The Edinburgh University astrophysicist behind research that predicts there could be 38,000 alien civilisations in our galaxy spells out how he came to that figure, and whether we are likely to get a message from outer space.
How did you work out that there could be between 361 and 37,964 alien civilisations in our galaxy?
We took the information we have learned from planets outside our solar system – such as how big they are, how far away they are from their parent star – and fed this into a computer model.
The model makes a synthetic galaxy, with billions of planets, and we can watch how life evolves in them.
How accurate are these estimates likely to be?
Sadly not very, yet. We are stuck with the problem that we don't really know how life begins, even how life began here on Earth. This is the main reason why our answers vary so much: it depends on how easy it is for life to appear.
As we learn, our answers improve – however, we have a lot of learning to do.
Is it likely we will be able to communicate with these other life forms?
Our results show that these civilisations are separated by vast distances – thousands of light years.
This would mean that any conversation would be almost impossible – it's even possible that civilisations may go extinct before messages arrive.
Are they likely to be similar to us?
Life is amazing: look at life on Earth, it takes so many shapes and forms, who knows what we might see on other worlds? It's very difficult to say.
What does the future hold – how long do you think it might be until we get proof of other civilisations existing?
Sadly, I think if aliens do decide to send a message, it would take so long to get here that we may have to wait for thousands of years.
Our results suggest that if aliens do exist, and we want to make contact, we have to be around for a long, long time.