Estimates of life in the universe

I ran across some estimates of the prevalence of life in the universe. There are 300 billion stars in an average galaxy and 100 billion galaxies in the observable universe so the total number of stars is 10^21.

1. Harvard astronomer Harlow Shapely assumed:

a. 1 star in a thousand has planets
b. 1 in a thousand of the stars in (a) has a planet at the right distance from it
c. 1 in a thousand of the planets in (b) is large enough to hold an atmosphere
d. 1 in a thousand of the planets in (c) has the right chemical composition to support life

and concluded there should be at least 100 million planets capable of supporting life in the universe.

2. Astronomer Su-Shu Huang made less limiting assumptions, so that he came to the conclusion that 5 percent of all solar systems in the universe should be able to support life. This means 100 billion life-bearing planets.

3. Harrison Brown assumed that almost every visible star posses a partially or wholly invisible panetary system. This means 100 billion solar systems in our own galaxy alone, and there are 100 billion galaxies in this universe.

4. Frank Drake, the originator of the Drake equation, estimated that 10,000 advanced technological civilizations are likely to exist in the Milky Way galaxy alone.

5. The Drake equation was updated and elaborated by Carl Sagan and colleagues. They estimated that up to one million intelligent civilizations could exist in our galaxy.

6. Robert Taormina applied the equations in (5) to a region within 100 light-years fro earth and found that more than 8 such civilizations should be present within hailing distance from us.

The estimates of the number of life bearing planets or intelligent civilizations vary quite a lot, but they are all significantly greater than 1.


--dkomo@xxxxxxxx

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