Harlow Shapley of Harvard University made an interesting series of calculations
If life could first arise spontaneously from non-living matter on Earth, could it arise elsewhere in the universe?
Few scientists interested in the origin of organisms think that life is unique to our planet Earth.
Most of those interested in the origin of life believe that some form of life has arisen many times in many places.
Those who hold this view point out that the origin of life on Earth was not a necessary and unrepeatable event.
On the contrary, all the events now hypothesized and all the known characteristics of life appear to be capable of occurring within the general laws of the universe.
Indeed, some argue that conditions permitting, biochemical evolution and life are an inevitable consequence of the total evolution of matter in the universe.
They argue that, given the infinite size of the universe, it would be inherently illogical to believe that life is confined to a small planet in a small solar system.
However, the prospects for any form of life in our solar system that is different from that on Earth are slim.
A few years ago, Harlow Shapley of Harvard University made an interesting series of calculations on the probability of life outside the solar system.
Shapley said that at least 1020 stars are visible with today’s telescopes (not counting those outside the field of view of our telescopes).
Most of these stars are binary stars orbiting in the same orbit. Because of the irregular gravitational forces and the extreme temperature cycle that this arrangement creates, life in these systems (binaries) is unlikely.
Even most single stars are too short-lived, too bright, or too dim. These many potentially suitable stars must also be devoid of planets.
Shapley thought it would be reasonable to assume that at least one in every thousand stars has a planetary system (that’s 101 stars with a total of planets).
Given theories about the formation of the universe, any place where life formed should be roughly similar to Earth, at least in terms of its basic chemistry.
Life could then only form on planets with an average temperature. The planetary systems of many stars may not have a planet with a suitable orbit.
Shapley argued that, as an average estimate, at least one in a thousand stars with planets has a suitable planet; this calculation gives a total of 1014 stars with at least one planet with a suitable temperature.
However, for life to develop, it is not enough for a planet to have an average temperature. It also needs to be of a certain size to hold a suitable atmosphere.
If one out of every thousand planets with a suitable temperature also has a suitable size, that is a total of 10¹¹. Even if a planet has a suitable temperature and a suitable atmosphere, life may still not arise for many reasons.
Again, Shapley, using the average one-thousandth estimate, hypothesized that there are 10⁸ (one hundred million) planets on which life could arise.
Today’s biologists working in this field consider Shapley’s estimate too conservative. Today’s biologists argue that the number would be 10¹⁶ or more.
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