A Next-generation Exoplanet Atmospheric Retrieval Framework for Transmission Spectroscopy (NEXOTRANS): Comparative Characterization for WASP-39 b Using JWST NIRISS, NIRSpec PRISM, and MIRI Observations

Abstract The advent of JWST has marked a new era in exoplanetary atmospheric studies, offering higher-resolution data and greater precision across a broader spectral range than previous space-based telescopes. Accurate analysis of these data sets requires advanced retrieval frameworks capable of navigating complex parameter spaces. We present NEXOTRANS , an atmospheric retrieval framework that integrates Bayesian inference using UltraNest / PyMultiNest with four machine learning algorithms: Random Forest , Gradient Boosting , K-Nearest Neighbor , and Stacking Regressor . This hybrid approach enables a comparison between traditional Bayesian methods and computationally efficient machine learning techniques. Additionally, NEXOTRANS incorporates NEXOCHEM , a module for solving equilibrium chemistry. We applied NEXOTRANS to JWST observations of the Saturn-mass exoplanet WASP-39 b, spanning wavelengths from 0.6 to 12.0 μ m using NIRISS, NIRSpec PRISM, and MIRI. Four chemistry models—free, equilibrium, modified hybrid equilibrium, and modified equilibrium-offset chemistry—were explored to retrieve precise volume mixing ratios (VMRs) for H 2 O, CO 2 , CO, H 2 S, and SO 2 . Absorption features in both NIRSpec PRISM and MIRI data constrained SO 2 log VMRs to values between −6.25 and −5.73 for all models except equilibrium chemistry. High-altitude aerosols, including ZnS and MgSiO 3 , were inferred, with constraints on their VMRs, particle sizes, and terminator coverage fractions, providing insights into cloud composition. For the best-fit modified hybrid equilibrium model, we derived supersolar elemental abundances of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mspace width="0.1em"/> <mml:mtext>O/H</mml:mtext> <mml:mspace width="0.1em"/> <mml:mo>=</mml:mo> <mml:mn>14</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>12</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>1</mml:mn> <mml:mo>.</mml:mo> <mml:mn>82</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>2</mml:mn> <mml:mo>.</mml:mo> <mml:mn>86</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> </mml:math> solar, <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mspace width="0.1em"/> <mml:mtext>C/H</mml:mtext> <mml:mspace width="0.1em"/> <mml:mo>=</mml:mo> <mml:mn>21</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>37</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>3</mml:mn> <mml:mo>.</mml:mo> <mml:mn>18</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>4</mml:mn> <mml:mo>.</mml:mo> <mml:mn>93</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> </mml:math> solar, and <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mspace width="0.1em"/> <mml:mtext>S/H</mml:mtext> <mml:mspace width="0.1em"/> <mml:mo>=</mml:mo> <mml:mn>5</mml:mn> <mml:mo>.</mml:mo> <mml:msubsup> <mml:mn>37</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.65</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.79</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> </mml:math> solar, along with a C/O ratio of <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:msubsup> <mml:mn>1.35</mml:mn> <mml:mrow> <mml:mo>−</mml:mo> <mml:mn>0.02</mml:mn> </mml:mrow> <mml:mrow> <mml:mo>+</mml:mo> <mml:mn>0.05</mml:mn> </mml:mrow> </mml:msubsup> <mml:mo>×</mml:mo> </mml:math> solar. These results demonstrate NEXOTRANS ’s potential to enhance JWST data interpretation, advancing comparative exoplanetology efficiently.