ATREIDES: Mapping Exo-Neptunian Migration Paths

• ATREIDES is an observational and modeling initiative that maps close-in Neptune-sized exoplanets, highlighting features like the Neptunian Desert, Ridge, and Savanna.
• The project employs high-precision spectroscopy and photometry, including RM effect measurements, to compute 3D spin–orbit angles that reveal planetary migration histories.
• Advanced modeling of internal structure and atmospheric parameters in ATREIDES refines theories on disk-driven versus high-eccentricity migration and the impact of atmospheric mass loss.

ATREIDES refers to a comprehensive observational and modeling initiative focused on the paper of close-in Neptune-sized exoplanets and the characterization of their formation and evolutionary histories. The project systematically maps the distribution of Neptune-size planets across the so-called Neptunian “Desert,” “Ridge,” and “Savanna” regions, and employs a combination of state-of-the-art observational techniques—including high-precision spectroscopy and photometry—with advanced internal structure and atmospheric modeling. It aims to disentangle the relative contributions of early disk-driven migration (DDM), high-eccentricity migration (HEM), and atmospheric mass loss in sculpting the observed exo-Neptunian population, thereby refining theoretical models of planet formation and evolution (Bourrier et al., 19 Sep 2025).

1. Exo-Neptunian Landscape: Desert, Ridge, and Savanna

A fundamental aspect of ATREIDES is its focus on the empirical features defining close-in Neptune-sized planet populations:

• Neptunian Desert: A pronounced scarcity of Neptune-size planets at very short orbital periods. This deficit is interpreted as the footprint of strong atmospheric erosion—in which intense stellar irradiation strips planetary envelopes, often leaving only rocky cores.
• Neptunian Ridge: An overdensity of Neptune-size objects at intermediate orbital periods, interpreted as marking a transition zone where evolutionary processes (including partial erosion and changes in internal structure) play a pivotal role.
• Neptunian Savanna: A mild deficit or lower occurrence of Neptune-size planets at longer periods. The organization of observable planets into these features is treated as a “map” encoding the interplay of high-energy irradiation, migration and dynamical histories.

The distribution of planets across these regions is diagnostic of the degree of atmospheric loss and the types of migrational pathways the planets have taken since formation.

2. Migration Pathways: Disk-Driven vs. High-Eccentricity

A central hypothesis explored within ATREIDES is that the present-day architecture and orbital configuration of exo-Neptunes encode their migration trajectories:

• Early Disk-Driven Migration (DDM): Occurs while the protoplanetary disk is present, causing smooth inward migration. Planets retain primordial spin–orbit alignment (i.e., low mutual inclination between stellar spin axis and planetary orbital plane).
• High-Eccentricity Migration (HEM): Takes place after disk dispersal. Induced by dynamical interactions (e.g., planet–planet scattering, Kozai–Lidov cycles), this process excites significant eccentricities and generates large misalignments.
• Population Consequences: Low-density Neptunes likely undergo DDM, experiencing full atmospheric stripping at short periods. Denser Neptunes are preferentially brought close-in via HEM, populating the Ridge and Desert.

By measuring present-day three-dimensional spin–orbit angles (Ψ), ATREIDES statistically attributes populations to these two migration channels, thus constraining the relative importance of each in shaping planetary demographics.

3. Observational Framework: Spectroscopy, Photometry, and RM Census

ATREIDES is executed as a large-scale, homogeneous survey of approximately 60 close-in Neptune-size planets, employing:

• High-Resolution Transmission Spectroscopy: Using the VLT/ESPRESSO spectrograph during planetary transits, spectroscopic time-series are leveraged to detect the Rossiter–McLaughlin (RM) effect—a velocity signature imprinted as a distortion in stellar absorption lines due to the occulting planet. This effect enables measurement of the projected spin–orbit angle (λ).
• Photometric Monitoring: Synchronous photometric observations from ground and space facilities are employed to refine transit ephemerides, measure orbital inclinations (iₚ), and characterize stellar activity.
• 3D Spin–Orbit Angle Calculation: By combining RM measurements (λ), stellar inclinations (iₛ), and photometric inclinations (iₚ), ATREIDES computes the true three-dimensional spin–orbit angle via:

$$\Psi = \cos^{-1}\left(\sin i_s \cos \lambda \sin i_p + \cos i_s \cos i_p\right)$$

This parameter lends direct constraints on migration processes.

