[Paper Review] The Origins of Planets for ArieL (OPAL) Key Science Project: the end-to-end planet formation campaign for the ESA space mission Ariel

Danae Polychroni, D. Turrini+8 more|arXiv (Cornell University)|Jan 23, 2026

Stellar, planetary, and galactic studies0 citations

TL;DR

OPAL builds an end-to-end, end-to-end campaign linking host stars, disks, planets, and atmospheres to produce realistic synthetic spectra for Ariel, sampling multiple systems and initial conditions with high-performance computing.

ABSTRACT

The growing body of atmospheric observations of exoplanets from space and ground-based facilities showcases how the great diversity of the planetary population is not limited to their physical properties but extends to their compositions. The ESA space mission Ariel will observe and characterise hundreds of exoplanetary atmospheres to explore and understand the roots of this compositional diversity. To lay the foundations for the Ariel mission, the OPAL Key Science Project is tasked with creating an unprecedented library of realistic synthetic atmospheres spanning tens of elements and hundreds of molecules on which the Ariel consortium will test and validate its codes and pipelines ahead of launch. In this work we describe the aims and the pipeline of codes of the OPAL project, as well as the process through which we trace the genetic link connecting planets to their native protoplanetary disks and host stars. We present the early results of this complex and unprecedented endeavour and discuss how they highlight the great diversity of outcomes that emerge from the large degeneracy in the parameter space of possible initial conditions to the planet formation process. This, in turn, illustrates the growing importance of interdisciplinary modelling studies supported by high-performance computing methods and infrastructures to properly investigate this class of high-dimensionality problems.

Motivation & Objective

• Define an end-to-end pipeline to link stellar, disc, planet formation, and atmospheric chemistry information to synthetic spectra relevant for the Ariel mission.
• Construct a library of realistic synthetic atmospheres spanning tens of elements and hundreds of molecules for Ariel data analysis testing and validation.
• Explore how initial disc and stellar conditions propagate to diverse planetary outcomes, highlighting parameter-space degeneracy.
• Leverage high-performance computing to run large suites of coupled simulations across multiple codes and scenarios.
• Characterize representative planetary systems (WASP-69, HD 209458, HIP 67522) from Ariel target lists to bound the parameter space.

Proposed method

• Integrate the Ar χ es suite codes (JADE, GroMiT, Mercury-Ar χ es, Hephaestus) with GGChem and the Exoclimes/ESP atmospheric tools to create a full astrochemical and dynamical chain from discs to planets to atmospheres.
• Use GGChem to initialize disc composition from host-star abundances and disc inheritance/reset scenarios for volatile inventories.
• Run JADE to simulate time evolution of protoplanetary discs with varying ionization, dust sizes, disc masses, and radii under an operator-splitting scheme.
• Apply GroMiT for population-synthesis of planet formation via pebble accretion to identify plausible formation/migration histories, then inform Mercury-Ar χ es N-body simulations of gas/planetesimal accretion and migration.
• Post-process with Hephaestus to compute bulk planetary compositions from disc chemistry and accretion histories.
• Derive atmospheric chemistries with FastChem and generate synthetic spectra with the Exoclimes suite.
• Execute simulations on the Leonardo Pre-Exascale Infrastructure with tailored compiler options to optimize parallel performance.
• Explore a parameter space including four JADE disc-chemistry scenarios, two disc masses, two radii, multiple grain sizes, and multiple core-formation times, plus 10^5 GroMiT Monte Carlo runs per system and ~240 n-body Mercury-Ar χ es simulations, yielding up to 11,520 chemical compositions per planet.

Experimental results

Research questions

• RQ1 How do different disc chemical inheritance versus reset scenarios affect the final planetary bulk and atmospheric compositions?
• RQ2 What range of planetary formation histories (growth, migration, and gas accretion) produce the observed diversity while remaining consistent with the three target systems?
• RQ3 To what extent does initial disc mass, characteristic radius, dust size, and ionization influence the resulting atmospheric spectra for Ariel?
• RQ4 What is the impact of including or excluding planetesimal accretion on the final planetary composition and its observable spectra?

Key findings

• OPAL demonstrates a broad diversity of planetary outcomes resulting from degeneracies in the initial conditions and evolutionary histories.
• The pipeline connects host-star chemistry, disc evolution, planet formation, and atmospheric composition to produce JWST/ Ariel-like synthetic spectra for validation and tool testing.
• The campaign leverages high-performance computing to enable large-scale, end-to-end simulations across multiple coupled codes and thousands of realizations.
• Three representative systems (WASP-69, HD 209458, HIP 67522) are used to sample equilibrium irradiation conditions representative of Ariel-observed planets.
• Large-scale sampling yields up to 11,520 possible planetary compositions per simulated planet when combining disc chemistry with formation histories.