0:00:18 > 0:00:23Why do we humans have such a connection to the night sky?
0:00:23 > 0:00:30The twinkling lights that seem like oases out there and yet we're not sure. Are there habitable worlds?
0:00:31 > 0:00:36Around the world there are a group of highly intelligent,
0:00:36 > 0:00:41highly trained scientists that share a surprising belief.
0:00:41 > 0:00:45There are a couple of hundred billion stars just in our galaxy and
0:00:45 > 0:00:50at least half of them probably have planets. That's 100 billion planetary systems.
0:00:50 > 0:00:53How many planets in each system? We've got eight in ours, let's say five.
0:00:53 > 0:00:58That's a half trillion, 500 billion planets out there.
0:00:58 > 0:01:02Keep in mind, there are 100 billion other galaxies!
0:01:02 > 0:01:07For these scientists the vastness of our universe can mean just one thing -
0:01:07 > 0:01:10the existence of life.
0:01:10 > 0:01:16So to think, "Look, man. This is the only place where anything interesting's happening",
0:01:16 > 0:01:19you've gotta be really audacious to take that point of view.
0:01:19 > 0:01:22But proving it has not been quite so simple.
0:01:24 > 0:01:28I have conducted many, many searches, none of which have produced a discovery.
0:01:30 > 0:01:32Until now.
0:01:34 > 0:01:38In our local neighbourhood, just 200 trillion kilometres from Earth,
0:01:38 > 0:01:42is a planet that we might find rather familiar.
0:01:42 > 0:01:48The discovery of Gliese 581c is a marvellous discovery.
0:01:48 > 0:01:52It shows how close we're getting to planets that remind us of the Earth.
0:02:22 > 0:02:27Occasionally you're sitting on a plane and they guy next to you says, "What do you do for a living?".
0:02:27 > 0:02:33"I look for aliens". I explain a little bit, and almost everyone is interested.
0:02:33 > 0:02:37Nobody says, "That's nice. I'll go back to my magazine now".
0:02:40 > 0:02:48In the desert, 300 miles north of San Francisco, Dr Seth Shostak is waiting for a message from an alien.
0:02:50 > 0:02:56This is SETI, the front-line in the search for extra-terrestrial intelligence.
0:02:56 > 0:03:01If a message ever comes our way, this is where it will be received.
0:03:03 > 0:03:08Anybody who can build a transmitter can send messages between the stars.
0:03:08 > 0:03:11If we can do that, maybe they can do it.
0:03:11 > 0:03:14Here we are, the Allen Telescope Array, - designed to do one thing -
0:03:14 > 0:03:21eavesdrop on any signals that might be being broadcast our way by some alien civilisation.
0:03:22 > 0:03:26This vast array of telescopes is the latest in a long line
0:03:26 > 0:03:31of experiments designed to eavesdrop on our nearest neighbours.
0:03:31 > 0:03:36As chief astronomer of the project, Shostak is more confident than most
0:03:36 > 0:03:39that he'll be on the receiving end of a close encounter.
0:03:41 > 0:03:46There are 42 antennae here now, you can count them up.
0:03:46 > 0:03:50But eventually the idea is to have 350 and then this thing
0:03:50 > 0:03:54will be able to scan big chunks of the sky,
0:03:54 > 0:03:58simultaneously observing five, six, maybe more stars at a time, looking
0:03:58 > 0:04:02for the signal that somebody's out there, trying to get our attention.
0:04:04 > 0:04:08It's the most powerful experiment humans have ever attempted -
0:04:08 > 0:04:13to discover if intelligent life is the exception or the rule in the cosmos.
0:04:18 > 0:04:22But although it's early days for Shostak and his team, the omens are not good.
0:04:29 > 0:04:34The SETI project has been casting its ear out to the universe for over 50 years.
0:04:38 > 0:04:43For the founding father of the search, Dr Frank Drake, the dream has never changed.
0:04:45 > 0:04:50Back in the 1950s, there were many scientists interested in ET life,
0:04:50 > 0:04:55but we were well aware that there were no means even to detect
0:04:55 > 0:04:58planets, let alone microbes,
0:04:58 > 0:05:02or any sign of non-intelligent life.
0:05:02 > 0:05:08In fact, the only thing open to us was radio transmissions from intelligent civilisations.
0:05:08 > 0:05:13Drake was the first scientist to believe that technology could answer the biggest question of all.
0:05:22 > 0:05:25Despite widespread scepticism,
0:05:25 > 0:05:29he believed that if there were intelligent life forms out there
0:05:29 > 0:05:34then the least we could do is to try and listen to any radio signals they may be sending out.
0:05:41 > 0:05:44In 1960, we conducted a search for radio signals from the two nearest
0:05:44 > 0:05:51stars to the Earth that are like the Sun - Tau Ceti and Epsilon Eridani.
0:05:51 > 0:05:55But to no-one's surprise, the search failed.
0:05:55 > 0:05:59We searched for two months. We didn't find anything.
0:05:59 > 0:06:04And that's actually an important result, because it showed not every star in the sky was radiating.
0:06:04 > 0:06:10It also demonstrated that a search was likely to be a very long and difficult one.
0:06:11 > 0:06:17Despite putting on a brave face, Drake and his ideas remained on the very fringes of astronomy.
