NIRPS), a Swiss-built spectrograph installed on the European Southern Observatory's 3.6-metre telescope at La Silla, Chile, together with the older HARPS instrument on the same telescope. Between the two instruments, the observations spanned wavelengths from visible light to the near-infrared, which was essential because red dwarfs emit most of their light at longer wavelengths that are too faint for older instruments to analyze with the necessary precision.

The researchers watched six transits of the planet in front of its star and used a technique called the Rossiter-McLaughlin effect: as a planet crosses the stellar disk, it blocks light first from the approaching, blue-shifted half of the star and then from the receding, red-shifted half, producing a brief, characteristic distortion in the star's measured velocity. The shape of that distortion reveals the sky-projected angle between the star's spin and the planet's path. Applying a refinement of the method known as "RM revolutions" to five years of accumulated high-resolution spectra, the team derived the full three-dimensional geometry of the orbit, not just its projection on the sky.

How such a planet gets discovered

The GJ 3090 system was first flagged by NASA's Transiting Exoplanet Survey Satellite (TESS), which searches for the periodic dimming that occurs when a planet passes in front of its star as seen from Earth. TESS discoveries around red dwarfs have proliferated since the satellite launched in 2018, in part because small, cool stars make the periodic dimming caused by a transiting planet comparatively easy to detect. Once a candidate is confirmed, ground-based spectrographs such as NIRPS and HARPS are needed to measure the finer details of an orbit, including its tilt.

Retrograde orbits are not unheard of. A handful of "hot Jupiters," giant planets orbiting close to sun-like stars, have been found circling backward, and in nearly every one of those cases astronomers could point to a nearby massive planet or a binary companion star whose gravity tipped the orbit off-kilter over time, through a slow gravitational process called Kozai-Lidov oscillation. What sets GJ 3090 b apart is that no such culprit exists. The team searched for wide stellar companions and massive outer planets using years of radial-velocity data and speckle-imaging observations and found none.

That absence rules out the most common explanation and pushes the team toward a different one: that the disk of gas and dust from which the planets formed was itself tilted from the outset, or that a second disk of material fell onto the young star later, misaligned with its spin. In the paper, the authors write that the data "disfavors scenarios involving gravitational perturbations from a massive body and instead points toward a primordial misalignment of the protoplanetary disk," and they propose that late accretion of a secondary, misaligned disk, followed by disk-driven migration of the planets inward, is the most likely explanation for the system's current architecture.

Why it matters for planet formation

The finding matters because red dwarfs, also called M dwarfs, are by far the most common stars in the galaxy, and their planets are prime targets in the search for potentially habitable worlds. Yet almost nothing was known, until now, about how often planets around red dwarfs form on tilted or backward orbits, because the faintness of these stars in visible light has made such measurements technically difficult. GJ 3090 b is both the first planet found on a retrograde orbit around an M dwarf and the smallest planet of any kind to have its three-dimensional orbital tilt measured around a cool star, a benchmark the authors say demonstrates that the RM revolutions technique can now probe small, close-in planets that were previously out of reach.

Theorists also point out that M dwarfs, because they have deep convective outer layers, are expected to be especially efficient at damping out orbital misalignments over time through tidal forces, gradually dragging close-in planets back into alignment with the star's spin. Finding a strongly misaligned system intact around exactly this kind of star suggests either that the tilt was established recently, on an astronomical timescale, or that the tidal realignment process is slower or less effective than current models assume.

"To our great surprise, not only is planet GJ 3090 b on a highly misaligned orbit but it also rotates retrograde, opposite to its star's rotation," said Yann Carteret, the study's lead author and a doctoral student at the University of Geneva.

Reaction and next steps

Co-author Vincent Bourrier, describing the puzzle the absence of a perturbing companion presents, said the missing culprit "will lead us to explore other hypotheses," pointing specifically to the possibility that "the star could have accreted a misaligned, retrograde secondary disk in which the system's planets then formed." Co-author Andrew Winter of Queen Mary University of London noted that the system is unusual precisely because the planet is not simply tilted out of the star's equatorial plane but is orbiting in the opposite direction entirely, a distinction that rules out several simpler formation stories at once.

The work was carried out within the National Centre of Competence in Research (NCCR) PlanetS, a Swiss research program dedicated to the study of planet formation, and draws on the wider TESS-Keck and NIRPS guaranteed-time observation collaborations that have been systematically following up small planets around nearby red dwarfs. Outside coverage of the result, including analyses published by Universe Today and Space.com, has focused on the same open question the paper itself raises: what, precisely, tipped this disk over in the first place.

The authors say the next step is to search for additional systems with similarly measured obliquities around other M dwarfs, in order to determine whether GJ 3090 b is a rare outlier or the first confirmed example of a more common, previously invisible population of misaligned planets around small stars. Because the RM revolutions technique required years of accumulated high-resolution spectra to resolve an orbit this small, expanding the sample will depend on continued observing time on instruments such as NIRPS and its counterparts at other observatories, as well as a growing catalog of transiting candidates from TESS and future surveys. Confirming or ruling out the proposed secondary-disk scenario will likely also require more detailed modeling of how young red dwarfs accrete late-arriving material and how quickly tidal forces can be expected to re-align a misaligned system over its lifetime.