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The TRAPPIST-1 planetary system is favourable for transmission spectroscopyand offers the unique opportunity to study rocky planets with possiblynon-primary envelopes. We present here the transmission spectrum of the seventhplanet of the TRAPPIST-1 system, TRAPPIST-1 h (R$_{\rm P}$=0.752 R$_{\oplus}$,T$_{\rm eq}$=173K) using Hubble Space Telescope (HST), Wide Field Camera 3Grism 141 (WFC3/G141) data. First we extracted and corrected the raw data toobtain a transmission spectrum in the near-infrared (NIR) band (1.1-1.7$\mu$m).We corrected for stellar modulations using three different stellarcontamination models; while some fit the data better, they are statisticallynot significant and the conclusion remains unchanged concerning the presence orlack thereof of an atmosphere. Finally, using a Bayesian atmospheric retrievalcode, we put new constraints on the atmosphere composition of TRAPPIST-1h.According to the retrieval analysis, there is no evidence of molecularabsorption in the NIR spectrum. This suggests the presence of a high cloud deckor a layer of photochemical hazes in either a primary atmosphere or a secondaryatmosphere dominated by heavy species such as nitrogen. This result could evenbe the consequence of the lack of an atmosphere as the spectrum is betterfitted using a flat line. We cannot yet distinguish between a primary cloudy ora secondary clear envelope using HST/WFC3 data; however, in most cases withmore than 3$\sigma$ confidence, we can reject the hypothesis of a clearatmosphere dominated by hydrogen and helium. By testing the forced secondaryatmospheric scenario, we find that a CO-rich atmosphere (i.e. with a volumemixing ratio of 0.2) is one of the best fits to the spectrum with a Bayesfactor of 1.01, corresponding to a 2.1$\sigma$ detection.

Near-infrared transmission spectrum of TRAPPIST-1 h using Hubble WFC3 G141 observations