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• Challenge to decipher spectroscopic signals of small exoplanets • Launch of JWST & Ariel will provide reams of high-quality data • Necessary for cross-sections used to model water to be equally as robust • Incorporating pressure self-broadening induces detectable transit depth differences • Imperative to include this into water cross-section calculations The discovery of super-Earth and mini-Neptune exoplanets means that atmospheric signals from low-mass, temperate exoplanets are being increasingly studied. The signal acquired as the planet transits its host star, known as the transit depth , is smaller for these planets and, as such, more difficult to analyze. The launch of the space telescopes James Webb (JWST) & Ariel will give rise to an explosion in the quality and quantity of spectroscopic data available for an unprecedented number of exoplanets in our galaxy. Accurately extracting the information content, thereby permitting atmospheric science, of such data-sets will require robust models and techniques. We present here the analysis of simulated transmission spectra for water-rich atmospheres, giving evidence for non-negligible differences in simulated transit depths when self-broadening of H 2 O is correctly accounted for, compared with the currently typically accepted standard of using H 2 and He-broadened cross-sections. Our case-study analysis is carried out on two super-Earths, focusing on water-based atmospheres, ranging from H 2 -rich to H 2 O-rich. The transit depth is considerably affected, increasing values by up to 60 ppm, which is shown to be detectable with JWST and Ariel. The differences are most pronounced for the lighter (i.e. μ ∼ 4) atmospheres. Our work illustrates that it is imperative that the field of exoplanet spectroscopy moves toward adapted cross-sections, increasingly optimized for high- μ atmospheres for studies of super-Earths and mini-Neptunes. [ABSTRACT FROM AUTHOR]

• Challenge to decipher spectroscopic signals of small exoplanets. • Launch of JWST & Ariel will provide reams of high-quality data. • Necessary for cross-sections used to model water to be equally as robust. • Incorporating water continuum induces detectable transit depth differences. • Imperative to include this into water cross-section calculations. Most of the exoplanets detected up to now transit in front of their host stars, allowing for the generation of transmission spectra; the study of exoplanet atmospheres relies heavily upon accurate analysis of these spectra. Recent discoveries mean that the study of atmospheric signals from low-mass, temperate worlds are becoming increasingly common. The observed transit depth in these planets is small and more difficult to analyze. Analysis of simulated transmission spectra for two small, temperate planets (GJ 1214 b and K2-18 b) is presented, giving evidence for significant differences in simulated transit depth when the water vapor continuum is accounted for when compared to models omitting it. These models use cross-sections from the CAVIAR lab experiment for the water self-continuum up to 10,000 cm − 1 ; these cross-sections exhibit an inverse relationship with temperature, hence lower-temperature atmospheres are the most significantly impacted. Including the water continuum strongly affects transit depths, increasing values by up to 60 ppm, with the differences for both planets being detectable with the future space missions Ariel and JWST. It is imperative that models of exoplanet spectra move toward adaptive cross-sections, increasingly optimized for H 2 O-rich atmospheres. This necessitates including absorption contribution from the water vapor continuum into atmospheric simulations. [ABSTRACT FROM AUTHOR]