Cool Gaseous Exoplanets: surveying the new frontier with Twinkle

Cool gaseous exoplanets ($1.75\ R_\oplus < R_\text{p} < 3\ R_\text{J}$, $200$ K $<T_\text{eq} < 1000$~K) are an as-yet understudied population, with great potential to expand our understanding of planetary atmospheres and formation mechanisms. In this paper, we outline the basis for a homogeneous survey of cool gaseous planets with Twinkle, a 0.45-m diameter space telescope with simultaneous spectral coverage from 0.5-4.5~$\mu$m, set to launch in 2025. We find that Twinkle has the potential to characterise the atmospheres of 36 known cool gaseous exoplanets (11~sub-Neptunian, 11~Neptunian, 14~Jovian) at an SNR $\geq$ 5 during its 3-year primary mission, with the capability of detecting most major molecules predicted by equilibrium chemistry to > $5\sigma$ significance. We find that an injected mass-metallicity trend is well-recovered, demonstrating Twinkle's ability to elucidate this fundamental relationship into cool regime. We also find Twinkle will be able to detect cloud layers at 3$\sigma$ or greater in all cool gaseous planets for clouds at $\leq$ 10 Pa pressure level, but will be insensitive to clouds deeper than $10^4$ Pa in all cases. With these results we demonstrate the capability of the Twinkle mission to greatly expand the current knowledge of cool gaseous planets, enabling key insights and constraints to be obtained for this poorly-charted region of exoplanet parameter space.