0:06:19 > 0:06:23Listening for aliens just wasn't science.
0:06:23 > 0:06:26Congress people would see that they could get publicity by
0:06:26 > 0:06:29attacking this project as a waste of tax-payers' money.
0:06:29 > 0:06:34Using tax-payers' money to search for little green men was a common theme.
0:06:36 > 0:06:40So Drake would have to wait for another generation of scientists
0:06:40 > 0:06:44to bring alien-hunting in from the cold.
0:07:00 > 0:07:03Being an astronomer is a bit sacrificial.
0:07:03 > 0:07:06My wife is at home and she misses me.
0:07:06 > 0:07:08I call her up and she says, "When are you coming home?"
0:07:08 > 0:07:12It's another four, five nights, I have to tell her.
0:07:12 > 0:07:19Professor Geoff Marcy is a planet hunter, an explorer of alien worlds.
0:07:22 > 0:07:25Staying up all night means you don't get much sleep.
0:07:25 > 0:07:30So it's a bit of a sacrifice, but I wouldn't give it up for anything. It's such a treasure.
0:07:39 > 0:07:42For the last ten years, Marcy has come here to
0:07:42 > 0:07:47use the planet's largest telescope in the hope of finding other worlds.
0:07:51 > 0:07:58The Keck Telescope high up on Mauna Kea, Hawaii is about as close as you can get to the stars.
0:08:01 > 0:08:06It's the world's largest, because the collecting area of the mirror is the largest in the world.
0:08:06 > 0:08:09The mirror is ten metres across,
0:08:09 > 0:08:131/10 the length of a football field, all to collect the starlight
0:08:13 > 0:08:17coming from hundreds or thousands of light years away.
0:08:20 > 0:08:22But even using the mighty Keck telescope,
0:08:22 > 0:08:26astronomers can barely make out the objects in our own back yard.
0:08:28 > 0:08:36Pluto, at the edge of our solar system, is a colossal 4½ billion kilometres away.
0:08:37 > 0:08:41And this is the best image astronomers have achieved.
0:08:44 > 0:08:48And yet Marcy wanted to look beyond our solar system,
0:08:48 > 0:08:56to find the hypothetical worlds that astronomers call exoplanets, which lie around other stars.
0:08:59 > 0:09:01Like our nearest star, Proxima Centauri,
0:09:01 > 0:09:07a staggering 40 trillion kilometres away, or four light years.
0:09:09 > 0:09:17Beyond lies the rest of our galaxy - an unimaginable 100,000 light years across.
0:09:17 > 0:09:21It's our local neighbourhood of 200 billion stars.
0:09:23 > 0:09:27Astronomers knew there had to be planets out there.
0:09:27 > 0:09:31We saw young stars with proto-planetary disks of gas and dust
0:09:31 > 0:09:34around them, surely making planets.
0:09:34 > 0:09:37But we couldn't detect the planets.
0:09:37 > 0:09:42And the reason is that even with the largest telescope, like this one,
0:09:42 > 0:09:47the mighty Keck, the planets were lost in the glare of the host stars.
0:09:49 > 0:09:53The problem is that compared to the light of a star,
0:09:53 > 0:09:57the reflected light from an exoplanet is all but invisible.
0:09:57 > 0:10:01The star burns a billion times more brightly.
0:10:05 > 0:10:13Indeed, even with the Hubble space telescope, we can't detect planets directly around nearby stars.
0:10:14 > 0:10:21Using direct observational methods, astronomers were confined within our own solar system,
0:10:21 > 0:10:26unable to prove even the existence of exoplanets, let alone life.
0:10:27 > 0:10:31So how do you look for something you can't see?
0:10:33 > 0:10:38The answer was first proposed in an obscure paper published in 1952,
0:10:38 > 0:10:42by a Russia astronomer called Otto Struve.
0:10:42 > 0:10:46Struve theorised that even though the planets themselves
0:10:46 > 0:10:50were invisible, there was still a way of unlocking their secrets.
0:10:54 > 0:11:01He knew that each planet was held in orbit around its star by an immense gravitational force.
0:11:01 > 0:11:04This force works in two directions.
0:11:04 > 0:11:09The star pulls on the planet but the planet also pulls back on the star,
0:11:09 > 0:11:12making the star move with the minutest wobble.
0:11:16 > 0:11:19It's not much, but this wobble is just big enough
0:11:19 > 0:11:23to make it theoretically detectable back here on Earth.
0:11:24 > 0:11:28If you have the right technology.
0:11:28 > 0:11:34The real tipping point was not in the telescopes, we've had big telescopes for several decades.
0:11:34 > 0:11:38The tipping point was having digital detectors, like the digital cameras
0:11:38 > 0:11:43that most of us enjoy, with the CCD light detectors at their backs - and computers.
0:11:45 > 0:11:47Let's see what kind of a night we're gonna have here.
0:11:47 > 0:11:52Marcy script underscore ETA underscore Earth.
0:11:52 > 0:11:55- Great.- First object on the list...
0:11:55 > 0:11:58Why don't you go ahead and open the dome slit?
0:12:01 > 0:12:08We're observing 80 stars, night after night, with one key goal, and that's to detect Earth-like planets.
