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# The ingredients for complex life

This week we’ve looked at extreme events on cosmological and geological timescales.

In Activity 2, Dr Morten Andersen explained how Earth was formed and took the shape we know today.

In Activity 3, Dr Ernest Chi Fru, focussed in on events that highlight the importance of CO2 in the atmosphere (without it the planet would be inhospitably cold) and oxygen - without which life would not exist.

Given that we need extreme, i.e. rare and large events to be able to support intelligent life, are we unique in the universe? Or did other intelligent lifeforms evolve on other planets? Humans have pondered about this question through the ages.

## The Drake equation

Let’s go back to the Drake equation. Earlier we calculated how many planets in our galaxy could support life. We can adopt the same equation with a couple of additions to consider whether other planets can and have supported intelligent life.

First, let’s redefine N as follows:

N = the number of civilisations (intelligent life) in our galaxy

The modified Drake equation is as follows:

N = R* x fp x ne x fl x fi x L

The first three terms are identical to the ones we used earlier. This is because in order to have intelligent life, a necessary (but not sufficient) condition is that we have a planet with the potential to support life. So we again have:

Value Description
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.

Once we have a planet with the potential to support life, there are two extra steps to get to intelligent life.

### Step 1

The first one is to determine, how many of these planets actually develop life.

fl = the fraction of planets that could support life that actually develop life at some point

We have very little data we can use to help determine this value. In our Solar System, three planets had the potential for life to emerge, Earth, Venus and Mars. We only know for sure that life developed on Earth. So, an educated guess for fl could be 1 in 3.

### Step 2

The next step is to consider, if life did emerge, what are the chances of the development of intelligent life. For that we define:

fi = the fraction of planets with life that actually go on to develop intelligent life (civilisations)

There is even less data on this. The only planet we know of, where life emerged (Earth), also developed intelligent life. So, if you are an optimist you can set this value equal to 1. We also know for sure that this cannot be zero, since our existence shows that the likelihood must be more than zero.

### Step 3

Finally, we have to redefine L. Again it is a length of time; however, it is no longer the time of the existence of a planet, but that of a civilisation:

L = the length of time for which such civilisations exist

To guess L, we again have little information to draw on. Humans have been around for at least 100,000 years, but it is unclear how long we will continue to exist. Since that is the only number we have. Let’s stick to 100,000 years.

### Calculating N

Bringing all of these numbers together, you can calculate N. What answer do you get?

We get the following two answers:

N = R* x fp x ne x fl x fi x L = 7/year x 1 x 3/8 x 1/3 x 1 x 105 year = 87500

If you are more inclined to accept that we are alone in our galaxy, this answer can be adjusted by changing our guess for the factor fc to 1/87500.

N = R* x fp x ne x fl x fi x L = 7/year x 1 x 3/8 x 1/3 x 1/87500 x 105 year = 1

Our answers are not meant to be precise, rather they help us reflect on the conditions that are necessary for life.

Based on these potential answers we could conclude that we expect roughly between 1 and 87500 (could be rounded to 100000 if we are generous) habitable planets in our galaxy that have life on them.

Although the most optimistic estimate might seem like a lot, our galaxy is so large it would be like finding a needle in a haystack.