GJ 436b is a prime target for understanding warm Neptune exoplanet atmospheres and a target for multiple JWST GTO programs .
Here , we report the first space-based optical transmission spectrum of the planet using two HST/STIS transit observations from 0.53-1.03 \mu m. We find no evidence for alkali absorption features , nor evidence of a scattering slope longward of 0.53 \mu m. The spectrum is indicative of moderate to high metallicity ( \sim 100 - 1000 \times solar ) while moderate metallicity scenarios ( \sim 100 \times solar ) require aerosol opacity .
The optical spectrum also rules out some highly scattering haze models .
We find an increase in transit depth around 0.8 \mu m in the transmission spectra of 3 different sub-Jovian exoplanets ( GJ 436b , HAT-P-26b , and GJ 1214b ) .
While most of the data come from STIS , data from three other instruments may indicate this is not an instrumental effect .
Only the transit spectrum of GJ 1214b is well fit by a model with stellar plages on the photosphere of the host star .
Our photometric monitoring of the host star reveals a stellar rotation rate of 44.1 days and an activity cycle of 7.4 years .
Intriguingly , GJ 436 does not become redder as it gets dimmer , which is expected if star spots were dominating the variability .
These insights into the nature of the GJ 436 system help refine our expectations for future observations in the era of JWST , whose higher precision and broader wavelength coverage will shed light on the composition and structure of GJ 436b ’ s atmosphere .