• Statistical RM Census: The survey aims to deliver the statistical distribution of spin–orbit angles across the mapped Desert, Ridge, and Savanna regions, correlating with planet and system properties to identify migration and atmospheric erosion signatures.

4. Internal Structure, Atmospheric Modeling, and Reduction Pipelines

ATREIDES interprets the observed data through integration with robust theoretical models:

• Internal Structure Models: These models often employ a polytropic equation of state to characterize the planetary envelope and core, facilitating constraints on composition and thermal evolution.
• Atmospheric Models: Interpretation includes inputs from codes such as ATES (ATmospheric EScape), a one-dimensional, open-source photoionization hydrodynamics solver. ATES computes steady-state temperature, density, velocity, and ionization profiles for strongly irradiated, primordial (H/He) atmospheres, producing mass loss rate estimates via hydrodynamic solutions to the Euler equations (Caldiroli et al., 2021).
• Data Reduction Pipelines: The project implements advanced reduction pipelines to efficiently process raw spectroscopic and photometric data, correcting for telluric and instrumental effects, and isolating planetary signals from stellar activity.
• Comprehensive Modeling: These computational tools collectively allow ATREIDES to map observed system architectures to evolutionary scenarios and to distinguish the relative roles of irradiation, internal structure, and dynamics.

5. Case Study: The TOI-421 Multi-Planet System

The first ATREIDES target is the TOI-421 system, containing two transiting planets—TOI-421 b (sub-Neptune) and TOI-421 c (warm Neptune):

• Measurement Outcomes: Using VLT/ESPRESSO and complementary photometry, ATREIDES measured 3D spin–orbit angles of Ψ_b ≈ 57${+11}_{-15}$° and Ψ_c ≈ 44.9${+4.4}_{-4.1}$°. These angles, well removed from alignment, indicate moderate mutual inclination and measurable orbital eccentricities.
• Dynamical Interpretation: The observed moderate misalignments and eccentricities suggest that the planets may have formed and migrated inward via DDM, then experienced subsequent dynamical instabilities (e.g., planet–planet scattering associated with HEM), accounting for the present-day orbital architecture.
• A Plausible Implication Is: The combination of measures hints at a chaotic dynamical origin possibly involving both smooth disk migration and later instabilities.

6. Implications for Exoplanet Formation and Evolution

ATREIDES addresses key questions within exoplanet science:

• Constraining Evolutionary Processes: By establishing a systematically measured catalog of spin–orbit configurations and planet properties, ATREIDES enables robust empirical discrimination between competing theories of planet migration and envelope loss.
• Informing Theoretical Models: The combined observational and modeling framework allows for more accurate initial conditions and evolutionary histories in simulations coupling planetary internal structure and atmospheric escape.
• Guiding Future Observations: The project’s homogeneous methodology yields a benchmark dataset for selecting and interpreting targets in future spectroscopic and photometric campaigns.

7. Summary and Future Prospects

ATREIDES contributes a rigorous, statistically substantial empirical framework for unraveling the relative roles of migration mechanisms and atmospheric mass loss in the evolution of close-in Neptune-size exoplanets. Its combination of spectroscopic RM measurements, precise photometry, and advanced modeling across a large, systemically observed sample addresses the physical drivers behind the gaps, ridges, and transitions of the exo-Neptunian landscape. The commitment to open data and collaboration will continue to provide constraints critical for refining models of planetary system formation and evolution (Bourrier et al., 19 Sep 2025).