Can you name the factors that make up the Rare Earth Equation? by ExileFromGroggs

Can you name the factors that make up the Rare Earth Equation?

created by ExileFromGroggs

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Source: Wikipedia article on the Rare Earth hypothesis
In Peter Ward and Donald Brownlee's book, "Rare Earth", they argue that Drake's estimate of the number of civilisations is optimistic, challenging the "Principle of Mediocrity".
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Factor	Abbreviation	Ward/Brownlee/Gonzalez estimate
Number of earth-like planets in Milky Way having complex lifeforms		Could be as low as 0 or 1
Number of stars in the Milky Way		Of the order of 100,000,000,000
Number of planets in a star's habitable zone		1 is a likely upper bound
Fraction of stars in galactic habitable zone		0.1
Fraction of stars with planets		Thought to be high
Fraction of rocky (not gaseous) planets		Relatively high
Fraction of habitable planets where microbial life arises		Thought to be high
Fraction of planets where complex life evolves		Thought to be low
Fraction of lifespan of planet where complex life present		Thought to be relatively low
Fraction of habitable planets with large moon		Thought to be low
Fraction of star systems with Jupiter-like planets		Thought to be high
Fraction of planets with sufficiently low extinction events		Thought to be high

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