Life On Other Planets
Delaane IV, which supports methane-breathing life.
Life is common. Almost all solar systems have at least one planet that supports life. However, over 99.9% of those planets are deadly to humans, so the mass of humankind forgets such places exist.
Sapience is extremely rare and separated by vast swaths of time and space. It may be that sapient life currently exists in other galaxies or even on the other side of our own, but we can't reach them. There have been at least two other sapient species that colonised this section of the galaxy, but both died out long ago.
It seems that sapient life is skewed towards air-breathers and Earth-type environments. The entomorphs are air-breathers, but they developed in an environment designed for humans. The Aquaans and Yogzarthu both adapted to Earth-type environments in their own ways. It is unknown what kind of environment the Qohathoth and the tsilok-Shilau required.
There is the theory that poison planets might support sapient life but that life so different from humanity that we cannot identify it as sapient.