Planet Goldilocks: The habitable zone brims with possibilities

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The late astronomer Carl Sagan used the phrase "billions and billions" to refer to the number of stars in our galaxy, which is home to an estimated 200 billion. As someone who devoted years to the search for extraterrestrial intelligence, Sagan would have welcomed the report Monday in the journal Proceedings, of the National Academy of Science, that puts the number of Earth-sized planets in the galaxy at 8.8 billion.

Using NASA data, scientists have calculated that there are 8.8 billion stars in the Milky Way galaxy that are similar to the sun. Each of these 8.8 billion suns hosts at least one planet that is similar to Earth in size. These planets are from one to two times the diameter of Earth and take between 200 and 400 days to circle their respective stars in a year.

Although these planets revolve in the not-too-hot, not-too-cold Goldilocks zone, they aren't necessarily places that Earthlings would want to spend their retirement years. Conditions likely range from too hot to unendurable because of poisonous atmospheres or rocky, waterless terrain.

Still, life could be as ubiquitous on some of these worlds as it is here -- just not the kind that humans are used to. It's possible that a small percentage of the planets could harbor intelligent life, although that discovery appears to be a long way off.

There will be no way of knowing whether life exists anywhere but here until humans can achieve deep space travel, detect signals from an alien civilization or observe distant worlds through super-powerful telescopes. After all, the nearest planet that fits some of the criterion for being Earth-like is 70 trillion miles away. That's a long, long way to go just to find out nobody's home.

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