ATREIDES I: Embarking on a trek across the exo-Neptunian landscape with the TOI-421 system

Abstract

The distribution of close-in exoplanets is shaped by a complex interplay between atmospheric and dynamical processes. The Desert, Ridge, and Savanna (respectively a lack, overoccurence, and mild deficit of Neptunes with increasing periods) illustrate the sensitivity of these worlds to such processes, making them ideal targets to disentangle their roles. Determining how many Neptunes are brought close-in by early disk-driven migration (DDM; expected to maintain primordial spin-orbit alignment) or late high-eccentricity tidal migration (HEM; expected to generate large misalignments) is essential to understanding how much atmosphere they lost. In this paper, we propose a unified view of the exo-Neptunian landscape to guide its exploration and speculate that the Ridge is a hot spot for evolutionary processes. Low-density Neptunes would mainly undergo DDM, becoming fully eroded at shorter periods than the Ridge. This is in contrast to denser Neptunes, which would be brought to the Ridge and Desert by HEM. We embark on this exploration via the ATREIDES (Ancestry, Traits, and Relations of Exoplanets Inhabiting the Desert Edges and Savanna) collaboration, which relies on spectroscopic and photometric observations of ~60 close-in Neptunes, their reduction with robust pipelines, and their interpretation through internal structure, atmospheric, and evolutionary models. We carried out a systematic Rossiter-McLaughlin census with VLT/ESPRESSO to measure the distribution of 3D spin-orbit angles, correlate its shape with the system properties (orbit, density, evaporation), and thus relate the fraction of aligned-misaligned Neptunian systems to DDM, HEM, and atmospheric erosion. The first ATREIDES target, TOI-421 c, lies in the Savanna with a neighboring sub-Neptune TOI-421 b. We measured for the first time their 3D spin-orbit angles (ψb = 57‑15+11∘; ψc = 44.9‑4.1+4.4∘). Together with the eccentricity and possibly large mutual inclination of their orbits, this hints at a chaotic dynamical origin that could result from DDM followed by HEM. Our program will provide the community with a wealth of constraints for formation and evolution models, and we welcome collaborations that will contribute to pushing our understanding of the exo-Neptunian landscape forward.