"Organisms and species do not have cosmological life spans. Gaia does, and this is perhaps a general property of living worlds. Influenced greatly by Lovelock and Margulis, I've argued that we are unlikely to find surface life on a planet that has not severely and flagrantly altered its own atmosphere. According to this idea, a planet cannot be "slightly alive" any more than a person can (at least not for long), and an aged planet such as Mars, if it is not obviously, conspicuously alive like Earth, is probably completely dead.7"