A Planet Rolling Against the Stellar Current express
The carousel spins all the horses in one direction, but one stubbornly gallops against the flow. That's exactly how the planet TOI-1710 b behaves around its star. Using the HARPS-N and GIANO-B instruments, astronomers measured its tilt at 149 degrees. That's nearly a 180-degree flip, meaning the planet orbits "backward" relative to the star's rotation.
Where did this flip come from? Most likely, the system's past resembled a chaotic intersection without traffic lights. Gravitational kicks from other planets (so-called planet-planet scattering) could have knocked TOI-1710 b off its normal orbit. Then the Kozai-Lidov mechanism kicked in: if a massive body is nearby, the planet's orbit starts to wobble like a pendulum, changing its tilt and eccentricity. In the end, tidal forces from the star rounded the orbit, but the tilt remained.
This discovery is important for understanding the "childhood" of planetary systems. Previously, it was thought that in the region where TOI-1710 b resides (the so-called savanna — the gap between rocky and gas planets), such violent migrations are rare. Now it's clear: even here, traces of real planetary catastrophes can hide.
Original abstract
as written by the authors arXiv:2604.19179
The obliquity between a planet's orbital axis and its host star's spin axis provides crucial insights into planetary formation and migration. Planets with scaled semi-major axes ($$a/R_\star$$) large enough to be unaffected by tidal alterations ("tidally detached"), offer a unique opportunity to study the original obliquity in which the system formed. We therefore observed TOI-1710 b ($$a/R_\star \approx 36$$) in-transit using HARPS-N + GIANO-B, collecting high-precision radial velocities to measure the Rossiter-McLaughlin (RM) effect. Spectral analysis of the H$$\alpha$$ and HeI triple lines was also pursued to evaluate atmospheric photoevaporation. Using our knowledge of the star rotation period ($$21.5 \pm 0.2$$ d), we estimated a true obliquity of $$\psi = 149 ^{+11}_{-10}$$ deg, which indicates a retrograde motion and places TOI-1710 b among the most misaligned systems -- and the only one known orbiting a cool star in retrograde motion. The strong misalignment favours a high-eccentricity migration (HEM) origin for this low-density super-Neptune planet in the savanna region, challenging previous findings that claimed a minor role of HEM in this period-radius(-density) domain. Moreover, the strong misalignment and lack of a detected close stellar companion suggests a purely planetary post-migration misalignment, likely due to planet-planet scattering followed by planet-planet Kozai-Lidov oscillations and tidal circularisation.
The star TOI-1710 completes a full rotation on its axis in 21.5 days — about three Earth weeks. In that time, its planet only manages to cover a small part of its 24-day orbit.
In Frederick Pohl's novel "Gateway," the protagonists find an alien ship on a retrograde orbit, and it's the first sign that something is wrong with the system.