GOOD PLANETS ARE HARD TO FIND
HUMAN-TYPE LIFE REQUIRES A
NARROW RANGE OF CONDITIONS: PLANET WITH RIGHT TEMPERATURES (LIQUID WATER) AND BREATHABLE OXYGEN ATMOSPHERESUN, SOLID EARTH (PLATE TECTONICS), OCEAN, ATMOSPHERE, LIFE
INTERACT IN COMPLICATED WAYS THAT KEEP EARTH HABITABLE
COMPARISON OF VENUS (TOO HOT), MARS (TOO COLD), AND EARTH (JUST RIGHT) SUGGESTS THAT SUCH PLANETS - AND HENCE INTELLIGENT LIFE IN THE UNIVERSE - MAY BE RARE
(Could warmer Mars have had life in past?)
NOT CLEAR HOW VULNERABLE EARTH CLIMATE IS
TO CHANGES THAT MIGHT MAKE IT SO HOT ("GREENHOUSE") OR SO COLD ("ICEHOUSE") TO SIGNIFICANTLY IMPACT CIVILIZATION