0:12:08 > 0:12:17Since the early 1990s, planet hunters like Marcy have been gazing to the stars for the tiniest wobble
0:12:17 > 0:12:20that could signal the existence of another planet.
0:12:20 > 0:12:22The theory seemed correct.
0:12:22 > 0:12:29They had the right technology, and yet after years of searching, the exoplanets were still missing.
0:12:31 > 0:12:33We were confronted with a contradiction.
0:12:33 > 0:12:39On the one hand, it appeared that young stars had the right kind of planet building material,
0:12:39 > 0:12:44but on the other hand, humanity had failed to find any.
0:12:46 > 0:12:52All the planet hunters had to keep them going was their faith.
0:12:52 > 0:12:56And the belief of the one man who has never given up hope.
0:13:02 > 0:13:07It occurred to me, we need to know how often does life arise, how often does intelligence arise.
0:13:07 > 0:13:12I recognised that all you had to do was multiply these factors together
0:13:12 > 0:13:17and you have a very prime important equation, of basic interest,
0:13:17 > 0:13:19because it tells us how many civilisations there are out there.
0:13:23 > 0:13:27Without any hard evidence, back in 1960, Frank Drake went
0:13:27 > 0:13:32about creating an equation that would answer the big question once and for all.
0:13:32 > 0:13:39We have an equation which gives us N, the number of detectable civilisations in our galaxy.
0:13:39 > 0:13:43It's based on what we know of the history of our galaxy
0:13:43 > 0:13:47and particularly the history of our solar system and of life on Earth.
0:13:48 > 0:13:54The equation defines all the necessary ingredients for intelligent life to arise.
0:13:54 > 0:13:58There are seven factors in the equation.
0:13:58 > 0:14:02Since life needs a home, it begins with a known observation.
0:14:02 > 0:14:05For the rate of star formation, we know that very well.
0:14:05 > 0:14:07It's about 20 stars per year.
0:14:07 > 0:14:11For the fraction of planets, we didn't used to know that at all.
0:14:11 > 0:14:16Everything else in the equation, from the number of stars with planets
0:14:16 > 0:14:22to the number of planets per star capable of supporting life, was a total mystery.
0:14:22 > 0:14:26But over the years, it hasn't stopped people from guessing.
0:14:26 > 0:14:30This is our number N.
0:14:30 > 0:14:34The number of technical civilisations in the galaxy.
0:14:34 > 0:14:39Into the now famous, or infamous, Drake equation goes everything, from astrophysics, through
0:14:39 > 0:14:46evolutionary biology to whatever it is that governs the lifetime of a detectable civilisation.
0:14:46 > 0:14:50Not surprisingly, no-one's solved it yet, but anyone can have a go.
0:14:50 > 0:14:52It's almost a game the whole family can play.
0:14:55 > 0:14:59If you sort of take the average of people's guesses,
0:14:59 > 0:15:03it gives you a total number of detectable civilisations,
0:15:03 > 0:15:05which is about 10,000.
0:15:07 > 0:15:08A big number.
0:15:11 > 0:15:18And yet with no call from ET, and no sign of another Earth, Drake's guess seemed wildly optimistic.
0:15:36 > 0:15:40When I was a child, I was living in a very small village without light.
0:15:40 > 0:15:46We spent evenings with my sister laying down on the grass and looking at the sky.
0:15:46 > 0:15:50And that's really good for the imagination.
0:15:51 > 0:15:57I'm sure there are other Earths similar to our own Earth.
0:15:57 > 0:16:01And on some of them, you even may have life developing.
0:16:03 > 0:16:07Professor Stephane Udry is part of a Swiss team of planet hunters
0:16:07 > 0:16:11who began searching for life-bearing planets in the mid-'90s.
0:16:14 > 0:16:17They had developed a new planet detector,
0:16:17 > 0:16:21which had just been installed at their observatory in central France.
0:16:21 > 0:16:23When you have a new instrument,
0:16:23 > 0:16:27you want to check the short-term precision of the instrument.
0:16:29 > 0:16:34Tests of their new detector were scheduled to last a few weeks.
0:16:34 > 0:16:39Among their target stars was one similar to our own sun, called Pegasi 51.
0:16:40 > 0:16:46The light from Peg 51 should remain constant, but there seemed to be a problem.
0:16:48 > 0:16:50The star...
0:16:50 > 0:16:52appeared to be wobbling.
0:16:54 > 0:17:01The thinking was, "Oh, that should be some crazy effect of the star that could explain the observations".
0:17:03 > 0:17:10They tried to reject, one after the other, all the possible explanations.
0:17:10 > 0:17:14And at the end, the best explanation was the presence of a planet.
0:17:18 > 0:17:21With ingenuity and a little bit of luck,
0:17:21 > 0:17:26the Swiss had discovered the first planet outside our solar system.
0:17:29 > 0:17:35It was massive, half the mass of Jupiter, but in a rapid orbit lasting only four days.
0:17:37 > 0:17:43The planet was so close to its star, that surface temperatures exceed 1,000 degrees Centigrade.
0:17:50 > 0:17:55Being part of these teams finding planets around other stars
0:17:55 > 0:17:57is very exciting in that sense.
0:17:57 > 0:18:02That's really a new domain that is opening in science.
0:18:04 > 0:18:08The discovery of this planet opened the flood gates.
