Quote:
Originally Posted by absael
There is an article on space.com in which Dr. Charles Lineweaver (research astronomer) guesses that there may be a billion Earth-like planets in our galaxy.
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Astronomers seem to have many different definitions of Earth-like. Some consider that any terrestrial planet within a certain range of masses and a certain distance of the local star to be Earth-like, no matter what conditions pertain on those worlds. By some definitions Venus and Mars would count as Earth-like. Other authorities, such as Ward and Brownlee, have a much more restrictive view of what an Earth-like planet is.
Our Galaxy contains maybe 200 to 400 billion stars; for there to be 100 billion Earth-like planets, Lineweaver must be adopting an inclusive definition of Earth-like, which includes tidally locked terrestrials in red dwarf systems and many other world quite different to our own. I think it is possible that life, even intelligent life, might evolve on such very different worlds, but each instance of life will have many differences to the life on our planet.