"In September a couple of years ago, there was news that UFO's were spotted around Mexico? Was this true and have there been any observations of UFO's?"
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Question
What are some of the astronomical events that make it possible for there to be life on Earth?
The most important requirements for life anywhere are liquid water and an energy source. The Sun is our main energy source through photosynthesis, and the water on Earth is liquid because we have the right combination of distance from the Sun and of atmospheric greenhouse effect to keep moderate surface temperatures. Much of our water is thought to have come from asteroid and comet bombardment early in the history of the Earth. Presumably we also need stability: a stable orbit, a star that does not vary much, and not too many lethal impacts from comets and asteroids. Beyond this short list, it is not clear which properties of the Earth are really necessary for life. Many of the other possibilities are discussed in the best-selling book "Rare Earth" by Peter Ward and Don Brownlee.
David Morrison
NAI Senior Scienetist
August 11, 2004
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