Alien's From...

The kind of enviroment required for life is a stable atmosphere, there would need to be water, breathable air, and heat that could hold a body temperature. There would also be medium mass for a normal amount of gravity for a body. Low mass planets have low gravity and high mass planets have high gravity. That is why scientists are searching for possible life on Mars. It's the closest thing to Earth's enviroment. It's close to the Sun, about Earth's mass, and it has a stable atmosphere. It's rumored that there has been water found on Mars, making it possibly able to sustain life.