16875

With no equivalent in our Solar system comma sub-Neptunes and super-Earths are the most common categories of planet discovered so far. Formation models predict that they are one and the samedoublePoint both composed of rocky cores comma with sub-Neptunes retaining extended atmospheres comma while super-Earths lose them early on. The puffy atmospheres of the smallest sub-Neptunes are outstanding candidates for atmospheric characterisation comma as they represent an important step towards the study of Earth-analogs. Particularly comma there is growing interest in exploring the atmospheres of temperate sub-Neptunes as these could be harbouring liquid water. In this context comma TOI-4336 b is of the highest interest due to the parameters of the system putting it at the top of the list of temperate candidates for atmospheric exploration. In anticipation of in-depth characterization with HST and the upcoming JWST comma it is essential to perform atmospheric reconnaissance in order to discriminate between a clear H-dominated atmosphere (e.g. K2-18b) and a cloudy one (e.g. GJ 1214 b) which prevents the detection of absorbers. We propose the observation of TOI-4336 b with HST.s WFC3 and the G141 grism in order to obtain a transmission spectrum covering the range between 1.1 and 1.7 microns. With this Mid-Cycle proposal we expect to probe the upper layers of the atmosphere and detect the presence of water vapour with only three transits at the 5 sigma level comma confirming our hypothesis this sub-Neptune is ideal for a full atmospheric characterisation.