Recent observations of cyanobacterial fossils on carbonaceous chondrites have conclusively established the presence
of fossil organisms on extraterrestrial bodies widely presumed to be comets. Likewise, the data from four
cometary flyby (and one impact) missions and the exploration of a peculiar S-type asteroid, show evidence of liquid
water in the past or present. In addition, sand grains returned from the tail of comet P/Wild-2 demonstrate
that comets accrete inner Solar System material. So it is a short step to propose the separate and independent
existence of a cometary biosphere, the ecosystem of organisms that exploit the niche of an extraterrestrial environment.
This paper attempts to lay the framework for such a hypothetical ecosystem, and establish criteria for
its continued existence and spread.