0:18:08 > 0:18:12Hundreds of exoplanets have since been discovered,
0:18:12 > 0:18:16but none of them have proved suitable for life.
0:18:25 > 0:18:31My favourite planet is a little planet that orbits the star Gliese 756.
0:18:31 > 0:18:34My favourite planetary system is called 55 Cancri.
0:18:34 > 0:18:37There are two planets which I have an emotional attachment to.
0:18:37 > 0:18:39It has an orbit of two days.
0:18:39 > 0:18:42The planets of the star Tau Ceti.
0:18:42 > 0:18:44That means its seasons occur in two days.
0:18:44 > 0:18:46And the star Epsilon Eridani.
0:18:46 > 0:18:49So summer-winter would alternate in two days.
0:18:49 > 0:18:53It's like the solar system because it's a planet like Jupiter on a Jupiter-like orbit.
0:18:53 > 0:18:56Those stars were my targets
0:18:56 > 0:19:01when I first searched for the first evidence of extra terrestrial life in 1960.
0:19:01 > 0:19:05It's unlike the solar system because it has three other planets that are
0:19:05 > 0:19:07very close into the star, hot Jupiter-type planets.
0:19:09 > 0:19:15In the last decade, astronomers have found over 260 exoplanets,
0:19:15 > 0:19:18most of them searingly hot gas-giants.
0:19:20 > 0:19:23As a biologist, I don't really have a favourite exoplanet at the moment,
0:19:23 > 0:19:28because the astronomers keep finding me hot Jupiters and they don't do much good for biology.
0:19:28 > 0:19:31What I'm looking for is something really Earth-like.
0:19:31 > 0:19:33Something that's got a good chance of liquid water.
0:19:33 > 0:19:35And then I'll have a favourite.
0:19:40 > 0:19:44So just how rare is our blue planet?
0:19:48 > 0:19:52Dr Lynn Rothschild is an astrobiologist who has studied our own solar system
0:19:52 > 0:19:56in an effort to understand what makes Earth so special.
0:19:59 > 0:20:03Let's pretend that this fire here is our sun and that this rock is Venus.
0:20:03 > 0:20:08It's about as close to the sun you can get and still have liquid water.
0:20:08 > 0:20:12So the orbit of Venus would be, say, like this.
0:20:14 > 0:20:18This is getting pretty hot, cos I'm awfully close to the sun here.
0:20:20 > 0:20:25Now on the other extreme, this is Mars, which is the farthest planet
0:20:25 > 0:20:28from the sun that has any chance of liquid water.
0:20:28 > 0:20:31There's no liquid surface water today, but we know
0:20:31 > 0:20:34that there used to be in the past. So let's trace the orbit of Mars.
0:20:37 > 0:20:42Now right between these two circles, where the orbit of Mars would be
0:20:42 > 0:20:47and the orbit of Venus, this is where liquid water is stable.
0:20:47 > 0:20:51And right in this habitable zone in our solar system is planet Earth.
0:20:51 > 0:20:54Our beautiful watery world that's just covered with life.
0:20:56 > 0:20:59Just 10% closer in,
0:20:59 > 0:21:03and Earth would no longer be capable
0:21:03 > 0:21:06of supporting liquid water.
0:21:15 > 0:21:20Almost miraculously, Earth slots right into the heart of the habitable zone.
0:21:25 > 0:21:30Giving this once lifeless rock just the right elements for life to take hold and flourish.
0:21:41 > 0:21:45Over billions of years, microbes, plants and animals
0:21:45 > 0:21:49have transformed Earth into a living, breathing world.
0:21:56 > 0:21:59A world where one evolutionary line
0:21:59 > 0:22:03has led to modern humankind and civilisation.
0:22:11 > 0:22:15But even with our civilisation's most advanced technology,
0:22:15 > 0:22:19finding other planets like Earth has proved impossible.
0:22:21 > 0:22:26Rotator is vertical, angle mode zero.
0:22:26 > 0:22:31The very factors that enable life, a small planet at a safe distance from
0:22:31 > 0:22:37the sun, means the telltale wobbles that these planets produce are tiny.
0:22:37 > 0:22:43Our Earth, when it orbits the sun, causes our sun to wobble with a speed
0:22:43 > 0:22:48of 1/10th of one metre per second, a smaller motion than we can detect.
0:22:51 > 0:22:57Or there could be another, more profound explanation for the missing Earth-like planets.
0:22:58 > 0:23:03It's possible that other stars didn't have planets around them.
0:23:03 > 0:23:07That we're just one of the freaks of nature that grew up on a rocky planet.
0:23:09 > 0:23:14Either way, despite decades of searching, until 2007,
0:23:14 > 0:23:17Earth remained entirely alone.
0:23:31 > 0:23:38Between the Andes mountains and the Pacific Ocean, on the remote southern edge of the Atacama desert
0:23:38 > 0:23:43lies one of the most extraordinary observatories on Earth.
0:23:47 > 0:23:50The high elevation and the low rainfall,
0:23:50 > 0:23:57just one millimetre a year, makes it the perfect place for uninterrupted views of the southern night sky.
0:23:57 > 0:24:00Please come in, I have something to show you in here.
0:24:04 > 0:24:11Professor Stephane Udry is the proud owner of a machine which could change the course of human history.
