You've seen how our self-image has gone from, "We are the center of the universe!" to, "The Earth is a very small stage in a vast cosmic arena." Now we move back to the question that the email pitch from SETI@home spurred: Is our Earth the only life-supporting stage in that arena? And if we do find another stage, how would that change us? To help with the first question, we have to consider the relevant unknowns, just like Frank Drake did in 1961:
- How frequently do stars form in the Milky Way?
- How many of those stars have planets?
- How many planets per star could support life?
- How many of those planets do develop life?
- How many of those planets develop intelligent life?
- How many civilizations release detectable signals?
- How long do those civilizations release those signals?
Or, in equation form respectively:
where N equals the number of civilizations in the galaxy with which communication might be possible.
More variables than this should be considered, and the relevance of even some of Drake's variables is open to debate, so Drake's equation shouldn't be a hard and fast rule for predicting how many communicative civilizations are in the galaxy. (I would argue it was never meant to be hard and fast. Drake's equation started simply as Drake's agenda for a 1961 metting on extraterrestrial intelligence.)
Still, like all Fermi problems, the Drake equation lays out ways to make reasonable assumptions, and it also forces us to think about what makes a communicative intelligence like us possible. For example, the human race developed in a system with a 5-billion-year-old medium-sized star about halfway through its lifespan, eight major planets and numerous minor planets, at least one life-friendly planet (two if Curiosity finds something), and one planet that, as evident in our experience, had enough atmosphere, gravity, basic elements, and evolutionary pressure to create beings that can send purposeful signals into space. (This doesn't consider the Jovian and Saturnian moons, some of which have shown quite livable conditions, for bacteria, at least.) Without one of those variables, life may have developed faster, more slowly, or not at all on our planet, greatly influencing the chances for communicative intelligence to arise.
The biggest question that the Drake equation and current expolanet studies attempt to answer is, "Are we normal?" We've already found that the Earth does not inhabit an exalted position in the universe, so the astronomers among us are skeptical of giving other special characteristics to us. The origin of life, however, is one that relies on so many variables that the chances of success could be very, very small, possibly one in a billion in our galaxy.
However, those miniscule chances do not rule out the possibility of over a billion communicative civilizations, at least in the whole universe. The BBC has an immersive interactive infographic that demonstrates this quite well (and in which I could spend all day!). Their version of the equation includes the optional variable of reappearance (nr), which considers the possibility that a new civilization could arise on a fertile planet after an older one develops and ends. So, it uses this:
I plugged in these values:
R* = 7/year (based on estimates from NASA and the ESA)
fp = 34% (from Kepler; on low end of estimates)
ne = 2.0 (based solely on our own experience [and some optimism re: Mars])
fl = 1% (just to prove a point; Drake meeting estimated 100%, but there are problems with that)
fi = 32% (in my opinion, a balance of the yeas and nays of the sieve of natural selection)
fc = 15% (midpoint of Drake meeting estimate)
L = 500 years (rounding-up of Michael Shermer's estimate)
nr = 2 (considering small-ish L that Shermer gave for humanity and the fact we've taken up about half of the sun's lifespan to get to this point)
N = 2.2848
That's about two communicating civilizations in the Milky Way. (In this case, they probably aren't sending signals at the same time, because it was the reappearance variable, nr, that got us over 2.) Given that there are likely over 150 billion galaxies in the universe, the BBC's infographic extended that figure to estimate about 343 billion communicating civilizations in the universe. For the purposes of the Drake equation, distances across the universe are probably too far to consider communication; the point here, however, is that even a very small chance of life popping up somewhere can create a LOT of life across the known universe.
Consider, though, if any of the variable values were larger. Maybe intelligence is inevitable on a lively planet, given enough time (fi = 100%). Maybe life's chances of arising are better, especially if it can develop in ways we can't imagine (fl = 13%). Maybe a civilization that makes it to interstellar communication can defend itself from all threats to its survival and transmit for millions of years (L = 5,000,000). In each of those cases, N changes from 2.3 to 7.1 in the first case, 29.7 in the second, and 22,848 in the third. Again, these are estimates (and very rough ones at that), but it does allow the mind to wonder: "If those larger estimates were to be true, how would we react if we found somebody out there?"
That's a question for tomorrow.
Bonus reading: If you really want to dive into the process of determining values for the Drake equation, I suggest this paper by Glade, Ballet, and Bastien. They use a stochastic process on the equation that largely goes over my head, but it's a great example of the lengths people go to estimate the likeliness of intelligent life in the galaxy.