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. So, the notable features of ExoJAX are summarized as

HMC-NUTS, SVI, Nested Sampling, Gradient-based Inference Techiques and Optimizers 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

Tutorials:

Gallery:

🎨 More Practical Samples

User Guide:

API:

exojax package

ExoJAX References

📄 Kawahara, Kawashima, Masuda, Crossfield, Pannier, van den Bekerom, ApJS 258, 31 (2022) (Paper I)
📄 Kawahara, Kawashima, Tada et al., ApJ 985, 263 (2025), (Paper II)

Publications that have used ExoJAX to date

Kasagi et al. AJ in press Unveiling the Atmosphere of HR 7672 B from the Near-Infrared High-Resolution Spectrum Using REACH/Subaru
🪐 AtmosphericRetrieval_HR7672B : An example of HMC-NUTS for a faint companion HR 7672 B by Subaru/REACH.
Kawashima et al. ApJ 988, 53 (2025) Atmospheric retrieval of Subaru/IRD high-resolution spectrum of the archetype T-type brown dwarf Gl 229 B
Hosokawa et al. ApJ 984, 92 (2025) Measurement of Methane Line Broadening in Hot Hydrogen/Helium Atmospheres at lambda = 1.60–1.63 mu m for Substellar Object Spectroscopy
🧪 HMC analysis of experimental spectroscopy data for hot methane gas Gascell_Exojax.
Tada et al. AJ 169, 255 (2025) Probing 2D Asymmetries of an Exoplanet Atmosphere from Chromatic Transit Variation
Kawahara et al. ApJ 985, 263 (2025) Differentiable Modeling of Planet and Substellar Atmosphere: High-Resolution Emission, Transmission, and Reflection Spectroscopy with ExoJAX2, Paper II
🪐 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
Kawahara et al. ApJS 258, 31 (2022) Autodifferentiable Spectrum Model for High-Dispersion Characterization of Exoplanets and Brown Dwarfs, ExoJAX Paper I (CoP)

License & Attribution

Hajime Kawahara (@HajimeKawahara, maintainer)
Yui Kawashima (@ykawashima, co-maintainer)
Shotaro Tada (@sh-tada)
Yui Kasagi (@YuiKasagi)
Kento Masuda (@kemasuda)
Tako Ishikawa (@chonma0ctopus)
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)
Ko Hosokawa (@KoHosokawa)
Hibiki Yama

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