0:24:11 > 0:24:16Inside this big box is an enclosure and inside there is a vacuum tank
0:24:16 > 0:24:21with the instrument, that is the most sensitive in the world now for planet detection.
0:24:21 > 0:24:30- With this instrument we can detect low mass planet five, ten times the mass of the Earth.- Can we go in?
0:24:30 > 0:24:36No. Of course not, because just opening the door will destroy the measurement for a few days.
0:24:36 > 0:24:44Because we need to have a very stable instrument to be able to repeat the measurement with the same precision,
0:24:44 > 0:24:47day after day, month after month, years after years.
0:24:53 > 0:24:57And that's exactly what they've been doing.
0:24:59 > 0:25:04They drew up a list of a thousand targets taken from the Gliese Catalogue of Nearby Stars
0:25:04 > 0:25:07and began measuring and re-measuring each candidate,
0:25:07 > 0:25:12hunting for wobbles that had previously been too small to detect.
0:25:23 > 0:25:28But one star caught Stephane's attention.
0:25:28 > 0:25:34Gliese 581 was in our target list since the beginning.
0:25:34 > 0:25:41Categorised as Gliese 581a, it's a red dwarf star, a third of the mass of our own sun.
0:25:45 > 0:25:52When the wobble was plotted it revealed 581b, a massive planet the size of Neptune,
0:25:52 > 0:25:54close into the star
0:25:54 > 0:25:58and orbiting once every 5½ days.
0:25:58 > 0:26:00It was no Earth,
0:26:00 > 0:26:05but the star's wobble held some fine detail that intrigued Stephane.
0:26:07 > 0:26:10We noticed that there was something else in the system.
0:26:10 > 0:26:16There seemed to another, smaller planet lurking in the detail.
0:26:16 > 0:26:23That something else could be a five Earth mass planet very close to the star.
0:26:23 > 0:26:31If Stephan's hunch was right, it would be the smallest planet ever detected around a distant sun.
0:26:31 > 0:26:33And this planet seemed to be habitable.
0:26:35 > 0:26:41We got excited because the distance was just right for the planet to possibly be in the habitable zone.
0:26:46 > 0:26:49After years of hunting, the search
0:26:49 > 0:26:53for the first "Second Earth" was over.
0:26:53 > 0:26:56European astronomers have spotted a new planet outside our solar system
0:26:56 > 0:26:58which closely resembles the planet Earth.
0:26:58 > 0:27:02The probability that there is life somewhere else in the Universe goes up a bit.
0:27:02 > 0:27:06This latest find has set the world of astronomy alight.
0:27:14 > 0:27:17For the Swiss team, the breakthrough was a triumph.
0:27:22 > 0:27:26It is always very exciting to be the first one to know.
0:27:26 > 0:27:31The discovery of Gliese 581c is a marvellous discovery.
0:27:31 > 0:27:36It shows how close we are were getting to planets that remind us of the Earth.
0:27:36 > 0:27:40It shows that potential life-bearing planets exist.
0:27:40 > 0:27:44When you know, when you realise it, and you are the only one, it's like being in the spaceship
0:27:44 > 0:27:48coming to a planet and being the first one to see the landscape.
0:27:50 > 0:27:54For those tempted to make the journey, pick a clear night
0:27:54 > 0:27:57and look for the constellation Libra.
0:27:57 > 0:27:59Invisible to the naked eye,
0:27:59 > 0:28:05Gliese 581 lies just north of the brightest star in the constellation.
0:28:08 > 0:28:14Remarkably, it's one of our closest neighbours, a shade over 20 light years distant.
0:28:17 > 0:28:20At the heart of the system is the parent star.
0:28:20 > 0:28:28Close by is 581b, 16 times more massive than Earth and too hot for life to survive.
0:28:30 > 0:28:34Beyond, just on the inner warm edge of the habitable zone,
0:28:34 > 0:28:38lies Gliese 581c -
0:28:38 > 0:28:43the smallest and most Earth-like exoplanet yet detected.
0:28:54 > 0:29:02At last, scientists have found another planet that may just be capable of supporting life.
0:29:05 > 0:29:07Not much out here.
0:29:07 > 0:29:10See if there's any under the rock.
0:29:10 > 0:29:14Nope. A lot of UV radiation.
0:29:22 > 0:29:25Nothing green, nothing coloured I can see. Very dry.
0:29:25 > 0:29:31For astrobiologists like Dr Lynn Rothschild,
0:29:31 > 0:29:37its discovery means they can begin to imagine what it would be like to spend a day on a Super-Earth.
0:29:43 > 0:29:48We're up here on the edge of the Atacama desert in Chile right near the Bolivian border.
0:29:50 > 0:29:55You can see it's very dry, in fact, one of the driest places on Earth.
0:29:59 > 0:30:04This is a great place to get an idea of what an extra-solar planet,
0:30:04 > 0:30:06for example Gliese 581c, might be like.
0:30:31 > 0:30:36Let's imagine that we're on Gliese 581c.
0:30:36 > 0:30:40There's an awful lot of rocks around. It's dry.
0:30:42 > 0:30:45The planet's mass is five times that of Earth.
0:30:45 > 0:30:49This means that gravity will pull twice as hard.
