Supercritical Exoplanetary Atmospheres as Potential Habitats for Extraterrestrial Aeroplankton
An increasingly familiar class of exoplanets amongst those being continually discovered is the "Super-Earth" (Valencia+2007), with a mass intermediate between Earth and Neptune. These have been found inside habitable zones (Udry+2007) and likely have dense atmospheres (Lopez+2014, Lammer+2012). With sufficient atmospheric pressure and temperature, these atmospheres are likely to enter a supercritical fluid phase with properties intermediate between gas and liquid. Supercritical fluid phases are typically considered for gas giant interiors (Trachenko+2014) or water oceans (Elkins-Tanton+2008), though any gas or liquid can form such a phase. For example, an Earth-like atmospheric composition will enter a supercritical phase at T > 132.6 K and p > 37.21 atm. This pressure is significantly lower than the ≳300 atm (Koschinsky+2008) experienced by life on the abyssal plain of Earth's oceans (Rona+1986). It is therefore possible for Earth-like life to tolerate the pressure-temperature conditions of a supercritical atmosphere.
In Earth's atmosphere, microbes have been found airborne as "aeroplankton" (Sharma+2007) and living microbes have been found at high altitudes (Sattler+2002), prompting planetary atmospheres to be considered as astrobiological targets (Smith2013). A denser atmosphere can support a greater number density of suspended particles, raising the possibility of aeroplankton ecologies in the dense supercritical atmospheres of Super-Earths.