Atmospheric Habitable Zones in Y Dwarf Atmospheres

We use a simple organism lifecycle model to explore the viability of an atmospheric habitable zone (AHZ), with temperatures that could support Earth-centric life, which sits above an environment that does not support life. We illustrate this idea using the object WISE J085510.83{0714442.5, which is a cool, free-foating brown dwarf. We allow organisms to adapt to their atmospheric environment (described by temperature, convection, and gravity) by adopting dierent growth strategies that maximize their chance of survival and proliferation. We assume a constant upward vertical velocity through the AHZ. We found that the organism growth strategy is most sensitive to the magnitude of the atmospheric convection. Stronger convection supports the evolution of more massive organisms. For a purely radiative environment we nd that evolved organisms have a mass that is an order of magnitude smaller than terrestrial microbes, thereby dening a dynamical constraint on the dimensions of life that an AHZ can support. Based on a previously dened statistical approach we infer that
there are of order 109 Y brown dwarfs in the Milky Way, and likely a few tens of these objects are within ten parsecs from Earth. Our work also has implications for exploring life in the atmospheres of temperate gas giants. Consideration of the habitable volumes in planetary atmospheres signicantly increases the volume of habitable space in the galaxy.