0:30:49 > 0:30:52Whereas on the moon, the astronauts could jump with no effort,
0:30:52 > 0:30:56on this planet you would be suffering from extra gravity.
0:30:56 > 0:31:02If you took a rock and you threw it, it would come crashing down,
0:31:02 > 0:31:04much faster than that of the Earth.
0:31:04 > 0:31:08High gravity will affect the look of the planet. No mountains.
0:31:08 > 0:31:11Just low hills and vast plains.
0:31:13 > 0:31:16And the last thing is it's close to the parent star,
0:31:16 > 0:31:22and so the radiation from the sun would be much stronger than on Earth.
0:31:22 > 0:31:25Here we're getting burned, there we would probably be fried.
0:31:34 > 0:31:37The planet's red dwarf star will dominate the sky -
0:31:37 > 0:31:41a fiery ball five times larger than our own sun back home.
0:31:41 > 0:31:43And a few hours into their trip,
0:31:43 > 0:31:48interstellar visitors will discover that this sun never moves.
0:31:55 > 0:31:57The planet is so close to its star
0:31:57 > 0:32:01that immense gravitational forces have united the two.
0:32:04 > 0:32:09They're tidally locked, with the planet presenting just one face to the light.
0:32:12 > 0:32:15On the Earth, we're used to getting up in the morning, the sun rises.
0:32:15 > 0:32:21We have our midday meal, in the evening we have dinner, if we're lucky, we get a nice sunset.
0:32:21 > 0:32:25But on something like Gliese 581c it would be totally different.
0:32:30 > 0:32:36If I wanted to see the equivalent of a sunset, I'd be the one who'd have to get into the car and move.
0:32:40 > 0:32:42Beyond this point is the dark side
0:32:42 > 0:32:48of the planet, perpetually turned outwards to the cold of space.
0:32:49 > 0:32:53I wouldn't want to live here, I wouldn't want to be a colonist
0:32:53 > 0:32:57on another world that was barren like this.
0:32:57 > 0:33:03I'd take even a year-long field trip, but I wouldn't sign up for the rest of my life.
0:33:07 > 0:33:12Comfortable as Gliese 581c may be for a day trip,
0:33:12 > 0:33:20for life to exist there, for it truly to be second Earth, it must have one other vital ingredient.
0:33:31 > 0:33:35Water is the one thing life on Earth has in common, so we think
0:33:35 > 0:33:37looking for water on other planets
0:33:37 > 0:33:39is a way to look for life on those planets.
0:33:47 > 0:33:50For astrophysicists like Sean Raymond, finding water
0:33:50 > 0:33:53on other worlds is the key to finding life.
0:33:56 > 0:34:02Every day in his laboratory, he makes new solar systems from scratch.
0:34:02 > 0:34:08So the way we do this is computer simulations of a disc of rocks orbiting
0:34:08 > 0:34:11a star, and we let them collide and let their orbits evolve, and such.
0:34:11 > 0:34:14And it turns out these take quite a long time to do.
0:34:14 > 0:34:18Over the months, Sean's computer calculates
0:34:18 > 0:34:22how alien planetary systems evolve over millions of years.
0:34:33 > 0:34:36Here's a movie of one of these simulations. You can see everything
0:34:36 > 0:34:39on the inner disc starts off red, meaning quite dry.
0:34:39 > 0:34:42All these guys start off being the size of the moon, or actually a little smaller.
0:34:42 > 0:34:47And then the number of bodies is going down as they collide and grow into larger things.
0:34:47 > 0:34:51And by about 10 million years or so, a planet almost the size of the Earth is formed right there.
0:34:51 > 0:34:53And you can see it's still red.
0:34:55 > 0:34:59These new planets are all dry. Only far out from the star
0:34:59 > 0:35:03are temperatures low enough for water to collect.
0:35:05 > 0:35:09It's not until a little later... you'll see in a second, it gets collided by something that's
0:35:09 > 0:35:15blue and turns - right there, it went from being completely dry to having some water.
0:35:15 > 0:35:20And that process of water delivery continues over the next 100 million years or so.
0:35:23 > 0:35:27Over this time, icy comets and asteroids from
0:35:27 > 0:35:32the outer solar system are drawn inwards towards the young planets.
0:35:32 > 0:35:37Shaun's theory is they bring with them vast amounts of water,
0:35:40 > 0:35:44transforming dead worlds into blue planets.
0:35:44 > 0:35:46That's a pretty good Earth analogue.
0:35:46 > 0:35:50And we think this is how the solar system terrestrial planets formed.
0:35:51 > 0:35:55Sean has run hundreds of simulations.
0:35:55 > 0:35:59And each time, something happens to the planets in the habitable zone -
0:36:01 > 0:36:06- they nearly all have water. - Water is very abundant.
0:36:06 > 0:36:12In the solar system, water is two to four times more abundant than rock and iron.
0:36:15 > 0:36:20It looks like Earth might, on average, be a little bit water-poor.
0:36:20 > 0:36:24And many planets may end up with a lot more water than the Earth.
0:36:24 > 0:36:27Including the newly discovered 581c.
0:36:27 > 0:36:34Gliese 581c especially, is very exciting, a very big discovery.
0:36:34 > 0:36:38These planets would have acquired some water-rich material, so they
0:36:38 > 0:36:41probably have water contents comparable to Earth at least.
