ExoJAX

Version 2.0 (History)

ExoJAX provides an auto-differentiable high-resolution spectrum model for exoplanets/brown dwarfs using JAX. It enables fully Bayesian inference for high-dispersion data, fitting line-by-line spectral computations to observed spectra end-to-end – from molecular/atomic databases to real spectra – by integrating with Hamiltonian Monte Carlo - No U Turn Sampler (HMC-NUTS), Stochastic Variational Inference (SVI), Nested Sampling, and other inference techniques available in modern probabilistic programming frameworks such as NumPyro <https://github.com/pyro-ppl/numpyro>. So, the notable features of ExoJAX are summarized as

• HMC-NUTS, SVI, Nested Sampling, Gradient-based Optimizer available

• Easy to use the latest molecular/atomic data in ExoMol, HITEMP, HITRAN, and VALD3

• A transparent open-source project; anyone who wants to participate can join the development!

For a more geek-oriented explanation

ExoJAX is a spectral model based on the Differentiable Programming (DP) paradigm! ExoJAX aims to provide building blocks for retrieval code, much like Minecraft 🧱.

🟢 If you have an error and/or want to know the up-to-date info, visit ExoJAX wiki. Or use the discussions form on github or directly raise issues.

Warning

Recently, logging into HITRAN has become mandatory to access HITEMP files, causing issues with automated HITEMP file retrieval by RADIS. For more details, please refer to this resource. (Feb. 3, 2025)

Contents

Introduction:

• 🌱 Introduction
  • Installation and References
  • Why Differentiable Spectral Modeling?
  • Why ExoJAX Prefers On-Demand Opacity Calculation?

Tutorials:

• Getting Started
  • Getting Started with Simulating the Emission Spectrum
  • Getting Started with Opart; GPU memory-efficient Emission Spectrum
  • Computes Transmission Spectra
• Retrievals
  • HMC-NUTS
  • SVI, Nested Samping via NumPyro
• Applications
  • Tips for Real Analysis
  • Reflection Spectrum
  • Molecular/Atomic/Continuum Databases
• Multi Molecule/Segments Mdb and Opa Handler
  • Multi Molecule Handler
• Manual Settings of Opacity (not using opa)
  • Cross Section using Various Opacity Calculators
• Others
  • Scattering
  • Micro Chemical/Physical Processes
  • Macro Physical Processes
  • Others

User Guide:

• Molecular Database
  • Molecular and Atomic Databases (mdb / adb)
  • ExoMol, HITEMP, HITRAN
  • Non-air broadening
  • VALD3
• Opacity Calculation
  • Opacity Calculator Classes (opa)
  • Voigt-Hjerting Function and Voigt Profile
  • MODIT
  • PreMODIT
  • Benchmark
  • 32 and 64 bit in ExoJAX
• Scattering and Clouds
  • Rayleigh scattering
  • Particulates Database (pdb)
  • Mie Scattering
  • Atmospheric Microphysics (amp)
  • Terminal Velocity of Cloud Particles
  • Comparison the AM geometric opacity with PieMieScatt
  • Comparison of the ExoJAX AM cloud model with VIRGA
• Radiative Transfer
  • Radiative Transfer in ExoJAX
  • Intensity-based Emission with pure absorption
  • Flux-based Reflection, Emission with Scattering
  • Transmission Spectroscopy
  • Flux-based Emission with pure absorption (optional)
• Spectral Operators
  • Spectral Operators (sop)
• Others
  • Compatibility to External Packages
• For Developers
  • Test codes for developers
  • Generates documents
• History
  • History

API:

• exojax package

ExoJAX examples (exojaxample)

• 🪐 exojaxample_WASP39b : An example of HMC-NUTS for actual hot Saturn (JWST/ERS, NIRSPEC/G395H)

• 🪐 exojaxample_jupiter : An example of HMC-NUTS for actual Jupiter reflection spectrum

• 📄 HMC-NUTS for Gl 229 B (T-dwarf) Emission Spectrum Kawashima et al. (arXiv)

• 🧪 HMC analysis of experimental spectroscopy data for hot methane gas Gascell_Exojax.

References

• 📄 Kawahara, Kawashima, Masuda, Crossfield, Pannier, van den Bekerom,

  ApJS 258, 31 (2022) (Paper I)

• 📄 Kawahara et al., 2025, under review. ArXiv (Paper II)

License & Attribution

Copyright 2021-2025, Contributors

• Hajime Kawahara (@HajimeKawahara, maintainer)

• Yui Kawashima (@ykawashima, co-maintainer)

• Kento Masuda (@kemasuda)

• Ian Crossfield

• Dirk van den Bekerom (@dcmvdbekerom)

• Daniel Kitzmann (@daniel-kitzmann)

• Brett Morris (@bmorris3)

• Erwan Pannier (@erwanp) and Nicolas Minesi (@minouHub) from RADIS community

• Stevanus Nugroho (@astrostevanus)

• Tako Ishikawa (@chonma0ctopus)

• Yui Kasagi (@YuiKasagi)

• Shotaro Tada (@sh-tada)

• Ko Hosokawa (@KoHosokawa)

• Hibiki Yama

ExoJAX is free software made available under the MIT License. See the LICENSE.