Earlier this week we reported on the astounding revelation that 22% of sunlike stars in the Milky Way are orbited by potentially habitable, Earth-sized worlds. Given that there may be billions upon billions of life-friendly planets out there, it's time to revise the numbers in the Drake Equation and estimate how many communicable alien civilizations may be out there.
The Drake Equation goes like this: R * fp * ne * fl * fi * fc * L = N where:
- R = the average rate of star formation in our galaxy
- fp = the fraction of those stars that have planets
- ne = the average number of planets that can potentially support life per star that has planets
- fl = the fraction of planets that could support life that actually develop life at some point
- fi = the fraction of planets with life that actually go on to develop intelligent life (civilizations)
- fc = the fraction of civilizations that develop a technology that releases detectable signs of their existence into space
- L = the length of time for which such civilizations release detectable signals into space
We don't know the precise numbers to fill in these variables, but we're getting a better idea. Let's go with the following assumptions:
Average rate of star formation in our galaxy: Drake originally went with the number 1 — a very conservative estimate — but it's probably closer to 7, so we'll go with that (R=7).
Fraction of those stars that have planets: Drake thought that only 50% of stars host planets, but we now know that 100% of stars have planets (fp=1).
Average number of planets that can potentially support life per star that has planets: Here's where we get to plug in the new data. According to the new study, 1-in-5 sunlike stars host an Earth-sized planet in the habitable zone. Depending on the total number of stars in the Milky Way, that could be as high as 11 billion planets, or a figure of roughly 20%. But there's also red dwarfs to consider. Recent surveys have shown that upwards of 40% of these dim stars host Earth-like planets in their habitable zones. That means there's as many as 40-60 billion habitable planets orbiting red dwarfs. That said, we do not know if red dwarfs can harbor life. These systems have poor magnetic fields, are tidally locked, and experience poor levels of "good" radiation. So let's prepare the equation to account for both possibilities. So our figures will be 34% (rl=0.34) in the optimistic case and 4% (fl=0.04) in the pessimistic case. Drake himself gave values between 1-5.
Fraction of planets that could support life that actually develop life at some point: This is a tough one, and we haven't got a clue. Drake thought it was 100%, but that can't possibly be right. Still, life on Earth started almost immediately once the proper conditions were established, so it's probably not a figure that's close to zero. Let's go with a number established by Charles Lineweaver who estimated that 13% (fl=.13) of planets have sprouted life.
Fraction of planets with life that actually go on to develop intelligent life (civilizations): Another tough one. There have been billions of species on Earth, yet only one has developed the capacity for radio communication. It's not immediately obvious that evolution favors human-like intelligence, preferring instead other sorts of adaptations. What's more, it not obvious that all human-like intelligences go on to form technological civilizations. Most estimates place this figure somewhere between 50 and 100% — but that seems absurdly high. Let's go with something more reasonable, like 1-in-10 (fi=0.1).
Fraction of civilizations that develop a technology that releases detectable signs of their existence into space: This one's probably quite high. Even pre-atomic civilizations are capable of this. Let's go with a figure of 80% (fc = 0.8). Drake himself said this figure should be between 10-20%.
Length of time for which such civilizations release detectable signals into space: This variable is challenging to assess because we ourselves are about to go radio silent. But let's assume that civilizations will engage in METI efforts and deliberately send signals into space — regardless of the potential risks. In fact, we're already doing this. But there's also the possibility that technological civs are short-lived owing to existential risks, including nuclear war, artificial superintelligence, and molecular assembling nanotechnology. Humanity could conceivably wipe itself off the intergalactic map at some point in the coming decades. But should we survive our technologies and the accompanying technological Singularity, we could be around for a tremendously long period of time — something on the order of millions of years. But because that's highly speculative, and because the Fermi Paradox and the Great Filter hypothesis tells us otherwise, let's go with 200 years (L=200). Drake gave values between 1,000 and 100,000 years.
Okay, with that out of the way, let's do some math:
7 x 1 x 0.34 x 0.13 x 0.1 x 0.8 x 200 = 5.
So, if both sunlike stars and red dwarfs host habitable planets, we get a figure of five communicable civilizations in our galaxy (including ourselves). This implies that, probabilistically speaking, our nearest radio-capable neighbor is 22,000 lightyears away. That's highly discouraging, to say the least.
But it gets considerably worse if we exclude red dwarfs. In that case, our equation reads:
7 x 1 x 0.04 x 0.13 x 0.1 x 0.8 x 200 = 0.58.
Which is a tough pill to swallow considering that we're here! What this might imply, however — aside from the fact that our variables may be wrong — is that we may in fact be the only ones in the Milky Way right now who are spewing radio signals out into the cosmos.
As for Frank Drake, he came up with values for N ranging between 1,000 and 100,000 civilizations in the Milky Way.