When we think of looking for extra-terrestrial life, we tend to focus on “earth-like”
planets, that is, planets with conditions that are similar to our own. “Life exists
on earth,” the logic goes, “so earth-like conditions are probably a good bet to find
more life.”
And yet.
There are hundreds of billions of galaxies in the observable universe, each with billions
if not trillions of stars, and most stars have planets orbiting them. That’s roughly
a million billion billion planets. The enormity of this number means it’s probably safe
to assume that there are many many *many* other planets with life - even intelligent
life.
And this, together with some basic physics and fancy statistics, implies it’s more
likely that species of intelligent extraterrestrials will live on habitable planets unlike earth
and will be unlike humans. More precisely, they’ll be bigger than us and live in smaller
groups on smaller planets.
Now, I know it sounds crazy that with only one data point – us – we can make any
predictions at all about aliens we don’t even know exist. But we can – here’s
how:
A basic result in statistics is that there’s a big difference between the properties of
a typical *individual* and the properties of an individual in a typical *group*. The
majority of humans, for example, live in countries with a population of at least 180 million
people. But the majority of countries have populations of less than 6 million.
And the majority of *religious* humans are members of religions with more than a billion
followers, while the majority of *religions* have fewer than a million followers.
And the majority of people who follow the English Premier league are fans of teams with
hundreds of millions of fans like Manchester United, while most *teams* have just a few
million fans each.
It doesn’t matter how many individuals you have, or how you make the groups – they
can be religions, or fans of sports teams, or the ingredients of this trail mix. It is
a mathematical fact that the group that the median individual belongs to will be at least
as big/bigger than the median group. Or, simply put, any time groups are not all the same
size, most individuals will be members of groups that are bigger than most of the other
groups.
The takeaway is that an individual should expect to be a member of a large group, not
an ordinary one. If you don’t know what group you fall into (like, I don’t know
what my blood type is), the most likely groups to be in are the biggest ones – I’m probably
O or A positive.
And when it comes to intelligent life forms, we humans don’t know what kind of group
we fall into. So statistics tells us that we, as individuals, should expect to be members
of a large group of intelligent beings. That is, we should expect that our species has
a higher population than most other species. And just knowing that we probably have a high
population tells us a lot.
For example, individual living beings require space to live – I mean, the countries with
the biggest populations tend to have large land areas – so earth, with its high population,
is probably bigger than most other planets with intelligent life.
Similarly, smaller living creatures need less space and energy per creature, and accordingly
tend to have higher population densities: that’s why there are way more ants on earth
than elephants. So humans, with our high population, are probably physically smaller than most
other species of intelligent life.
In fact, we should expect to be abnormal among intelligent aliens when it comes to anything
that influences overall population size. Like, easily available energy makes it easier to
maintain higher populations, so we should expect our sun to be somewhat hotter and brighter
and closer than the stars of most intelligent alien species; and we should expect our atmosphere
to be more transparent to our star’s light; and so on.
If this all sounds a bit unspecific, well, with just a few more simple and reasonable
assumptions based on basic physics, we can be more precise. Researchers have predicted
that the population of most intelligent alien species should be below 20 million individuals;
the majority of planets with intelligent life should have less than 80% the radius of the
earth; and the individuals of most intelligent alien species should be at least as massive
as polar bears.
So instead of looking for nearby intelligent extraterrestrials on “earth-like planets”,
the intelligent approach might be to look for habitable planets slightly smaller, darker,
and hazier than our own. In short, we should expect to be the Manchester United
of the universe, searching for AFC Wimbledon.