they are microbes that can exist anywhere .
Practically ,life could exist anywhere. Scientists have discovered extremepholites. A rocky planet has no limit to it's size. Unless it's gravitational pull turns itself into a black hole. the answer is ∞
Mars had the possibilty of life thousands of years ago. The space probes spirit and opportunity browsed the surface of the crimson planet and found nearly indisputable evidence of water. Given that, and its ice caps, Mars is thought to have maybe harbored life many years ago.However, as we can detect no traces of it today, Mars is no longer a candidate. Instead, Europa is what astronomers are placing their telescopic sets on today. (My apologies for the terrible pun.) It is thought to, under its thick ice surface, and above the volcanic vents of its core, contain a temperate underground ocean, which may or may not contain carnivorous, or otherwise, life.