As we discussed in the series so far (1 2 3), the Drake Equation gives an estimate of the number of civilizations in our galaxy with which communication might be possible, N. After entering the first two values, we have:
N = 5.625 * n_e * f_l * f_i * f_c * L
Today, we’ll talk about the third term, n_e = the average number of planets potentially supporting life per star that has planets.
(Credit: NASA / Jenny Mottar)
What does a planet need to potentially support life? Three things:
Elements capable of forming a wide variety of chemical bonds
A solvent for those elements
An energy source to drive otherwise unfavorable bonds formations
On Earth, those requirements are primarily met by:
Carbon, hydrogen, oxygen, nitrogen, sulfur, and traces of other elements
Let’s be carbon- and water-chauvinists and assume we need the same elements and solvents to potentially support life off-Earth. After all, while silicon can form the same number of bonds as carbon, silicon is about 900-fold more prevalent in Earth’s crust, yet life is built with carbon. As for water, it has a huge advantage over other plausible solvents for biochemistry: its solid form is less dense than its liquid.
Regarding an energy source, though, sunlight isn’t the only game in town. Geothermal energy can support life, and all planets have molten cores early in their existence.
The question then becomes, on average, how many planets per star have carbon, water, and sunlight or geothermal energy? Answer: probably several. In the early years of our solar system, Venus, Earth, Mars, and probably Europa had all three requirements for life. It’s also possible Mercury, Io, and Ganymede did as well. Is our solar system typical? Tough to say, until we know a lot more about extrasolar planets.
Based on all that, we’ll write on the back of our envelope a value for n_e of 3. With n_e = 3, our current value for the Drake equation is:
N = 16.875 * f_l * f_i * f_c * L
So far, we’ve given values to the terms that are favorable to a hypothesis of a galaxy full of high-technology alien civilizations. We’ll see if the fractions of planets that develop life (f_l), particularly intelligent life (f_i), and particularly high-technology civilizations (f_c), will further support that hypothesis in future posts.