By Gideon Bradshaw
BBC Horizon producer
The giant Allen Telescope Array will listen for an alien message
"There are 200 billion stars in our galaxy and at least half of them probably have planets," ponders Dr Seth Shostak, chief astronomer at Seti, the Search for Extraterrestrial Intelligence.
"How many planets in a system? Well we've got eight in ours, so let's say five.
"All right, that's a half a trillion, 500 billion planets out there; and keep in mind there are a hundred million other galaxies," he relates.
"So to think 'hey, look man, this is the only place where there's anything interesting happening'; I mean you've gotta be really audacious to take that point of view."
For scientists like Seth Shostak, the scale of the Universe means it is nearly impossible that human beings are alone.
The sheer number of stars makes it virtually inevitable that there are other planets around other stars that are just like Earth - planets with atmospheres, oceans of liquid water and ambient conditions conducive to life.
1960: First attempt to listen in on alien messages made by Frank Drake
Dr Drake creates an equation to motivate future searches
The equation is an estimate for the number, N, of detectable civilisations in our galaxy
If true, then Shostak believes it inevitable that life will have begun elsewhere; and that the laws of evolution mean that given enough time, life could have organised itself into civilisations capable of transmitting radio messages.
The trouble is that after 50 years of listening, Seti has not heard a whisper from our galactic neighbours.
And even worse - right up until 1995, no-one had any idea if planets outside our Solar System, the so-called exoplanets, even existed.
That all changed with the discovery of a huge planet orbiting the star Pegasi 51.
It was on a fast orbit, whizzing around its star in just four days, compared to the 12 Earth-years it takes our own Jupiter to orbit the Sun.
Dr Frank Drake: The "founding father" of SETI
Half the mass of Jupiter, its close proximity to its star means that surface temperatures are around 1,000C.
Indeed, the planet is so weird that it has forced astrophysicists to rethink their models of planetary formation.
Since its discovery, more than 260 other exoplanets have been discovered, but none appeared to be a second Earth.
With no planets capable of supporting life, Earth and its residents were looking more and more alone in the Universe.
Then, in early 2007, a Swiss team of astronomers led by Professor Stephan Udry working at the European Southern Observatory in northern Chile made an amazing discovery.
They identified the smallest planet orbiting a main sequence star yet found in our galaxy. It is called Gliese 581c.
It was only five and a half times the mass of our Earth and seemed to be at just the right distance from its star to be habitable.
At last, here was a planet where there was a good chance of liquid water flowing on the surface; a so called "Goldilocks" planet where surface conditions might just be right for life to begin and survive, just as it has here on Earth.
No-one knows for sure if life is there. For now, all we know of this planet is its location, its minimum mass and the distance it lies from its star.
Professor Geoff Marcy: More than 100 planets and counting
Even so, astrobiologists like Dr Lynn Rothschild are able to deduce a number of the planet's characteristics.
Gravity on G581c will be twice that of Earth. Astronauts would feel heavy on the surface, and falling would be a major hazard.
The landscape too will be different; no high mountains, just vast plains and low hills.
The star of this system is a red dwarf. It will be much larger in the sky than our Sun here on Earth and will cast reddish light.
Curiously, due to the phenomenon known as Raleigh scattering, the skies will still be blue, but clouds will have a pinkish tinge.
The red dwarf star is cooler than our own Sun, and the planet's orbit is smaller than Earth's. So in theory surface temperatures should be similar, though if the planet's atmosphere is rich in naturally occurring greenhouse gases, then it may be a bit too hot for life.
Even so, Rothschild thinks it remains a possibility, since here on Earth, biologists have found living organisms that are able to survive in temperatures up to 121C.
Other scientists are more sceptical.
Professor Geoff Marcy is the world's most prolific planet hunter.
His team has found more than 100 planets, but none of them remotely resembles Earth. He too has studied G581c and is convinced that it is not habitable.
So, the hunt for the first incontrovertible Earth-like planet continues, and a new competitor is about to enter the race.
In 2009 Nasa will launch Kepler, a space telescope with a mission to seek out new worlds. Horizon visited the factory where Kepler is being built, in the company of its creator, Dr Bill Boruki.
The Allen array will eventually comprise 300 dishes
Kepler is designed to be sensitive enough to detect Earth-like planets from day one. It will scan an incredible 100,000 stars day and night for four years.
After this time, we will know for sure just how common Earths are in the Milky Way.
Nasa's most pessimistic calculations predict that at least 50 Earth-like planets should exist within this collection of stars.
This would be the first galactic map of Earth-like planets, a "phone-book for ET".
Meanwhile, back on Earth, Horizon joins Seti scientists as they gear up to make use of this shortlist of Earth-like worlds.
Out in the desert, 300 miles north of San Francisco, the giant Allen Telescope Array is being built.
Rather than the giant radio telescopes that Seti astronomers have traditionally used, the array is a collection of off-the-shelf radio antennas; all computer-controlled and designed to work as one.
Eventually they aim to build it out to 300 dishes, making it the most powerful attempt yet to listen for an alien message.
And this time, rather than the entire galaxy, they will focus all their efforts on stars where there are planets capable of supporting life.
Horizon: Are we Alone in the Universe? is on BBC Two at 2100GMT, Tue 4 March or afterwards from BBC iPlayer