Are we Alone? Listening for extra-terrestrial life

“Is that the Earth straight ahead?”

That's the Earth, in all its glory. Only five seconds and the best part of a million miles to go.

“Pretty place, with all those bluey, greeny whitey bits. A bit small though.”

At this distance it certainly looks small. Amazingly no earthling has ever seen the Earth from this distance with their own eyes. The farthest humans have been is the moon which is only a quarter of a million miles away, just over a light second.

“We’ve been 3000 light years across space on this journey and human astronauts have only travelled one light second! That’s a bit pathetic isn’t it?”

Well, earthlings are only just starting to explore space. NASA is back talking about going to Mars which is at least a planet away.

“And how far is Mars?”

At its nearest a few light minutes.

“A few light minutes? And they’re still only talking about that? I guess it’ll be a while before humans make the return visit to our star.”

They have sent machines, or space probes as they call them, off into the outer solar system to Saturn and beyond. Remember we talked about voyager and the golden disc a while back?

“Not exactly Star Trek is it? Will humanity ever travel into space and meet aliens?”

We might but it is more likely they will visit us than the other way around.

“Why is that?”

Well, we have only just started wondering about life on other planets in the last hundred years – a mere blink of the eye in terms of the age of the universe. Humans only discovered the first planet around another star ten years ago. If there are other civilisations out there asking the same question as us, the chances are they are more advanced.

“I don’t see why?”

If there are lots of intelligent civilisations who know that planets exist around other stars and we have only just discovered this, then we must be the newest member of the galactic civilisations club. So it stands to reason that the other members have been in the club for longer and so will be more technologically advanced.

“Unless we are the only member of the club.”

True, but that is the real question - are we alone?

“How can we answer that question without just waiting for someone to visit.


We can try and listen for their signals. When we talked about this last time, I explained how radio and TV signals from earth can’t have travelled very far into the galaxy because we’ve haven’t been transmitting for that long. The signals from other planets, if they are more advanced and using radio waves for thousands of years, should have been travelling for far longer so could have spread out across the whole galaxy by now. This is what the organisation SETI, which stands for Search for Extra-Terrestrial Intelligence, does – listen for radio signals from space.

“Have they found anything yet?”

Not yet but there is a lot of space out there. They have been getting ordinary people to help by using their own computers, almost 3 million people are helping in the search.

“But if they have found nothing so far is there any point in keeping trying?”

Heavens above, what a question, of course. Scientists have tried to calculate the chances of there being other civilisations in our galaxy and it seems unlikely that we are completely on our own.

“How can they know that?”

One of the founders and now president of SETI, Dr. Frank Drake, came up with a way of came up with a way of caculating the chances of finding intelligent life in our galaxy by listening to radiowaves from space – The Drake Equation. To work it out you need to first answer seven questions. I’ll take you through a slightly simplified version of the Drake Equation and we can answer them together as they are tricky enough. First, we need to know how many stars there are in the galaxy. Any ideas?

“A hundred million?”

Oh, a lot more than that, perhaps a hundred billion or more. Now the next question is how many stars have planets around them. When Dr Drake thought up his equation in 1961 astronomers had no idea how to answer that question. With all the planets discovered in the last few years we now know planets are common so it might be as much as half of all stars.

“Just what I was about to say.”

The next question is how many planets are capable of supporting life, with a temperature that is not too hot or cold and an atmosphere. If there are lots of planets around these stars then the chances of one in the right zone goes up. So there might be one suitable planet around every one or two stars that have planets. So can you guess what’s the next thing we need to know?

“How often life starts on these planets?”

Exactly, well done. Even though it seems remarkable that life could start out of nothing, if it could start on earth then there is no reason that it couldn’t start anywhere else. So life might be very common, so common that some form of life might start whenever there is a suitable planet.

“But would it be intelligent life?”

Good question. On earth chimpanzees and dolphins undoubtedly have some form of intelligence. So if there are at least three intelligent species on earth then given enough time evolution should be able to create intelligent life anywhere. The sixth question is the chance that this intelligent life could build machines to send radio signals into space. This could be difficult for dolphins even if they evolved to be ten times smarter than humans because they only have flippers, so not every intelligent life form may be able to send radio waves or want to.

“How about 10% of intelligent life being able to send radio waves?”

Not a bad guess. Now the seventh and last question is how long an intelligent alien species would use radio waves for.

“Once they discover radio waves, why would they stop?”

They might blow themselves up in a war if they are like humans, or die from an asteroid hitting the planets like the dinosaurs. The other reason is that there are probably much better ways of sending signals across space that we haven’t even discovered yet. Imagine how advanced human civilisation might be in a thousand or a hundred thousand or even a millions years? A really advanced civilisation would probably have stopped using radio waves and TV-like signals thousands of years ago, in the same way that we’ve moved on from painting pictures on cave walls. They could be sending signals to us right now and we wouldn’t know because we aren’t advanced enough to know what to listen for.

“How long will humans be using radio waves for?”

We discovered them about a hundred years ago, but I can’t imagine we won’t have discovered something better than radio waves in another thousand years. So earth might be sending out radio waves for only 1000 years. For a planet that will exist in total for 10 billion years this means that there is only a one in a ten million chance of anyone listening in with a radio telescope of catching us at the right moment in our evolution. So that’s the last number we need - the number of years a civilisation sends radio signals into space.

“So what’s the answer? How many planets in the galaxy have intelligent life?”

At least one.

“That’s’ a daft answer, of course there’s at least one, the earth.”

Well you can put in your own numbers and calculate it yourself, but before you do have a wild guess.
When you are ready CLICK HERE.


p.s. If you are interested in helping SETI in the search for extra-terrestrial life please visit http://setiathome.berkeley.edu/

p.p.s If you are an extra-terrestrial life form please leave a message in the comment section.

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