Around 40 years ago, the astronomer Frank Drake estimated that there were 100 civilisations within our galaxy, the Milky Way, capable of communicating with us. Now his estimate has gone up to 10,000. Drake was the founder of SETI (Search for Extra-Terrestrial Intelligence), whose scientists are the world’s principal alien hunters.

As part of my new series, The Cosmos: A Beginner’s Guide, I visited their new telescope, the Allen Telescope Array (ATA), in northern California, which will listen for alien messages 24 hours a day. When it is complete, the ATA will comprise 350 aluminium dishes, each six metres in diameter, which will scan the heavens. The SETI astronomers believe that any intelligent aliens are most likely to live on planets orbiting around stars, so they will start with the nearest stars and listen to each in turn. They hope to scan more than a million stars. There are a hundred billion stars in our galaxy so it will be like looking for a needle in a haystack. But their chief astronomer, Seth Shostack, has bet me a pint that they will pick up a signal by 2030 Supposing that they do pick up a few beeps coming from another planet. How will they know whether it really is a signal, rather than just noise? How will they attempt to work out what it means? Should they reply? What should they say? Or should they act first and send out their own signals? Fortunately there is a team, the InterStellar Message Design Group, working on these very questions. With luck they will be ready when the time comes. [[ In our solar system there are eight planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. We can be sure that there are no intelligent creatures on seven of them – the first two are too hot, and the outer ones are too cold. But what about planets orbiting other stars? The problem is that they are a long way away and very dark. Unlike stars, which shine brightly, planets are dim and are lit only by their parent stars – looking for them is rather like trying to catch a cricket ball when the sun is shining in your eyes. However, the technology has advanced so much in the past decade that astronomers have been able to find more than 200 of these so-called exo-planets.

My favourite planet hunter lives in a garage with a sliding roof, perched on the rim of an old volcano in the Canary Islands. Super WASP (Wide-Angle Search for Planets) is an array of eight digital cameras mounted on a robotic arm. They photograph the night sky, capturing 50,000 stars in each picture; then the arm moves around, and they take another set. By the end of the night they have taken 600 photographs. The brightness of every star in every picture is measured, and if any star has become slightly dimmer for a while then there might have been a planet passing in front of it, blocking out some of the light. In its first year, Super WASP looked at seven million stars, found 18,000 possibles, 100 probables and what turned out to be two new exo-planets.

All the exo-planets found so far have been “hot Jupiters” – giant planets orbiting close to their suns. These are unlikely to harbour life, but the fact that there are so many suggests that there may be planets around most of the stars in our galaxy, and surely some of those must be a bit like Earth.

All life that we know about is based on water, so life is most likely to be found on planets where there is liquid water on the surface – in other words, with a temperature between about 0 and 60 degrees Celsius. This puts them in the “habitable zone”, where life might be able to evolve. We know that at least one of the exo-planets has water in its atmosphere, so the chance of finding life seems to be increasing.

Many people remember Carl Sagan’s 1980 series Cosmos. Since then there have been immense advances in technology and computer power, and as a result the subject has moved on. Sagan had no knowledge about exo-planets, for example. That is why I wanted to tackle cosmology again, to bring the ideas up to date. The Cosmos: A Beginner’s Guide starts with the story of the alien chasers, and goes on to look at the other remarkable areas of research. We look at some of the spacecraft and robots that have been sent out to explore our solar system. I was amazed by the story of Voyager I, which was launched by Nasa in September 1977 and is still sending back a message every night – even ET didn’t phone home that often. After whizzing around Jupiter, Saturn and Saturn’s biggest moon, Titan, Voyageris now zooming out of the solar system – it is heading for the stars at a million miles every day. But even at that colossal speed, reaching the nearest star will still take 40,000 years – the universe is vast.

And beyond our galaxy, there are a hundred billion more galaxies. Surely, somewhere, there must be life. The only question is, will we find it? Or will it find us?

The Cosmos: A Beginner’s Guide, Tues, BBC Two, 7.30pm

THE UNIVERSAL TRUTH

If we sent a radio message to aliens orbiting our nearest star, it would reach them in four years.

Our nearest star, Proxima Centauri, is four light-years away, or 40 million, million miles.

The universe is about 100 billion light-years across.

If the solar system were the size of Wembley Stadium, the Earth would be as big as a pea.

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