I recommend reading _How to Find a Habitable Planet_. The first half of the book is about why Earth is habitable, and then it goes to project that against what

I recommend reading _How to Find a Habitable Planet_. The first half of the book is about why Earth is habitable, and then it goes to project that against what the search for a similar world requires. The "snowball earth" scenarios were terrifying (basically, you get an ice ball that is very reflective and thus stays ice, unless you have something like plate tectonics to dirty-up the atmosphere to the point where some insolation can be retained).