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How many rocks from the sun?

Are we alone in the universe?

This is a very big question, and one that is very difficult to answer! Even if the universe was teeming with life, how hard would it be to find? The answer to this question is – very hard indeed! The main reason for this is that the universe is so big and so empty.

So, just how big and how empty is the universe?

Human beings have a real problem grasping any kind of scale. We evolved in a society where until very recently most people would never have travelled more than a few miles from their villages or towns in a whole lifetime. If you ask most people how big the Earth is, or how far away Australia is you’ll get a lot of pretty vague answers. Try it!

Where are we in relation to our neighbours?

We live on a small rock planet about 12000Km (8000 miles) in diameter. This sounds pretty big. If you tried to walk a distance equal to the circumference of the Earth it would take you about two years if you walked twelve hours a day.

Around the world
  • The nearest object to the Earth in space is the moon: this is about 384000Km (236000 miles). It would take about 13.5 years to walk this distance.
  • The distance to our nearest star, the sun, is about 93,000,000 miles. It would take about 5300 years to walk this distance.
  • The distance to the nearest star after the sun is about 26,000,000,000,000 miles (4 light years). It would take about 1.5 billion years to walk this distance.
  • Our local cluster of stars, the Milky Way Galaxy, is about 100,000 light years across. It would take 70,000 billion years to walk this distance.

As you can see, the universe really doesn’t do things on a human scale. The numbers become very silly very quickly.

The Galaxy contains lots of stars: about 300 billion stars is a reasonable estimate. If each star was an M&M you would pack 6 double decker buses to contain them all. Yum!

M&Ms Busses

The problem is with the stars is that they are all rather spread out. The distance between each M&M, if they were packed at the same density as the stars in our galaxy, would be on average about 386.25km (240 miles). This means that although there is a lot of stuff out there it is spread out very, very thinly. If the Galaxy was scaled to the size of a large football stadium it would appear like a very wispy cloud. It would be like fine smoke, and if you walked through it you would feel nothing.

The distances between the stars are vast. If we travelled at ten times the speed of the space shuttle (281635.2km or 175,000 miles per hour) it would still take us 16,000 years to reach the nearest star. So visiting any potential neighbours is probably a long way into the future.

Any aliens in the house?

Erich Von Daniken wrote a best selling book called ‘Chariots of the gods’ claiming that aliens had visited the Earth in the distant past. He said that the ancient people’s had recorded their appearance in pictures and carvings, often on temple walls. Much of his ‘evidence’ has since been disproved. It didn’t stop him selling over 60 million books though! So how are we going to contact ET when there is so much space to explore?

Ancient Drawings

Bring on the big ear!

Just about our only chance of detecting ET at the moment is to listen out for radio signals. This is an incredibly difficult task. Radio signals get weaker the further they have to travel. In fact, the radio energy picked up by all the radio telescopes in the world since the dawn of radio astronomy amounts to less energy than the single beat of a butterfly’s wing.

The signals can come from any point in the sky and at any one of billions of frequencies. We also have the problem of man-made radio signals. A mobile phone produces a signal billions of times stronger than the one we’re listening for. And there are quite a few mobile phones about.

This is a really difficult problem to crack. The things you really need are:

  • a radio telescope that is really big so that it can pick up as much of this incredibly weak signal as possible
  • it must be able to point in any direction and see a large area of sky at any one time
  • it must be able to see a wide range of frequencies
  • it must be able to pick out the one important signal from massive amounts of background noise

This is what the Allen Telescope Array (ATA) is designed to do. It is funded largely by Paul Allen, hence the name. Paul Allen (the clever one) and Bill Gates (the one with the dodgy jumpers) co-founded Microsoft in the 1970s.

Allen Telescope

The ATA is being built in California about a hundred miles north of San Francisco. Once it’s completed it will have about 350 antennae, each with a collecting area of over 28 m2. The whole thing will have a collecting area of about 10,000m2. This is the equivalent of a huge single radio telescope over 110m in diameter; this is about the length of football pitch! The big advantage of this design and its size is that each component is small enough to steer and can therefore cover lots of sky very quickly. Eventually it should be able to look at around 100,000 nearby stars. The ATA should be able to listen to over a 100 million channels simultaneously. It does require a fair bit of computing power to do this, around the same as 100,000 PC’s!

What’s the chance of finding a signal?

This is a very good question. To answer this we first need to ask ourselves a few questions. This is what Frank Drake did back in 1961. He produced the famous Drake equation.

The questions go like this:

How many stars are in the Milky Way Galaxy?

(Answer – about 300,000 million)

What percentage of these stars have planetary systems?

(Answer – recent searches for extra solar planets suggest more than 90 per cent)

For each star that does have a planetary system, how many planets are capable of sustaining life?

(Answer - tricky one this, about 1-10 maybe?)

On the planets that are capable of sustaining life, what perceentage of life does actually evolve?

(Answer – even more tricky this one. It could be anywhere between 100 per cent and 0 per cent. If we do find traces of life on Mars in the next few years, and we may, it would put the figure close to 100 per cent. Take a guess.)

On the planets where life does evolve, what percentage evolves intelligent life?

(Answer- Very tricky one this! The Earth has had advanced life forms on it for around 500 million years. As we far as we know humans are the only example to have evolved out of the millions of species that have existed. Say 0.0000001 per cent)

What is the fraction of intelligent life that can communicate using radio transmitters?

(Answer- Dolphins may be quite bright but they wouldn’t be much good with screw drivers. This is another really difficult one. Shall we say about 10 per cent. What do you think?)

For each civilization that does communicate, what fraction of the planet's civilization survive?

(Answer- probably the most difficult question of all. The only example we have is Earth. The Earth has been around for about 4.5 Billion years. We have only been producing radio signals ourselves for around 100 years. Our civilization has nearly destroyed itself once already in the 1960s during the Cuban missile crisis. How long are we likely to survive? We could destroy ourselves tomorrow with nuclear or biological weapons. A large meteor strike would achieve the same result; so would unchecked population growth. Think this one out for yourselves. If our civilization survives for a thousand years it would still only be 0.000001 per cent of the age of the Earth).

If I put my best guesses in and multiply the numbers together to get the number of civilizations in our Galaxy that we may be able to communicate with I get about 300. How many do you get?
If my guess is correct we are looking for one star in every billion. If we checked a thousand stars a day it could still take 2,500 years to find the one with the civilization!

Don’t despair though. The rate at which computers are progressing we will be able check loads more than that soon enough. I suspect that if you are in school at the moment we will certainly have tuned into ET before you reach middle age. How will it change our view of ourselves to know that we are not alone? Now that really is an interesting question! Don’t look down at your boots, look up and claim your place in the Galaxy.


Produce a PowerPoint explaining the true scale of the universe. Include as many real images as possible. The NASA Hubble web site is a fantastic place to start.

The big questions:

Are we alone in the universe?

Does it matter one way or another?

Would contact with another civilization change the way humanity thinks about itself?

How would different religions respond to contact with intelligent aliens? Would they have a different God?

Is it worth the money? None of this research is cheap. If we did tune into a alien Wikipedia it may just to able to show us how to solve the worlds energy, climate and food problems.