In the solar system, all planets orbit in one direction: they follow the direction of rotation of the sun. The reason for this lies in their creation. Planets form in the protoplanetary disk of their star and this rotates with the star. But there are also exceptions: Some exoplanets orbit their stars in a heavily tilted orbit, and a handful of planets even orbit completely the wrong way around – they are retrograde. The cause of such misalignments is usually a collision with a massive planet or the gravitational influence of a second star.
Near sub-Neptune in sight
But now astronomers have identified a retrograde exoplanet that defies conventional explanations. It is the innermost planet of the red dwarf star GJ 3090, which is around 73 light-years away. The planet GJ 3090 b, identified by the TESS space telescope in 2022, is around 2.2 times as large and 4.5 times as heavy as Earth. Sub-Neptune takes around 2.9 days to orbit its star. Further out in this system there are two more sub-Neptunes with orbital periods of 13 and 16 days – so far, so known.
However, what the orbit of GJ 3090 b looks like was previously unknown. Yann Carteret from the University of Geneva and his team therefore evaluated spectral data recorded by telescopes at the European Southern Observatory in Chile from the GJ 3090 planetary system. In the resulting light spectra, tiny shifts in the spectral lines provided information about the direction in which the innermost planet of this system orbits its star.
Retrograde, but no cause in sight
The analyzes showed: Planet GJ 3090 b does not orbit its star in a “normal” orientation, but at an angle of 136 degrees – this corresponds to a retrograde orbit. “GJ 3090 b is therefore one of the planets that is most strongly deviated from its normal orbit,” report the astronomers. The innermost planet of this system is orbiting the wrong way around – it is the first evidence of such a retrograde planet around a red dwarf star. GJ 2090 b is also only the sixth exoplanet with an orbital deflection of more than 70 degrees, as the team explains.
Comparison of the orbital rotation axis of known exoplanets with GJ 3090 b (star). Pentagons indicate multi-planet systems, the colors show the temperature of the central star. — © Carteret et al./ Astronomy and Astrophysics, CC by 4.0
But how did this exoplanet end up in its reverse orbit? “We looked for a massive companion in the system that could have forced the planet into such an extreme orbit – but we couldn’t find one,” reports Carteret. No trace of a potential collision partner or other disruptive influence was seen in the close vicinity of the star GJ 3090 and its planets. “GJ 3090 b is the first retrograde planet in a multiplanet system in which there is neither a massive outer planet nor a second star,” said the astronomers.
Was there a second disk of dust around the star?
This raises the question of what else could have put the planet on its retrograde orbit. “The architecture of the GJ-3090 system suggests that the protoplanetary disk is already misaligned,” explain the astronomers. “We therefore need a mechanism that either tilted the protoplanetary disk or created a second, misaligned disk.” According to theoretical models, such a secondary disk of gas and dust could arise if a triple star system collapses or if material is subsequently attracted to a young star and then collects around it in a different orientation.
“The idea that a planetary system could also arise from a secondary, differently oriented disk is particularly exciting,” says co-author Vincent Bourrier from the University of Geneva. “This suggests that the environment of a young star plays a larger role in the structure of its planetary system than previously thought.” According to astronomers, such a secondary dust disk is the most likely scenario in the case of GJ 3090 b.
But this is still just an assumption. The astronomers hope that further observations will provide more detailed information about the orbits of GJ 3090 b’s two outer neighbors and also about the star’s past environment. This could help clarify whether this scenario of a secondary protoplanetary disk in GJ 3090 applies. It would also be helpful if other planetary systems with similar planets orbiting the wrong way round could be discovered.
Source: Yann Carteret (University of Geneva) et al., Astronomy and Astrophysics, 2026; doi: 10.1051/0004-6361/202661989