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# How many planets exist that can support life?

There was a cascade of extreme events that led to Earth forming and becoming able to support life.

But in general, how likely or unlikely do you think it is that a planet can support life?

Are there other planets in our Solar System that had the potential to support life?

Are there other planets in our galaxy that could support life?

## Planets in our Solar System

Let’s start with our Solar System. There are eight planets in our Solar System. Given that liquid water and plate tectonics are two essential ingredients, which of our neighbouring planets have the potential for life (at least in their early days)?

Well, Venus and Mars are thought to have had both plate tectonics and liquid water early in their development.

## Planets in our galaxy

What about elsewhere in our galaxy? This question obviously becomes more speculative and we can’t expect to find a definitive answer. But, we can think about the ingredients needed and the probability that they could exist.

## The Drake equation

There’s a well known equation developed by the American astronomer and astrophysicist Frank Drake, which in part seeks to answer these questions.

The equation (or our expression of it) aims to provide a number for N:

N = Number of planets in our galaxy that can support life

Our version of the equation is as follows:

N = R* x fp x ne x L
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.
L the average lifetime of a planet.

### Working it out

We’ve covered the steps and events that led to Earth being able to support life, so we can begin to work through these to estimate what N might be.

1. Average rate of star formation in our galaxy.

In the previous steps we said that there are roughly 7 new stars formed in the Milky Way each year.

2. Fraction of those stars that have planets.
3. We discussed the formation of planets and concluded that it is reasonable to assume that nearly all stars have planets orbiting them.

4. Average number of planets with potential to support life.
5. Here we could take our own Solar System as an example. We decided that three (Venus, Earth, and Mars) out of eight planets might be able to support life.

We could also look back to a previous step and the estimate based on the recent discoveries of planets outside of our Solar System. It was estimated that 1 in 5 planets could exist in the habitable zone of their star.

6. Average lifetime of a planet.
7. Planets only exist for a finite length of time. We haven’t discussed this previously but you can assume that the average lifetime of a planet is 10 billion years.

### Calculating N

Bringing all of these numbers together, here's the answer we get for N.
N = R* x fp x ne x L = 7/year x 1 x 3/8 x 109 year = 7/year x 1 x 0.375 x 109 year = 1.4 x 109 = 2.625 billion
An alternative answer: N = R* x fp x ne x L = 7/year x 1 x 1/5 x 109 year = 7/year x 1 x 0.2 x 109 year = 1.4 x 109 = 1.4 billion

We may expect to find either 2.625 or 1.4 billion planets with the potential to create and maintain life. The exact number is not important as we are dealing with an estimate that contains large uncertainties.

But the number does indicate we could be dealing with vast numbers of planets that have characteristics similar to Earth, Venus, and Mars.