Since the report of the first exoplanet in 1995, astronomers have used various techniques to discover over 1700 exoplanets, with another 4200 candidates, orbiting a wide variety of stars, mostly less massive and cooler than our Sun. Some of these planets are touted as being potentially habitable. This appellation relies solely on the luminosity of the star, and the distance of the planet from its star. By this criterion alone, Earth is not a habitable planet.
I will discuss the definition of the habitable zone for life as we know it. With the assumption that life requires billions of years of stable conditions to become complex, as it has on Earth, I will compare conditions on Earth over its history to those on planets of other stars. The conclusion is that conditions on most other planets seem to be far less conducive to the development of complex life than they were on Earth.
Prof. Walter is starting is 26th year at Stony Brook University, and has earned the right to be opinionated and curmudgeonly. He researches the birth, lives, and death of stars like the Sun, and also exploding white dwarfs. He lives in East Setauket.