Alien life is a staple of science fiction. In my latest novel, First Command, a crew of astronaut cadets crash land on a habitable planet and must survive an array of hostile life forms native to that world. But how likely is it that there actually is life on other planets?

Are we alone in universe?

The Drake Equation

In 1961 at the first Search for Extra Terrestrial Intelligence (SETI) meeting, Astronomer Frank Drake proposed an equation to stimulate scientific dialogue on the topic. The equation is a means of estimating the number of intelligent alien civilizations that humans might make some form of contact with. While scientists don’t really expect it to come up with a precise value, it can still help guide efforts in this search. It also serves as a convenient way of breaking down the different dimensions of the problem–dimensions that span a wide array of sciences.

The Drake equation looks like this:

N – The number of alien civilizations that humans could communicate with.
How many alien intelligences are we likely to be able to communicate with? 10,000? 1? If it’s much less than 1, then there isn’t much point in trying to communicate with anyone else. But if it’s a lot larger than 1, it rather begs the question so eloquently asked by the Italian-American physicist Enrico Fermi… where is everybody?

To estimate this number, we consider seven (mostly) independent factors.

R_* – The mean rate of star formation in our galaxy
About 10 billion years in, the Milky Way galaxy has converted roughly 90 percent of its initial mass into stars. Initially R_* was estimated at about 1 star per year, but some more current astronomical research puts this at about 7 per year and some estimates are as high as 20. It’s important to bear in mind though that there’s also a lag factor at play in the game. The relevant R_* that was 5 billion years ago when our own sun was formed, may have been different than it is now.

f_P – The fraction of those stars that have planets
It turns out that stars with planets are relatively common in the universe. To a rough approximation f_p ~ 1. Digging a little deeper, according to NASA, about one in six stars has as Earth-sized planet orbiting it.

n_e – the mean number of planets that could support life per star with planets
These are the “Goldilocks” planets. Most often this factor is associated with the physical conditions required for water–too cold and you live on an ice cube, too hot and you’re in a cloud of steam, but just right and you have conditions for liquid water. Astronomers define a “circumstellar habitable zone” as the range of distances from a given star where liquid water could exist. Sometimes this is called the “Goldilocks” zone after the character who needed her chair, porridge and bed to be just right. Initial estimates of n_e were as high as 5, but I think it’s important to recognize that water’s not the only element necessary for life. The planets would also need to have the other necessary raw materials too, which cuts this factor down considerably.

f_l – the fraction of life-supporting planets that actually develop life
Now we get into the variables with much wider ranges of uncertainty. That fact of the matter is that in terms of planets that have actually developed life, we have precisely one data point. That’s not much to go off of. Still, that hasn’t stopped scientists from attempting to address the idea. If even microbial life were to be found on Mars or on one of Jupiter’s moons, that would imply that f_l is relatively close to 1. There is also an argument that since life began on Earth soon after the geological conditions were favorable, that the emergence of life must be relatively common. Really I think the basic transition here is going from having the correct set of molecules and getting precisely the right conditions where they can spontaneously start to replicate themselves (i.e. RNA and DNA).

f_i – the fraction of planets with life that go on to develop intelligent life
Once you’ve got basic microbial life, then you go through billions of years of Darwin Awards. New species arise and go extinct. Life survives through a series of mass extinction events (we’ve had five so far and are arguably in the midst of a sixth). And eventually you get something as complex as a primitive human. On one hand of the argument, you have a group that argues this factor should be very low. After 3.5 billion years of evolution we have only one intelligent species out of billions of branches on the tree of life. On the other hand are scientists who argue that complexity in living things has increased over time and given billion year timescales, intelligence is rather inevitable and therefore f_i ~ 1. (But try telling that to your grade seven math teacher!)

f_c – the fraction of intelligent civilizations that become capable of communicating across space
Now comes the tricky part of going from making fire and wheels to building microprocessors and radio-telescopes. Oh, and not annihilating ourselves somewhere along the line. Again given our one data point, humans have only just reached this point. Some might argue we’re not even there yet. Sure, we’ve been emitting somewhat coherent radio waves for the last century, but it’s really only been since the Cold War that we’ve emitted radio signals powerful enough to make it beyond our own atmosphere and the radio noise from the sun. Personally I think once a species gets to “wheel and fire” it’s just a matter of surviving mass extinctions until we become capable of long range communication, so I’d estimate f_c ~ 1.

L – the mean length of time that those civilizations could communicate
And back to not blowing ourselves up. Or making our planet unfit for human habitation. Or consuming all our food. Or not wearing masks and not getting vaccinated during a global pandemic. Or producing enough Bruce Willis’s to save us from rogue meteors. This factor basically argues that it’s all just a matter of time folks. The optimists would argue that once we get to the communication stage we’re likely to expand beyond many of these threats and therefore if even a small fraction of civilizations make it to this “immortal” stage, L can reach billions of years.

According to Wikipedia, on the low end we have N ~ 9.1 x 10^-13. For anyone who doesn’t understand scientific notation that’s about one in a trillion, or a very very very small number. For all practical purposes… we’re alone. On the optimistic end however, you’ve got N ~ 1.56 x 10⁷ (15,600,000). We’re like on person amidst double the population of New York City. If that’s the case, there are all kinds of parties going on out there. We just don’t seem to have been invited yet. Maybe humans just aren’t cool enough. Or maybe there are some aliens hanging out at the galactic punch bowl staring at us and quickly looking away whenever we look up, trying to work up the nerve to talk to us.

As a science fiction writer and scientist myself, I tend to favor the more optimistic side of things. But one of the big challenges we face, again comes back to our single current data point. We have only life on Earth as our example to guide us in our search. If and when we do encounter some kind of alien civilization, will we even be able to recognize each other?

So, what do you think? Is there anyone out there?

*Photo above is attributed to:
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