Some bacteria thrive in watery environments such as volcanic vents that would kill humans and most other creatures and plants. But not even the hardiest of creatures or plants on Earth can survive outside this range of about 80 degrees Celsius. This water must be in liquid form, not as ice or steam, for creatures and plants to use it. Planets have to be the right size. A planetary body as small as the Moon does not have strong enough gravity to keep its atmosphere, so the gas escapes into space.
Planets have to be the right distance from their sun.
Parent Star Single stars are also probably important. Many stars exist as partners with one or more other stars, with each star exerting a gravitational pull on any planets that form around other stars in the system. Planets orbiting far from a star would eventually be slung out into deep space, and any planets occupying a complicated orbit around both stars in a binary system would not be continuously in a habitable zone. Planets close to one of the stars might stay in stable orbits in the habitable zone long enough for life to arise, but they might experience irregular and dramatic changes as the stars orbit each other. ET' stands for extraterrestrial, meaning a life form from somewhere other than the Earth.