NARRATION
If you want to get a feel for just how special Planet Earth is, take a look at the place next door. Pretty bleak, unless frozen Martian deserts are your thing. Or visit the planet on the other side - Venus. It's so hot here, lead, zinc and tin would all melt. There's nowhere in the solar system remotely like our planet. It makes you wonder - are there other Earths out there?

Dr Graham Phillips
People have been pondering the question for centuries. But now, for the first time, we've got a chance of answering it. For the last three years, NASA's had a telescope orbiting in space looking for other Earths. They've started finding some exciting results.

NARRATION
We went off to NASA's Ames Laboratories in Silicon Valley, which are the home of this Kepler Space Telescope.

Dr Natalie Batalha 
Kepler is funded by NASA to answer one very specific question, and that is - what fraction of stars in our galaxy harbour planets that are like Earth in size and potentially habitable?

Dr Seth Shostak
One of the most exciting science experiments going on today, in my mind, is the Kepler Mission.

Professor Geoff Marcy
It's the Columbus, it's the Armstrong step on the moon, for our first steps out into the galaxy.

NARRATION
The plan to look for other Earths in the galaxy began a long time ago.

Bill Borucki
Starting in 1984, I started trying to put that idea into practice. How would we actually implement that?

NARRATION
After all, finding tiny planets in the vastness of space would be an enormous challenge. Eight years later, the Kepler Telescope was proposed to NASA.

Bill Borucki
They rejected it '92 and '94, '96, '98, so year after year we would go back and show that we COULD do all these things.

NARRATION
Kepler finally got the go-ahead to build in 2000.

Dr Graham Phillips
This is the instrument the researchers worked on to develop Kepler. It's basically a replica of the space telescope. They shone artificial starlight up from the bottom, through the telescope to the detectors at the top. Now here's the thing - they showed they could detect variations in starlight of ten parts per million - incredible accuracy, but just what you need to detect Earth-size planets orbiting other stars.

NARRATION
The reason such extreme light sensitivity is needed, is because of the way planets are detected.

Dr Natalie Batalha 
The Kepler spacecraft is actually orbiting the sun, staring at one patch of the galaxy continuously.

Professor Geoff Marcy
It watches thousands of stars to see if any of those stars dim a little bit as an Earthlike planet crosses in front, blocks a little of the starlight, dimming the star, and so we watch for stars that dim over and over and over again.

Bill Borucki
If the star dims by 1 percent, that planet's the size of Jupiter. It it dims by a tenth that, it's the size of Neptune. 100th that - that's an Earth-size planet.

NARRATION
At last, in 2009, Kepler was in space.

Bill Borucki
And almost immediately we got a stream of data coming down. It's just a wonderful feeling - an elation, you know? You're sort of walking on clouds because of all these years - 30 years or more - of putting this together, it works.

NARRATION
Big planets are easiest to find because they block the most starlight. But excitingly, Earth-size ones have started being detected, like Kepler 20e.

Dr Natalie Batalha 
Kepler 20e is orbiting very close to the star, so this whole star-facing side is just extremely hot. What you're seeing is literally a lava world, right, where one side is molten because of those very high temperatures.

Dr Graham Phillips
And the other side is cold?

Dr Natalie Batalha 
The other side can be very cold. It depends on the exact orbital dynamics.

Dr Graham Phillips
It's a really whacky world, by the sounds of it.

Dr Natalie Batalha 
It is. That's exactly right.

NARRATION
So whacky, 22e may be Earth-sized, but hardly Earth-like. But, Kepler 22b is more interesting. It's not too close to its sun, nor too far away.

Bill Borucki
You want the place that's between fire and ice - the Goldilocks zone where we think life could evolve.

NARRATION
In the Goldilocks zone, the amount of sunshine hitting the planet is just right.

Dr Natalie Batalha 
That's going to dictate the surface temperature of the planet, and the surface temperature is important because it tells us whether or not liquid water could pool in the surface.

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Liquid water is the key, because it makes the chemistry of life work.

Professor Geoff Marcy
Liquid water takes ordinary carbon-based molecules - ethane, methane, that sort of thing, amino acids - breaks them apart into the carbon atoms, the hydrogen atoms, the oxygen atoms - and once broken apart those atoms can reassemble into new, more complex hydrocarbons.

NARRATION
And complex hydrocarbons are the basis of life. Astronomers are keen to learn just how Earth-like Kepler 22b is.

Bill Borucki
It's in the habitable zone. If there's a surface on it, the temperature's pretty much like that of Earth but a little bit warmer - it'd be a little bit more pleasant there. Of course, it might be a water world or it might be a gas world - we're not sure, and so, this summer, we're trying to make more measurements from ground-based telescopes to see if it has a rocky core or not. But maybe we go there and you find all the life that is there are fish and whales.

NARRATION
Kepler IS homing in on its goal - measuring the percentage of stars that have Earth-like planets.

Dr Seth Shostak
Look we'll probably tell you, it's gonna be somewhere between maybe 0.01% and a few per cent, okay? Nobody knows yet, but if that range is correct, it still means there are hundreds of millions of habitable planets at least, and that's just in our galaxy. If for some reason our galaxy doesn't measure up to your demanding needs for habitability, there are 150 billion other galaxies we can see. So there's plenty of real estate.

NARRATION
Kepler's incredible observations are making our vast universe seem a little friendlier.

STORY CONTACTS

Professor Geoff Marcy 
Professor of Astronomy, 
University of California, Berkeley

Dr Seth Shostak 
Senior Astronomer
SETI Institute, California

Bill Borucki 
Space Scientist
NASA