0:36:43 > 0:36:45Far from being a barren rock,
0:36:45 > 0:36:49this new planet may be awash with liquid water.
0:36:54 > 0:36:58But in their rush to tell the world of another world,
0:36:58 > 0:37:03the Swiss had overlooked one thing - the planet's atmosphere.
0:37:05 > 0:37:08We got very excited about Gliese 581c
0:37:08 > 0:37:14when we realised that it was just at the right distance from the star.
0:37:14 > 0:37:17But then, talking with specialists of the evolution of atmospheres
0:37:17 > 0:37:21on the planet, they told us that maybe the greenhouse effect could be big.
0:37:21 > 0:37:25And so the temperature could be too high for the development of life.
0:37:25 > 0:37:31If the planet's atmosphere contains too much water vapour or carbon dioxide...
0:37:31 > 0:37:36a runaway greenhouse effect could take hold.
0:37:36 > 0:37:41Rather than resembling Earth, 581c could be a super-Venus.
0:37:46 > 0:37:48Instead of liquid water,
0:37:48 > 0:37:53steam would shroud a searingly hot world, incapable of supporting life.
0:38:00 > 0:38:04It's probably too hot to be habitable.
0:38:04 > 0:38:12If it has water at all, which is doubtful, that water would be boiled off, evaporated and gone.
0:38:12 > 0:38:17581c may, after all, lie on the hot side of the habitable zone,
0:38:17 > 0:38:22but the light the Swiss team were collecting from the planet's star held another surprise.
0:38:24 > 0:38:30After decades of fruitless searching for habitable worlds, out popped another one.
0:38:32 > 0:38:34We had to wait for one more year
0:38:34 > 0:38:40before being able to actually find another planet a bit further out.
0:38:40 > 0:38:42There is a third planet in the system.
0:38:48 > 0:38:54They'd discovered a second super-Earth in the same system -
0:38:54 > 0:38:57Gliese 581d.
0:38:58 > 0:39:02This world lies on the far, cold edge of the habitable zone.
0:39:02 > 0:39:07On first calculations, this would make it a giant frozen world.
0:39:07 > 0:39:14But if it too enjoys a greenhouse effect, then it could be just warm enough for liquid water.
0:39:16 > 0:39:20If there is some atmosphere, and a greenhouse effect, then the temperature could
0:39:20 > 0:39:23be even better on that planet for the development of life.
0:39:28 > 0:39:31Perched on opposite edges of the habitable zone,
0:39:31 > 0:39:35the conditions on the planets in this system will be harsh.
0:39:37 > 0:39:40Perhaps too harsh for life to survive.
0:39:46 > 0:39:53Here on Earth, Dr Lynn Rothschild is investigating places where conditions mirror the extreme
0:39:53 > 0:39:57environments found on both the G581 planets.
0:39:59 > 0:40:03We're up here in the altiplano in Bolivia. Up at about...
0:40:03 > 0:40:06well over 4,000m, or 15,000ft.
0:40:06 > 0:40:12In the winter it's frozen - it's not a whole lot warmer in the summer,
0:40:12 > 0:40:14and yet life lives up here.
0:40:14 > 0:40:19Every place we've gone that's cold - the Antarctic, the ice caps, we've found life.
0:40:19 > 0:40:23And even under here, there's plenty that's growing. It's just amazing.
0:40:29 > 0:40:34So life in the freezing conditions of the outer planet is a possibility.
0:40:43 > 0:40:49And even the on the inner, hotter world, where temperatures could exceed the boiling point of water,
0:40:49 > 0:40:53scientists are beginning to understand how life could survive.
0:40:53 > 0:40:56We don't know how life actually got started on Earth.
0:40:56 > 0:41:00But we do know that when we look at modern organisms,
0:41:00 > 0:41:03and at their evolution, the most ancient ones
0:41:03 > 0:41:07seem to be the ones that live at extremely high temperatures,
0:41:07 > 0:41:09just like these areas around here.
0:41:15 > 0:41:17Indeed, the more scientists look,
0:41:17 > 0:41:23the wider the range of habitats they find in which living organisms can thrive.
0:41:23 > 0:41:28So this gives us hope, this gives us optimism that when we go elsewhere
0:41:28 > 0:41:31to other worlds, that there might be life.
0:41:31 > 0:41:36For now, no-one knows for sure if life could survive in the massive,
0:41:36 > 0:41:38strange worlds of the G581 system.
0:41:43 > 0:41:48And Earth-bound planet hunting may have reached the end of the line.
0:41:48 > 0:41:51Because to find true Earth-sized planets,
0:41:51 > 0:41:54the hunt is moving into space.
0:42:11 > 0:42:15This is a spaceship factory.
0:42:18 > 0:42:22In these category A clean rooms, machines are built
0:42:22 > 0:42:26that their designers hope will unlock the secrets of the universe.
0:42:26 > 0:42:29That's the interferometer.
0:42:33 > 0:42:38- There's the focus mechanisms right here.- Here's one focus mechanism.
0:42:38 > 0:42:43- This is the actual focus mechanism. - This is the flight hardware. - Wonderful.
0:42:43 > 0:42:48Today the team are midway through assembling their latest mission -
0:42:48 > 0:42:51the giant Kepler space telescope.
