Anjali Piette
Atmospheric observations of mini-Neptunes orbiting M-dwarfs are beginning to provide constraints on their chemical and thermal properties, while also providing clues about their interiors and potential surfaces. With their relatively large scale heights and large planet-star contrasts, mini-Neptunes are currently ideal targets for atmospheric characterisation in the low-mass regime. Understanding the thermal structures and spectral appearances of mini-Neptunes is important to understand various aspects of their atmospheres, including radiative/convective energy transport, boundary conditions for the interior, and their potential habitability. We explore these aspects of mini-Neptunes using self-consistent atmospheric models to explore the effects of irradiation, internal flux, metallicity, clouds and hazes. Building on recent suggestions of habitability of the mini-Neptune K2-18 b, we find a range of physically-motivated atmospheric conditions that allow for liquid water under the H2-rich atmospheres of such planets. We find that observations of thermal emission with JWST/MIRI spectrophotometry can place important constraints on the habitability of temperate mini-Neptunes such as K2-18 b, and provide more detailed constraints on the chemical and thermal properties of warmer mini-Neptunes such as GJ 3470 b. These results underpin the potential of temperate mini-Neptunes such as K2-18 b as promising candidates in the search for habitable exoplanets.
Effects of atmospheric parameters: We calculate model atmospheres of mini-Neptunes considering a wide range of possible conditions. We vary the following atmospheric properties in our models: the internal temperature, stellar irradiation, infrared opacity in the atmosphere and the presence of haze. These parameters all have significant impacts on the temperature profile of the atmosphere, e.g. a higher IR opacity leads to a hotter temperature profile. We also find that mini-Neptunes as cool as ~350 K could be potentially be observable with the James Webb Space Telescope (JWST) in the mid-IR. Our models are computed using an adaptation of the GENESIS self-consistent atmospheric model [1].
What lies beneath the atmosphere?: Mini-Neptunes could have a water-rich layer beneath the hydrogen-rich atmosphere (e.g. [2]). The phase of the water surface depends on the atmospheric temperature profile; we compare our model temperature profiles to the phase diagram of 100% H2O to determine the phase of the H2O layer beneath the atmosphere. For example, the surface of the water layer could be super-critical, or liquid, or even liquid at habitable temperatures. We also show here that the phase of the atmospheric water is different to the phase of the water-rich layer as it has a lower partial pressure. In our models, the atmospheric water is in the vapour phase, except when water ice clouds are present.
Case study: K2-18 b: K2-18 b is a habitable-zone mini-Neptune and its density suggests a water-rich interior [2]. Its observed transmission spectrum suggests an atmospheric water abundance of ~1-100✕solar [2,3,4]. We calculate atmospheric models for K2-18 b with atmospheric water abundances in this observed range, and with different amounts of haze. We find several model solutions (with lower metallicity and/or haze) which allow for a liquid water ocean beneath the atmosphere.
Case study: high humidity above a water ocean: Above a liquid ocean, the atmosphere could be saturated with water vapour due to surface evaporation. But a higher atmospheric water abundance also results in a hotter atmospheric temperature profile – so can this still allow for a liquid water ocean beneath the atmosphere? Yes! Even with 100% relative humidity above the ocean surface, atmospheric solutions cool enough for a liquid water ocean exist.
Temperate mini-Neptunes with JWST/MIRI: MIRI photometry can constrain the photospheric temperature of temperate mini-Neptunes, which we show with our model atmospheres and simulated data. The photospheric temperature is a lower limit on the surface temperature, so MIRI photometry can therefore be used to select promising candidates for further atmospheric characterisation of potentially-habitable exoplanets.
Warm mini-Neptunes with JWST: MIRI LRS and photometry can constrain the composition and temperature profiles of warmer mini-Neptunes. E.g. we show simulated MIRI LRS data assuming 1 eclipse (error bars from Pandexo, [5]) Molecular features such as H2O and CO2 are present in this spectral range.
Takeaway points:
-A range of atmospheric conditions could allow mini-Neptunes to host liquid water oceans under their hydrogen-rich atmospheres
-With the right atmospheric conditions (e.g. clouds/hazes), this ocean could even be habitable
-JWST will be able to place constraints on the habitability of some temperate mini-Neptunes (and more detailed constraints on the atmospheres of warmer mini-Neptunes)
References: 1) Gandhi & Madhusudhan (2017), MNRAS, 472, 2334, 2) Madhusudhan et al. (2020), ApJL, 891, L7,
3) Tsiaras et al. (2019), NatAs, 3, 1086, 4) Benneke et al. ( 2019), ApJL, 887, L14, 5) Batalha et al. (2017), PASP, 129, 064501