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

TL;DR

OPAL tackles the degeneracy in inferring planet formation histories by building an end-to-end pipeline that links host-star chemistry to protoplanetary discs, planet formation, and planetary atmospheres, using the Arχies suite and HPC. The pipeline combines GGChem, JADE, GroMiT, Mercury-Arχes, Hephaestus, FastChem, and Vulcan to produce JWST/Ariel-like synthetic spectra for a representative sample, providing a controlled testbed for Ariel data reduction and interpretation. Early results reveal enormous diversity in bulk and atmospheric compositions, showing that simple tracers like $\mathrm{C/O}$ alone cannot uniquely identify formation pathways, whereas combinations including $\mathrm{C/N}$ and other elemental ratios help break degeneracies. The OPAL program demonstrates the practicality of integrated, high-dimensional modelling for exoplanet science and establishes a ready-to-use spectral library to maximise Ariel's scientific return.

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.

Figures (10)

• Figure 1: Scatter plots of the currently known exoplanets from the NASA Exoplanet Archive. In the top panel we plot the planet orbital period around the parent stars versus the planet mass, while in the bottom one we plot the planet mass versus the planet density.
• Figure 2: Infographic of the OPAL campaign simulations in the ambit of the Ariel mission Dry Run of 2025. The green box contains the codes presented in this paper that make up the Ar$\chi$es and the Exoclimes suites. We use as inputs provided homogenised information for the stellar and planetary samples and we produce the elemental bulk composition and the molecular atmospheric composition of these planets. These are used to make highly detailed and realistic synthetic atmospheric spectra.
• Figure 3: Flowchart showing the initialisation of Jade (disc Setup and the operations performed in a single iteration of the code, following an operator-splitting scheme. Reproduced with permission from Pacetti2025.
• Figure 4: Evolutionary tracks of the elemental ratios over 3 Myr across all considered scenarios. Ratios are shown in pairs across four key compositional regions: within the H$_2$O snowline (region 1, top-left), between the H$_2$O and CO$_2$ snowlines (region 2, top-right), between the CO$_2$ and CH$_4$ snowlines (region 3, bottom-left), and beyond the CH$_4$ snowline (region 4, bottom-right). Values represent gas-phase elemental ratios, calculated from total elemental abundances averaged over the radial extent of each region and weighted by the surface density of the gas. The colour bars indicate the four chemical scenarios, with time progressing from lighter to darker shades. The three different markers represent the selected grain sizes and are placed along each track every 2$\times$10$^5$ yr. The dark marker on each track denotes 1 Myr. Dashed grey lines highlight regions of the parameter space where pairwise comparisons of the elemental ratios provide constraints on specific scenarios or subsets of scenarios. Reproduced with permission from Pacetti2025.
• Figure 5: Illustrative example of the results of the population synthesis version of the GroMiT code. We plot the final semimajor axis of our simulated planets versus their final mass. The colour bar represents the time, from the beginning of the simulation, that the planetary embryo was inserted in the disc as its original semimajor axis position. The simulations are run for 5 Myr for a total of 10$^5$ synthetic planets.