Again, looking at what we’ve found in our Milky Way alone with these, 1.5 thousand confirmed planets so far, we know there must be billions of so-called habitable planets, just in our Milky Way galaxy. Not in the Andromeda galaxy the one I’ve shown you this another galaxy. And not in the hundred billion other galaxies that we can see in the observable universe.
Just in our own, there must be billions of habitable planets, potentially habitable planets. In other words, ones that are not too close to their star, not too hot; not too far away, not too cold so they’ve got liquid water.
Now, have aliens visited earth?
Who thinks aliens have visited the earth? Oh, nobody’s daring to put their hands up. OK, there’s quite a lot of people around the planet who do believe aliens visit the earth and visit the earth all the time and indeed claim to have met them.
But I don’t think there’s actually much good evidence for that. But, it’s sort of an interesting question, might they’ve done – you know, we’ve only been around for… Well, modern humans have not been around for that long at all, in terms of the history of the planet, just a few hundred thousand years.
So, might aliens have visited the earth at some point? Well, here’s an interesting sort of thing to think about. And it’s something we might do in the future, which is to send out a fleet of what are called von Neumann Probes. They’re space probes that you send off and they go out to search around other stars to see what they can find around other stars.
We’ve just discussed the fact that these stars are very far away, even light takes four years to reach just the nearest another star. We can’t make things travel at the speed of light, the fastest spacecraft travels at something like 60,000 km an hour or that sort of speed, a small fraction of the speed of light.
So, imagine sending out one of these space probes traveling at… that’s a fraction of the speed of light. That’s the sort of speed we can send things out at the moment. It would take it, you know, maybe 50,000 – 100,000 years to reach the nearest another star.
But when it gets there, what it does is it self-replicates. So, it gets there, and maybe along the way it gathers material from the stuff between the stars maybe, when it gets to a planet around one of those stars, it gathers more material and it builds a copy of itself.
And then, actually, those spacecrafts set off again and they head off in different directions and they go to the next another stars. And when they get there, each of those self-replicates and sets off again. This sounds a bit like science fiction, but, in fact, it’s possible to imagine that we’ll be able to do this in the not-too-distant future.
And again, if you look at how long this takes — you can calculate how long this process would take. You can actually explore the whole galaxy in less than a hundred million years.
Now, to an astronomer that sounds quite quick, because the earth is about 4.5 billion years old, 4.5 thousand million years. This was less than a hundred million years and you can speed it up by making multiple copies or making them travel a bit faster. So, this is actually short on the lifetime of a planet like the earth or on the lifetime of the stars that are in the galaxy.
So if you imagine that there might be life on one of these planets doing this, one interesting question is, where are the aliens? If there had been a civilization before us, that was able to do this, might they’ve done this, might we’ve actually been visited by these fleets probably robot spacecraft self-replicating and spreading through the galaxy. We haven’t seen any evidence for that, but it’s an interesting question.
We’ve not been here for very long in the history of earth, this is something we’d be looking for. It’s called the Fermi paradox: if Aliens exist where are they? because, really, this is actually not too long to populate the galaxy.
How are we going to look for life on other planets? Well, one of the ways to look for intelligent life, and I’m only going to talk about intelligent life here, and that’s because we know that on the history of life on earth most of that time life was bacterial life. So, it’s basically microbes for billions of years before it evolved into anything complicated like plants or animals or even intelligent life like us.
So it may well be that bacterial life is common but if we’re interested in intelligent life, if we wanted a conversation with some of these aliens, one of the ways we might do it is by using a radio telescope. This is the one at Jodrell Bank where I work, the Lovell Telescope and this is just a recording of the sort of signal we pick up with that telescope.
(Radio signal sounds)
But, you may get a little bit bored. It just sounds like hiss. It’s just noise. It’s actually radio waves. In that case, it’s radio waves that came from a star that exploded in about the year 1670, as it happens. You know, you can’t really hear much. It just sounds like noise.
If you were expecting to pick up a signal from aliens, you’d probably expect something a bit more complicated in that signal than just that noise.