The “Rare Earth Hypothesis” suggests that complex life is rare in the universe, and may even be unique to Earth. If life does occur elsewhere, it will only be in the form of single-celled microbial life such as bacteria. Originally proposed by Ward and Brownlee in their book Rare Earth: Why Complex Life Is Uncommon in the Universe, it has been much debated among astrophysicists. As the Authors say: “The main conclusion of Rare Earth is that Earth is a very special place. Many circumstances and events had to happen just right for Earth to remain a healthy habitat for advanced life. It appears the our planet won the cosmic lottery and we should cherish our very special place and time in the Universe.”
It appears now that Earth may even be rarer.
As reported in Space.com this week:
One of the most important characteristics of an alien planet is whether or not it falls into what’s called the habitable zone — a Goldilocks-like range of not-too-close, not-too-far distances from the parent star that might allow the planet to host life.
Now scientists have redefined the boundaries of the habitable zone for alien planets, potentially kicking out some exoplanaets that were thought to fall within it, and maybe allowing a few that had been excluded to squeeze in.
“This will have a significant impact on the number of exoplanets that are within habitable zone,” said research team leader Ravi Kumar Kopparapu of Penn State University.