The Neptunian ridge planet WASP-156 b does not have a polar orbit

The population of short-period exo-Neptunes is thought to be shaped by an interplay between different dynamical mechanisms, such as orbital migration and tidal effects, and photoevaporation. We can gain insight into these processes by studying observables such as the stellar obliquity. Here, we study the Rossiter–McLaughlin (RM) effect and measure the projected obliquity, λ⁠, of the Neptunian ridge planet WASP-156 b. We analyse new ESPRESSO and MAROON-X spectroscopic transit observations, and new Next-Generation Transit Survey (NGTS) photometry simultaneous to the ESPRESSO data. Our analyses show an aligned orbit (⁠λ = -8 ± 16°⁠, based on the ESPRESSO observations), in contrast to a previous report of a highly misaligned orbit. We also find the star’s projected rotational velocity to be ν sin i_*  ≤ 2 km s^-1 from spectral line modelling and ν sin i_* = 0.40 ± 0.11 km s^-1
from the RM modelling. This is lower than the previously reported value of _˜ 4 km s^-1⁠, which could partly explain the previously derived polar orbit. We also update the system’s orbital parameters and rule out Jupiter-mass companions within 5 au using long-term radial velocity data. The planet’s aligned and circular orbit (⁠e < 0.16 at 3σ⁠), and lack of nearby massive companions, are consistent with in situ formation or early disc-driven migration. Our findings move WASP-156 b from a tentative cluster of close-in Neptunes in polar orbits to the group of aligned Neptunes.