Page "Extraterrestrial life" Paragraph 3

Alien life, such as bacteria, has been hypothesized to exist in the Solar System and throughout the universe.

This hypothesis relies on the vast size and consistent physical laws of the observable universe.

According to this argument, made by scientists such as Carl Sagan and Stephen Hawking, it would be improbable for life not to exist somewhere other than Earth.

This argument is embodied in the Copernican principle, which states that the Earth does not occupy a unique position in the Universe, and the mediocrity principle, which holds that there is nothing special about life on Earth.

In it, they discussed the Rare Earth hypothesis, in which they claim that Earth-like life is rare in the Universe, while microbial life is common.

Ward and Brownlee are open to the idea of evolution on other planets which is not based on essential Earth-like characteristics ( such as DNA and carbon ).