  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.
