21/06/2013 12:55 BST | Updated 21/08/2013 06:12 BST

We Are Not Alone

With the closure of the Ministry of Defence desk specifically dealing with claimed alien encounters in the UK, we now have no formal department to analyse - or respond to - any potentially legitimate claim. In other words, if E.T. phones home, all he'll get is 'number unobtainable.' But how likely is a call from E.T. in the first place?

Well, let me put it this way: I'm not a conspiracy theorist but I do believe in aliens. I also believe in interstellar spaceships and life on other planets. Is this rational? Far from being outlandish, this is the only logical conclusion to draw about the universe and our place in it.

Think about the size of our cosmos. We live on a single planet in a galaxy which may have about six trillion planets - that's 6,000,000,000,000 planets - a huge number. It's inconceivable to me that out of all those other planets, not one is like earth. More probably, there are many earth-like planets. Since life started on our earth - and probably more than once before it took hold - it's reasonable to believe life also started on at least one of those other planets too.

What if life only started here on earth? Well, even then, there's a good reason to believe it can spread. There's a respectable theory which explains how meteorites could transport bacteria and the basic building blocks of life from one place to another, even through the harsh environment of space. Across billions of years that gives life a long, long time to drift from one place to another, seeding suitable habitats, just as weeds seed a playground, eventually growing in any crack or habitable part of it.

Given multiple life-bearing planets, it's fair to suggest some life evolves to intelligence. Then it's a question of curiosity: will they, like humans, want to explore? We reached the moon a mere 66 years after mankind's first powered flight. No doubt we'll be capable of extraordinary voyages in 1,000 years, let alone in a million years' time. Again, it's plausible this is true of other civilizations, which may have reached technological maturity a billion years ago or more.

So, what does that mean for the chances of anyone visiting? Well, if my first two points hold true, then it would seem likely that somewhere along the way they've visited us. After all, if you've got a hundred million years to look around, you can explore a large proportion of the entire galaxy even at a relatively sedentary pace.

So where are they? Well, three possibilities here. The first one is that UFO sightings are genuinely extra-terrestrial in origin. I regard this as unlikely, given the very large proportion of them which are subsequently explained in more conventional ways. Next, perhaps, they have visited, but in keeping with 'Starfleet Command's' directive in Star Trek, they carefully avoid interfering with evolving and primitive civilizations such as ours. Thus they watch from a careful distance without revealing themselves. This, by definition is impossible to prove but also makes sense in terms of biological safety: there's a real chance of accidental catastrophe from mutual diseases against which one or both species have no immunity.

The third option is the darkest. Maybe we really are alone. Since I believe intelligent life forms would evolve into space travellers, this suggests either there are no other intelligent life forms - or that they don't survive long enough to explore. Humans are more than capable of annihilating themselves in what the astronomer Carl Sagan called an 'unforgivable act' of nuclear neglect. In this scenario, civilizations come and go through their own suicidal warring tendencies - a prospect also loaded with religious connotations of Armageddon.

But, like Sagan, I choose to be optimistic - to believe we're going to make it through, and that others have too. Then aliens have quite possibly paid a visit - but kept themselves quiet for now.

And finally, consider this: that's probably what we'd do if we make it across the next 10,000 dangerous years. By then, we will be interstellar travellers and find habitable planets, and life. Then, to loosely quote Samuel Beckett, 'we have found the extra-terrestrials - and they is us.'