0:42:57 > 0:43:05But it's not scheduled to fly until 2009, so currently the spaceship is in bits.
0:43:05 > 0:43:09This is where the primary mirror is gonna sit, on top of this.
0:43:09 > 0:43:13So that measures how well you've got the optics aligned?
0:43:13 > 0:43:17That's right, you can measure how well it's working.
0:43:17 > 0:43:21Leading the NASA team assembling the space telescope is Bill Borucki.
0:43:21 > 0:43:26It's magnificent, it's just wonderful to see it come together.
0:43:26 > 0:43:30We've been planning this for years and years.
0:43:30 > 0:43:31So to actually see it here...
0:43:31 > 0:43:34This is the flight equipment, this will go into space.
0:43:34 > 0:43:36It's this that will make our discovery.
0:43:36 > 0:43:41I'm delighted to see all the details that seem to be right.
0:43:46 > 0:43:52When Kepler flies, it will undertake a four-year mission to seek out new worlds.
0:43:54 > 0:43:57But it won't be looking for wobbles.
0:43:57 > 0:44:04Instead, Kepler will be hunting for planets that pass in front of their stars, creating a tell-tale wink.
0:44:04 > 0:44:08Looking at the star, it seems to wink, it gets dimmer for a while.
0:44:08 > 0:44:12Like it closed its eye for a second and then opened it.
0:44:12 > 0:44:16This is because the planet moved in front of it and blocked some of its light.
0:44:16 > 0:44:18It happens in our solar system too.
0:44:18 > 0:44:25We had a transit, Mercury going in front of the sun fairly recently, we could see that with a telescope.
0:44:27 > 0:44:31For the wink technique to work,
0:44:31 > 0:44:33a space telescope is essential.
0:44:33 > 0:44:37Free from the interference of Earth's atmosphere, it gives
0:44:37 > 0:44:41Kepler an uninterrupted view of a very special part of the galaxy.
0:44:41 > 0:44:43Kepler only looks at one area of the sky.
0:44:43 > 0:44:48It's a good area for us, in that it has a huge number of stars.
0:44:48 > 0:44:51Kepler will scan the same 100,000 stars
0:44:51 > 0:44:54over its entire four-year mission,
0:44:54 > 0:44:58constantly measuring the brightness of each one.
0:44:58 > 0:45:05And from day one, it will be sensitive enough to detect the wink
0:45:05 > 0:45:09of an Earth-sized planet crossing its sun, tens of light years away.
0:45:12 > 0:45:18It's always very exciting, because we've always wanted to know - are there lots of Earths out there?
0:45:18 > 0:45:22Geoff Marcy and Stephane Udry and all these other people are extremely competitive.
0:45:22 > 0:45:25They want to find planets, they want the answers too.
0:45:25 > 0:45:29Well, we all do, and the best way to do that is to co-operate.
0:45:29 > 0:45:33There's a bit of a race going on, but it's a delightful race.
0:45:33 > 0:45:38The competition is lovely, and it makes us get up in the morning, go to work, and work a little harder.
0:45:38 > 0:45:41So who's gonna find the first Earth-sized object?
0:45:41 > 0:45:46We are. Kepler's going to find the first Earths in the habitable zone.
0:45:49 > 0:45:56Between them, the planet hunters are beginning to define the first galactic map of Earth-like worlds.
0:46:01 > 0:46:08At last, a phone directory for those listening for a message from ET.
0:46:11 > 0:46:14They're gonna allow us to sharpen our gaze of the heavens,
0:46:14 > 0:46:16where we're pointing these antennas,
0:46:16 > 0:46:20trying to pick up a signal, they're gonna tell us, "You don't have to look at every star,
0:46:20 > 0:46:24"these ones have planets", and eventually they'll be able to say,
0:46:24 > 0:46:28"These are the ones that have planets the same size as Earth."
0:46:28 > 0:46:31And ten years after that, they'll be able to say,
0:46:31 > 0:46:36"These are the ones with oxygen or methane in their atmosphere.
0:46:36 > 0:46:41"So they have some biology, and it's up to you to find out if any of that biology is smart or not."
0:46:43 > 0:46:49Rather than the entire galaxy of 200 billion stars, in the future, SETI
0:46:49 > 0:46:56need only tune into the handful of star systems that Kepler discovers.
0:47:01 > 0:47:05Everything has caused us to become more optimistic.
0:47:05 > 0:47:09We really believe in the next 20 years or so, we're going to learn
0:47:09 > 0:47:12a great deal more about life beyond Earth
0:47:12 > 0:47:16and very likely we'll have detected that life
0:47:16 > 0:47:21and perhaps even intelligent life elsewhere in our galaxy.
0:47:21 > 0:47:26Remember, there's a flip side to this - it could be that advanced
0:47:26 > 0:47:33technological civilisations, species, are a rarity, one in a million, maybe one in a billion.
0:47:33 > 0:47:38If so, we humans could be quite a precious rarity in the Milky Way galaxy.
0:47:38 > 0:47:42Maybe, in fact, they're not out there watching us.
0:47:42 > 0:47:47We may be the ones to be the first to go out and explore the galaxy.
0:48:05 > 0:48:08Subtitles by Red Bee Media
0:48:08 > 0:48:11E-mail subtitling@bbc.co.uk