JWST’s 0.6 to 5µm transmission spectrum of the Neptune-like planet HAT-P-26b

We present a 0.6-5 μm feature-rich JWST transmission spectrum of the warm Neptune-like planet, HAT-P-26b (7R_Earth,22M_Earth, T_eq = 900K) using one transit of JWST NIRISS SOSS (0.6 to 2.8μm) and one transit of NIRSpec G395H (2.9 to 5μm) instruments. These measurements allow us to put remarkable atmospheric constraints on this planet from 0.6 to 5μm. This investigation is part of the JWST-TST DREAMS GTO program 1312, led by PI Nikole Lewis, which aims to construct the 0.6-14 µm transmission spectrum of HAT-P-26b. HAT-P-26b is a warm Neptune-mass exoplanet located at the upper edge of the radius valley, exhibiting low density and making it an ideal target for JWST spectroscopy. HST near-infrared observations have revealed a prominent water feature at 1.4μm, suggesting an extended primary H/He envelope and low metallicity (Wakeford et al., 2017). The addition of JWST observations complements and extends the valuable insights gained from previous HST studies. The extended wavelength coverage and increased sensitivity significantly enhance our understanding of this planet’s chemistry and formation processes by providing strong constraints on the oxygen and carbon content. We provide a comprehensive discussion of the data reduction process for the two transits and present results from multiple pipelines, retrievals, and forward modeling tools. Our study demonstrates the strength of combining JWST NIRISS SOSS and G395H instruments in the interpretation of exoplanetary atmospheres. Our final spectrum interpretation reports the detection of H₂O, CO₂, and SO₂, contributing to our understanding of this exoplanet’s atmosphere and the atmospheric chemistry of Neptune-like planets. Particularly striking is the unexpectedly large absorption feature of SO2 in the atmosphere of HAT-P-26b. Previously detected in the atmospheres of hot Jupiters, this is the first time it has been identified in a Neptune-mass exoplanet, constituting the first robust evidence of photochemistry in a Neptune-like planet.