GOOD PLANETS ARE HARD TO FIND

HUMAN-TYPE LIFE REQUIRES A NARROW RANGE OF CONDITIONS: PLANET WITH RIGHT TEMPERATURES (LIQUID WATER) AND BREATHABLE OXYGEN ATMOSPHERE

SUN, 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