Mapping the exo-Neptunian landscape

A ridge between the desert and savanna

¹ Centro de Astrobiología, CSIC-INTA, Camino Bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
² Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
³ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
⁴ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
⁵ CFisUC, Departamento de Física, Universidade de Coimbra, 3004-516 Coimbra, Portugal
⁶ IMCCE, UMR8028 CNRS, Observatoire de Paris, PSL Université, 77 Av. Denfert-Rochereau, 75014 Paris, France

Received: 1 June 2024
Accepted: 4 August 2024

Abstract

Context. Atmospheric and dynamical processes are thought to play a major role in shaping the distribution of close-in exoplanets. A striking feature of such distribution is the Neptunian desert, a dearth of Neptunes on the shortest-period orbits.

Aims. We aimed to define the boundaries of the Neptunian desert and study its transition into the savanna, a moderately populated region at larger orbital distances. Our goal was to acquire new insight into the processes that carved out the Neptunian landscape, and to provide the exoplanet community with a framework for conducting studies on planet formation and evolution.

Methods. We built a sample of planets and candidates based on the Kepler DR25 catalogue and weighed it according to the transit and detection probabilities. We then used the corrected distribution to study occurrences across the period and period-radius spaces. Results. We delimited the Neptunian desert as the close-in region of the period-radius space with no planets at a 3σ level, and provide the community with simple, ready-to-use approximate boundaries. We identified an overdensity of planets separating the Neptunian desert from the savanna (3.2 days ⪅ P_orb ⪅ 5.7 days) that stands out at a 4.7σ level above the desert and at a 3.5σ level above the savanna, which we propose to call the Neptunian ridge. The period range of the ridge matches that of the well-known hot Jupiter pileup (≃3–5 days), which suggests that similar evolutionary processes might act on both populations. We find that the occurrence fraction between the pileup and warm Jupiters (ƒ_pileup/warm = 5.3 ± 1.1) is about twice that between the Neptunian ridge and savanna (ƒ_{ridge/savanna} = 2.7 ± 0.5). This indicates either that the processes that drive or maintain planets in the overdensity are more efficient for Jupiters, or that the processes that drive or maintain planets in the warm region are more efficient for Neptunes.

Conclusions. Our revised landscape supports a previous hypothesis that a fraction of Neptunes were brought to the edge of the desert (i.e. the newly identified ridge) through high-eccentricity tidal migration (HEM) late in their life, surviving the evaporation that eroded Neptunes having arrived earlier in the desert. The ridge thus appears as a true physical feature illustrating the interplay between photoevaporation and HEM, providing further evidence of their role in shaping the distribution of close-in Neptunes.