Possibilities for an Aerial Biosphere in Temperate Sub Neptune-Sized Exoplanet Atmospheres

Abstract

The search for signs of life through the detection of exoplanet atmosphere biosignature gases is gaining momentum. Yet, only a handful of rocky exoplanet atmospheres are suitable for observation with planned next-generation telescopes. To broaden prospects, we describe the possibilities for an aerial, liquid water cloud-based biosphere in the atmospheres of sub Neptune-sized temperate exoplanets, those receiving Earth-like irradiation from their host stars. One such planet is known (K2-18b) and other candidates are being followed up. Sub Neptunes are common and easier to study observationally than rocky exoplanets because of their larger sizes, lower densities, and extended atmospheres or envelopes. Yet, sub Neptunes lack any solid surface as we know it, so it is worthwhile considering whether their atmospheres can support an aerial biosphere. We review, synthesize, and build upon existing research. Passive microbial-like life particles must persist aloft in a region with liquid water clouds for long enough to metabolize, reproduce, and spread before downward transport to lower altitudes that may be too hot for life of any kind to survive. Dynamical studies are needed to flesh out quantitative details of life particle residence times. A sub Neptune would need to be a part of a planetary system with an unstable asteroid belt in order for meteoritic material to provide nutrients, though life would also need to efficiently reuse and recycle metals. The origin of life may be the most severe limiting challenge. Regardless of the uncertainties, we can keep an open mind to the search for biosignature gases as a part of general observational studies of sub Neptune exoplanets.

A transiting, temperate mini-Neptune orbiting the M dwarf TOI-1759 unveiled by TESS

“…In addition, these cooler worlds allow us to make connections with the planets in our own Solar System, all of which have equilibrium temperatures smaller than 500 K (hereon referred to as "temperate" exoplanets). These connections, in turn, have key implications for the search for life outside the Solar System, and have the potential to help us improve and refine the concept of planetary habitability itself (Tasker et al 2017;Meadows & Barnes 2018;Seager et al 2021).…”

TOI-2257 b: A highly eccentric long-period sub-Neptune transiting a nearby M dwarf

“…Of course, such exoplanets do not allow surface habitability, given their immense atmospheric pressure and heated surface. However, life in the clouds could be a possibility-as first proposed by (Morowitz & Sagan 1967;Sagan & Salpeter 1976) and recently discussed for sub-Neptune-sized exoplanets by Seager et al (2021) (see also Fig. 11).…”

“…Different symbols/colors designate different planet size categories. TOI-2257 b lies in the middle of the blue shaded area, which highlights sub-Neptunes that are (i) amenable for atmospheric characterization (TSM>12) and (ii) for which life in the clouds is possible (T eq between 200 and 320 K, followingSeager et al 2021). Equilibrium temperatures were estimated using an albedo of 0 and emissivity of 1, followingKempton et al (2018).…”

Can Carbon Fractionation Provide Evidence for Aerial Biospheres in the Atmospheres of Temperate Sub-Neptunes?

“…So far the search for signs of life on other worlds has focused on terrestrial planets, but recent work has explored whether life could originate and survive in the liquid water clouds of temperate sub-Neptune-sized planets (Seager et al 2021). Sub-Neptunes are more readily observable than terrestrial planets owing to their larger size and low mean molecular weight (MMW), H 2 -dominated atmospheres (e.g., Bean et al 2021).…”

“…Sub-Neptunes are more readily observable than terrestrial planets owing to their larger size and low mean molecular weight (MMW), H 2 -dominated atmospheres (e.g., Bean et al 2021). Passive microbial-like particles could persist aloft in regions with liquid water clouds for long enough to metabolize, reproduce, and spread before sinking to altitudes that may be too hot for life of any kind to survive (Seager et al 2021). Isotope fractionation is thought to be among the strongest signs of life that theoretically can be remotely detected in a planet's atmosphere (Neveu et al 2018).…”