2025ApJ...986....1F

Our brains are hardwired for pattern recognition as correlations are useful for predicting and understanding nature. As more exoplanet atmospheres are being characterized with JWST, we are starting to unveil their properties on a population level. Here we present a framework for comparing exoplanet transmission spectroscopy from 3 to 5μm with four bands: L (2.9-3.7 μm), SO₂ (3.95-4.1μm), CO₂ (4.25-4.4μm), and CO (4.5-4.9μm). Together, the four bands cover the major carbon-, oxygen-, nitrogen-, and sulfur-bearing molecules including H₂O, CH₄, NH₃, H₂S, SO₂, CO₂, and CO. Among the eight high-precision gas giant exoplanet planet spectra we collect, we find strong correlations between the SO₂ - L index and planet mass (r = -0.41 ± 0.09) and temperature (r = -0.64 ± 0.08), indicating SO₂ preferably exists (SO₂ - L > -0.5) among low-mass (∼ <0.3 M_J) and cooler (∼ <1200 K) targets. We also observe strong temperature dependency for both CO₂ - L and CO - L indices. Under equilibrium chemistry and isothermal thermal structure assumptions, we find that the planet sample favors supersolar metallicity and a low C/O ratio (<0.7). In addition, the presence of a mass-metallicity correlation is favored over uniform metallicity with the eight planets. We further introduce the SO₂ - L versus CO₂ - L diagram, similar to the color-magnitude diagram for stars and brown dwarfs. All reported trends here will be testable and will be further quantified with existing and future JWST observations within the next few years.