Disentangling atmospheric compositions of K2-18 b with next generation facilities

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.

Keywords: Occultations; Radiative transfer; Techniques: spectroscopic; Telescopes.

Fig. 1

Simulated observed spectra for our three scenarios of K2-18b with 1σ uncertainties obtained by combining a number of transits recorded with JWST. Top: 2 stacked transits. Bottom: 10 stacked transits. For solution 1, we also display a scattered instance of our simulated observation. Un-scattered spectra are used for the retrievals

Fig. 2

Simulated observed spectra for our three scenarios of K2-18b obtained by combining 50 transits recorded with Ariel. If 20 transits are combined, the three scenarios are difficult to distinguish. For solution 1, we also display a scattered instance of our simulated observation. Un-scattered spectra are used for the retrievals

Fig. 3

Posteriors related to the three atmospheric scenarios for 10 combined transits recorded with JWST. The three scenarios can be easily distinguished by inspection of the posterior distributions of the parameters

Fig. 4

Posteriors related to the three atmospheric scenarios for 50 combined transits recorded with Ariel. The three scenarios can be easily distinguished by inspection of the posterior distributions of the parameters

Fig. 5

Best fit spectra from retrievals assuming different surface pressures to interpret the HST-WFC3 observations (black) as published in [1]. Blue plot: 10 bar; red plot: 1 bar; purple plot: 0.7 bar

Fig. 6

Simulated forward spectra assuming different surface pressures for JWST simulated observations (20 combined transits). Blue plot: 10 bar; Red plot: 1 bar; Purple plot: 0.7 bar. Un-scattered spectra are used for the retrievals

Fig. 7

Posteriors for the retrievals where we attempt to recover the surface pressure. Blue: the forward model was using a surface pressure of 10 bar; Purple: the forward model was using a surface pressure of 0.7 bar. The forward models correspond to the ones in Fig. 6

Fig. 8

Observed spectra for the atmospheric scenario 1 with the different JWST instruments: NIRISS, NIRSpec and MIRI. The Error bars are displayed for a single transit with each instrument. The averaged model and 1σ error are indicated by the shaded blue region

Fig. 9

Retrieval posteriors for the atmospheric scenario 1 with different JWST setup. The NIRISS only scenario seems to provide similar performances than the NIRISS + NIRSpec case. If only NIRSpec is used, the water-to-hydrogen ratio is much more difficult to constrain, since only a single broad spectral modulation is present in NIRSpec wavelength coverage

Fig. 10

Retrieval posteriors for the atmospheric scenario 1: secondary atmosphere composed of H/He and H₂O

Fig. 11

Retrieval posteriors for the atmospheric scenario 2: secondary atmosphere composed of H/He and another undetectable gas, i.e. N₂. Traces of H₂O are also present

Fig. 12

Retrieval posteriors for the atmospheric scenario 3: primary atmosphere composed of H/He and clouds. Traces of H₂O are also present

Fig. 13

Retrieval posteriors for the 3 scenarios in the case of 10 Ariel transits. The posteriors indicate a departure from a unique solution, however some of the atmospheric parameters are still correlated