My opinions on extra terrestrial space aliens
One of the occupational hazards of being an amateur astronomer is the evil curse of the space aliens. I'm often asked for my opinion on the prospect of life beyond Earth. "Invaders from another planet", "extra-terrestrials" "space aliens", "beings from another dimension", call them what you will, these little green men have cost me some serious time in pubs dispensing my ideas to those who, perhaps mistakenly, value my view on the topic because I spend do much of my time looking up at the stars.
So what is my opinion on life beyond Earth? I think it is best summed up as "Definitely, yes. Maybe"
Simple extra-terrestrial life
The first point I make is that there are several levels to "life". By life we could mean single celled organisms living by a hydrothermal vent in a deep ocean, maybe under a crust of ice. Definitely. I have no doubt in my mind that this level of life is prolific around the universe. I would not be surprised one iota if such life was discovered in our own solar system on Europa or other planetary moons.
With more complex organisms, again, I am pretty confident these exist out there. The problem with primitive life of this nature is it is very difficult to detect it at astronomical distances. To find life in our solar system, we will have to go there, dig it up, tunnel in, and look at what is there. Whilst this is possible for bodies in our own solar system, it is utterly beyond the state of the art to do this for other stars in our galaxy - largely because they are such a chuffing long way away.
Spotting the ameba
Technological advances on the horizon may make this slightly less impossible. Some of the very large aperture telescopes, such as the OWL telescope (overwhelmingly large telescope) will potentially make it possible to see the light from planets around other stars. Once this is possible, astronomers will be able to detect the chemicals of life (such as O2) in the atmospheres of these other worlds using spectroscopy.
As before, I won't be in the slightest surprised at these discoveries being made in due course.
One thought that always strikes me when thinking about this is that the odds are that any advanced civilization out there is vastly more developed than our own. As well as being technologically advanced, they are also going to be infinitely more mature than us. We have only been broadcasting our presence for the last 50-100 years or so. Even today we are still mindlessly killing our fellow men, destroying our natural resources and generally behaving like cave-men. Only a few thousand years ago we were cave-men and scarcely able to communicate with the next swamp without throwing rocks and spears.
So, because of that train of thought, I often end up thinking about the "zoo hypothesis" - they are out there, they can see us, but they are isolating themselves from us until we reach a certain level of civility. At this point they will be able to risk communicating with us because we have grown up to the point where we are not going to lob a couple of nukes at the first visiting spacecraft. Although this theory is attractive, I'd be impressed by any civilization that can police that. In our society, a fairly good way of guaranteeing a certain behavior is to outlaw it.
Are we alone?
These days I am of the opinion that I would not be surprised either way. There may be bountiful civilizations out there waiting to be discovered. This wouldn't surprise me. However, I'd be just as unsurprised by the news that we are completely alone: The only technologically advanced civilization in the universe.
Mind you, there is a fairly good chance that even if we did discover an alien civilization, our governments would keep a very firm lid on the news. Communicating, or even detecting, with extra-terrestrial civilizations would be the single most momentous event in the whole of human history - and I don't think for one minute we'd be the same afterwards. People who run governments generally, as a rule, like to control such forces.
How Big is the galaxy?
If I am not being gloomy and decide that today I think that other civilizations do exist, my normal excuse for us not seeing them is down to the scales involved. Just for laughs, let's look at our galaxy. We float in space with about 200 billion other stars in our galaxy. The Milky Way is about 100,000 light years across, and about 3000 light years thick. These numbers are very rough, and are always being revised!
It all comes down to how rare we think life is. If there are only a few civilizations scattered around the galaxy, say 20, then things become rather spaced out. If you simply calculate the volume of the galaxy as 2.3x1013 cubic light years, and then divide by the number of civilizations, you can work out, on average, how far apart they are. For 20 civilizations they are going to be a huge 6500 light years apart. For 2 million civilizations, this figure becomes 141 light years. We have only been seriously transmitting and receiving for less than this, maybe only a few tens of years. Even with a fairly well populated galaxy, we simply haven't been around long enough to see anybody else, or let on about the fact we are here.
Primitive extra-terrestrial life: Yes. No doubt.