A planet orbiting backwards around a red dwarf has no giant neighbor to blame

A planet 72 light-years from Earth is circling its star the wrong way. GJ 3090 b, a world a little over twice the width of Earth, travels around its red dwarf against the direction the star itself spins, tilted about 136 degrees from where planets are supposed to sit. It is the first planet ever found moving backwards around a red dwarf, the most common kind of star in the galaxy.

Backwards planets are rare, but they are not new. In every earlier case of an orbit this badly out of line in a system with several planets, astronomers could point to a culprit: a companion star or a giant outer planet whose gravity, over millions of years, dragged a neighbor off course. Around GJ 3090 there is no such bully. That absence is the real news, because it suggests the planet may have been born in a disk of gas and dust that was already spinning the wrong way.

“To our great surprise, not only is the planet GJ 3090 b on a highly misaligned orbit, but it also orbits retrogradely,” said Yann Carteret, a doctoral student at the University of Geneva and first author of the study in Astronomy & Astrophysics.

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The planet is a sub-Neptune, about 2.2 times Earth’s radius and 4.5 times its mass, and it completes an orbit in roughly 2.9 days. Its star, an M-type red dwarf in the southern constellation Phoenix, is smaller and cooler than the Sun. The system also holds at least one more planet, a confirmed sub-Neptune on a 16-day orbit, plus a candidate at 13 days.

How they caught a planet going the wrong way

Nobody can photograph the orbit directly. The team read it from starlight instead, using a trick known as the Rossiter-McLaughlin effect. A spinning star has one side turning toward us and one side turning away, so light from the first half is slightly shifted toward blue and light from the second toward red. When a planet crosses the star’s face, it blocks a patch of one half and then the other. The order in which it covers them reveals the direction of travel.

For a planet going the normal way, the approaching half is covered first. GJ 3090 b covers them in the reverse order.

The signal is tiny. A transit here lasts about an hour and a quarter, the planet is small, and the star turns slowly. The team used NIRPS, a near-infrared spectrograph on the European Southern Observatory’s 3.6-meter telescope at La Silla in Chile, built by a consortium led by the University of Geneva and the Université de Montréal. Red dwarfs shine far more brightly in the infrared than in visible light, which is why an infrared instrument could do what optical ones struggle with.

The researchers observed six transits with NIRPS, four of them simultaneously with the older HARPS spectrograph, collecting 193 spectra over about 23 hours. Rather than looking for the classic wobble in the star’s overall speed, which was too faint to detect, they used a method called RM Revolutions that analyzes the light from the exact strip of star hidden behind the planet at each moment. GJ 3090 b is now the smallest planet around a red dwarf with a full three-dimensional tilt measurement.

Where 136 degrees sits

In our Solar System, the planets orbit within about 7 degrees of the Sun’s equator, and every one of them goes the same way the Sun turns. An angle of 90 degrees would mean a polar orbit, passing over the star’s poles. Anything above 90 is retrograde. At 136 degrees, GJ 3090 b is closer to fully upside down than to sideways.

That places it among the most tilted planets for which astronomers have a three-dimensional measurement, alongside two gas giants, KELT-19 b and TOI-1710 b. It is also only the sixth confirmed system with several planets that contains one tilted by more than 70 degrees. In the other five, the team notes, there is always either a wide binary companion star or a massive outer planet capable of doing the damage.

Red dwarfs make the result stranger still. Their deep, churning outer layers are thought to exert strong tides on close-in planets, gradually pulling tilted orbits back into line. GJ 3090 b appears to have escaped that correction, most likely because it is small and sits just far enough out, at about 13 times the star’s radius, for the tidal pull to stay weak.

The missing culprit and a second-hand disk

The team went looking for a hidden perturber. Combining years of radial-velocity data from HARPS and NIRPS, high-resolution speckle imaging and measurements from the Gaia space telescope, they ruled out any companion star within about 3 astronomical units, and any planet heavier than Jupiter within 15 astronomical units, assuming it orbits in the same plane as GJ 3090 b. Anything heavier than 13 Jupiter masses, the line above which objects count as brown dwarfs, is excluded out to 100 astronomical units under the same assumption.

“The absence of a massive companion to explain this unusual orbit will lead us to explore other hypotheses,” said co-author Vincent Bourrier of the University of Geneva.

Their preferred explanation is a second-generation disk. After the star’s original disk had cleared, the young red dwarf may have swept up fresh gas and dust from interstellar space. That material would arrive with a random orientation, and in this case it arrived spinning against the star. The planets would then have formed inside this tilted disk and migrated inward together. The authors link the idea to so-called Peter Pan disks, red dwarf disks that keep feeding their stars for tens of millions of years, far longer than models expect.

Their calculation adds a constraint. For the star not to realign with the new disk, it could have swallowed less than about 1,500 Earth masses of material, which at standard proportions means roughly 15 Earth masses of dust. The planets alone account for a large share of that, so most of the dust in the disk must have ended up in planets rather than falling onto the star.

What it doesn’t settle

The 136-degree figure carries wide error bars, from roughly 118 to 160 degrees. The team excludes a polar orbit at 2.5 sigma, a meaningful result but well short of the 5-sigma standard physicists reserve for discoveries. The full three-dimensional tilt also depends on estimates of the star’s rotation period, radius and inclination, not only on the transit data.

The final fit rests on five nights from a single instrument. One NIRPS transit with poor signal was dropped, and the HARPS data were too weak to constrain the angle, so no second instrument independently confirms the direction.

The companion search has a loophole the authors state plainly. Their limits assume any hidden giant orbits in the same plane as GJ 3090 b. A massive body on a steeply inclined orbit is not fully excluded. The second-disk scenario is an inference from what is missing, not something anyone has observed.

Common questions about backwards planets

What is a retrograde planet?

A retrograde planet orbits its star in the opposite direction to the star’s own rotation. Planets form from the same spinning disk that feeds their star, so they normally share its direction. A retrograde orbit means something tilted the planet or its birth disk by more than 90 degrees.

How do astronomers know which way a planet orbits?

They watch the planet cross in front of its star and measure how the color of the starlight shifts. Because one side of a rotating star approaches us and the other recedes, the order in which the planet blocks each side shows its direction of travel. This is the Rossiter-McLaughlin effect.

Does anything orbit backwards in our Solar System?

No planet orbits the Sun backwards, and all eight stay close to the Sun’s equator. Some moons do: Neptune’s largest moon, Triton, orbits against the planet’s rotation and is thought to be a captured object. Venus spins backwards on its axis, but its orbit runs the normal way.

Could GJ 3090 b support life?

The study says nothing about life. The planet circles its star in under three days, far closer than Mercury is to the Sun. At about 4.5 Earth masses and 2.2 Earth radii it is a sub-Neptune, a class usually thought to carry a thick gas envelope rather than an exposed rocky surface.

The planet was first identified by NASA’s TESS space telescope and confirmed in 2022. The NIRPS transits behind the new measurement were recorded in 2024, and the paper was accepted by Astronomy & Astrophysics in August 2026 and published in September. The authors’ scenario makes a testable prediction: if every planet in the system formed in the same tilted disk, the siblings of GJ 3090 b should share its backwards tilt. The next step is measuring their orbits too, which will be hard because the 16-day planet never crosses its star from our point of view.

Reference: Carteret et al., “Upside down: GJ 3090 b the first retrograde exoplanet around an M dwarf detected with NIRPS,” Astronomy & Astrophysics, 2026. DOI: 10.1051/0004-6361/202661989

Tags: astronomy, ESO, GJ 3090 b, La Silla Observatory, NIRPS, red dwarf

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