Playing with the Drake Equation

Thinking about the existence of extraterrestrials is always fun. One way to be self-indulgent is with the Drake Equation. While criticized often, it’s at least a reasonable tool for engaging in thought experiments until we discover who our neighbors are. Here is my shallow thoughts on the topic. After a quick web search I settled on the calculator from, since it looks pretty.

# R* = 7: Number of stars born per year

According to, seven new stars are created in the galaxy per year.

# ƒp = 100% : Percentage of stars with planets

It seems that practically every star has at least one planet orbiting it.

# ne = 3 : Number of habitable planets per system

I’m not a fan of the term “habitable” here, so I’ll interpret this to mean planets with enough warmth to support liquid water. If this definition is further extended to include liquid hydrocarbons (to include the satellite Titan), three planets on average seems to be a reasonable number.

# ƒl = 100% : Percentage of habitable planets developing life

The definition of “life” is glossed over or vague in most places I’ve seen. The definition I prefer is the one given by Erwin Schrödinger: Anything which prevents decay into equilibrium. In other words, things that resist entropy. Given that life on Earth is composed of elements in a ratio consistent with the ratio of those same elements in the universe, no additional thought is needed on what composition the planets must have. Additionally, life on Earth started nearly immediately. Thus, life seems inevitable given appropriate conditions.

# ƒi = 1%: Percentage of life developing intelligence

Once again, a vaguely defined term: “intelligence”. I could spend all day pulling that apart, but since this post is just a half-assed indulgence I’ll arbitrarily choose the Bronze Age as a marker. On Earth we know humans fit the definition, including our now extinct humanoid cousins. Some dinosaurs may be considered intelligent by this definition as well if I’m being generous.

Additionally it should be noted that intelligent life is significant in size and multi-cellular. It seems that an oxygen rich environment is a requirement to achieve these body sizes.

Is Eusociality an additional requirement? I’m not sure, so I’ll ignore it. At this point the percentage is already small, but a little bigger than I first thought given the mentioned extinct species. I’ll choose 1% as a liberal estimate

# ƒc = 0.01: Percentage of life that can communicate across space

Can communicate or does communicate? In either case it’s near zero. Humans have barely started broadcasting themselves. To be liberal again, I’ll choose 0.01%.

# L = 10,000,000: Length of time a civilization broadcasts

This is a blind guess based on the fact that large species like mammals have a species lifespan of approximately 1,000,000 – 10,000,000 years. I took a more liberal number as we’re the only species having achieved our level of technology and the fact we started broadcasting relatively soon in our civilization.

Cynics would say we’re going to kill ourselves off relatively soon due to one reason or another, but the long-term trend from 10,000 BC to now has been positive. Is past performance an indicator of future results?

# nr = 3: Number of times civilization could re-develop

A giant meteor, a black plague, a nuclear war, or some other giant eraser can wipe the board clean and civilization would have to start again. We know what happened to the Dinosaurs, and it might have happened to Humanity at the end of the last ice age. Assuming extraterrestrial impacts as the most likely cause of civilization death, and assuming these occur primarily during the younger days of star system formation, I’m going to make a guesstimate of 3.

# Are we alone

Fill in the blanks and turn the crank and I get an undoubtedly overestimation of:

840 Communicating Civilizations in the galaxy

Given the 200,000 light year diameter of the Milky Way, no wonder it seems so empty… If we are distributed evenly across the galaxy, there would be nearly 250 light years between each of us.


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