Abstract
Recent analysis of the planet K2-18 b has shown the presence of water vapour in its atmosphere. While the H2O detection is significant, the Hubble Space Telescope (HST) WFC3 spectrum suggests three possible solutions of very different nature which can equally match the data. The three solutions are a primary cloudy atmosphere with traces of water vapour (cloudy sub-Neptune), a secondary atmosphere with a substantial amount (up to 50% Volume Mixing Ratio) of H2O (icy/water world) and/or an undetectable gas such as N2 (super-Earth). Additionally, the atmospheric pressure and the possible presence of a liquid/solid surface cannot be investigated with currently available observations. In this paper we used the best fit parameters from Tsiaras et al. (Nat. Astron. 3, 1086, 2019) to build James Webb Space Telescope (JWST) and Ariel simulations of the three scenarios. We have investigated 18 retrieval cases, which encompass the three scenarios and different observational strategies with the two observatories. Retrieval results show that twenty combined transits should be enough for the Ariel mission to disentangle the three scenarios, while JWST would require only two transits if combining NIRISS and NIRSpec data. This makes K2-18 b an ideal target for atmospheric follow-ups by both facilities and highlights the capabilities of the next generation of space-based infrared observatories to provide a complete picture of low mass planets.
Highlights
Despite biases in observational techniques towards large, gaseous giant planets, current statistics from over 4000 confirmed planets show a very different picture: low Experimental Astronomy radius planets are the most abundant exoplanets, especially around late-type stars [2–6]
Direct observations of the atmospheric composition may help to remove some of the degeneracies associated with the bulk composition and nature of these planets (e.g. [9, 18]), and provide additional constraints to the formation and evolution scenarios currently considered in the literature [15, 16, 19]
We find that James Webb Space Telescope (JWST) and Ariel will be able to distinguish among the three scenarios presented in [1]
Summary
Despite biases in observational techniques towards large, gaseous giant planets, current statistics from over 4000 confirmed planets show a very different picture: low Experimental Astronomy radius planets are the most abundant exoplanets, especially around late-type stars [2–6]. The frequency of these planets seems to follow a bimodal distribution when plotted against the planetary size [5], with most planets clustering around two peaks at Rp ∼ 1.3 R⊕ and Rp ∼ 2.4 R⊕. Direct observations of the atmospheric composition may help to remove some of the degeneracies associated with the bulk composition and nature of these planets (e.g. [9, 18]), and provide additional constraints to the formation and evolution scenarios currently considered in the literature [15, 16, 19]
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