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  • If things don't work out on this planet...

  • Or if our itch to explore becomes unbearable at some point in the future...

  • Astronomers have recently found out what kind of galactic real estate might be available

  • to us.

  • We'll have to develop advanced transport to land there, 20 light years away.... But that's

  • for later.

  • The question right now: is it worth the trip? The destination is a star that you can't see

  • with your naked eye, in the southern constellation Libra, called Gliese 581.

  • Identified over 40 years ago by the German astronomer Wilhelm Gliese, it's a red dwarf

  • with 31% of the Sun's mass... and only 1.3% of its luminosity.

  • Until recently, the so-called M Stars like Gliese 581 flew below the radar of planet

  • hunters.

  • They give off so little energy that a planet would have to orbit dangerously close just

  • to get enough heat.

  • Now, these unlikely realms are beginning to show some promise... as their dim light yields

  • to precision technologies...

  • ...as well as supercomputers... honed in the battle to understand global changes on this

  • planet... Earth.

  • Will we now begin to detect signs of alien life?

  • Or will these worlds, and the galaxy itself, turn out to be lifeless... and Earth, just

  • a beautiful, lonely aberration?

  • To some, like astronomer and author Carl Sagan, the sheer number and diversity of stars makes

  • it, as he said, "far more likely that the universe is brimming over with life."

  • This so-called "many worlds" view can be traced back to ancient observers... in China, India,

  • Greece and Egypt. The Qur'an, the Talmud, and many Hindu texts all imagined a universe

  • full of living beings.

  • In the 16th Century, this view got a boost from astronomer and mathematician Nikolas

  • Copernicus... who came to believe that Earth is not the center of the universe, but revolves

  • around the Sun.

  • Seven decades after Copernicus, Galileo Galilei used his newly developed telescope to show

  • that our Sun was just one among countless other stars in the universe.

  • By the modern era, the "many worlds" view held sway in scientific circles. A variety

  • of thinkers considered what and who inhabited worlds beyond our own.

  • From Martians desperate to get off their planet... to alien invaders intent on launching pre-emptive

  • strikes against ours... or simple life forms on an evolutionary track to complexity.

  • But other thinkers have been struck by a different view.

  • The Greek philosophers Aristotle and Ptolemy believed that humans and Earth are unique.

  • With the spread of Christianity, this Ptolemaic system became widely accepted.

  • The latest variation on this theme is what's called the "Rare Earth" hypothesis. It holds

  • that Earth and sophisticated life were the result of fortuitous circumstances that may

  • not be easy to find again in our galaxy.

  • Does the current search for planets shed light on this debate... sending it in one direction

  • or the other?

  • So far, our only good reference for recognizing an Earth-like planet is... Earth.

  • It does have some fortuitous characteristics... it's dense, it's rocky - with a complex make-up

  • of minerals and organic compounds - and it has lots and lots of water.

  • It's also got a nearly circular orbit around the Sun, at a distance that allows liquid

  • water to flow... not too close and not too far away, in the so-called "Habitable Zone."

  • That's defined as the range of distance from a parent star that a planet would need to

  • maintain surface temperatures between the freezing and boiling points of water.

  • Of course, that depends on the size of the planet, the make-up of its atmosphere, and

  • a host of other factors.

  • And whether the parent star is large; medium like the Sun; or small.

  • Some scientists also believe we live in a "Galactic Habitable Zone." We're close enough

  • to the galactic center to be infused with heavy elements generated by countless stellar

  • explosions over the eons...

  • But far enough away from deadly gamma radiation that roars out of the center.

  • If there is a galactic habitable zone... it's thought to lie 26,000 light years from the

  • center... about where we are... give or take about 6,000 light years.

  • According to one estimate, only about 5% of the stars in our galaxy fall within it.

  • But even out here - in the galactic suburbs - there are hazards. Some researchers have

  • linked mass extinctions in the past to the Sun's passage through one of the spiral arms,

  • where other sources of dangerous gamma radiation lie in wait.

  • Somehow, though, we've made it through asteroid impacts, climate changes, solar eruptions

  • and everything else the galaxy has thrown at us.

  • Now we want to know... are there kindred spirits, somewhere out there, to share our survival

  • stories with?

  • This image shows the single biggest obstacle faced by planet hunters. We're looking at

  • Earth, as photographed by the Voyager spacecraft, from a distance of 3.7 billion miles.

  • Our mighty world occupies only about one tenth of one pixel.

  • Try seeing something this small at hundreds of thousands of times that distance.

  • And try seeing it through the bright glare of a star.

  • Faced with these obstacles, astronomers have developed ways to see them indirectly.

  • Since 1995, Swiss and American teams have discovered over 400 planets by looking for

  • subtle shifts in the motion of their parent stars.

  • Here's what they'd see if they used this technique ...

  • ...to monitor our Sun for a period of 30 years beginning in 1990.

  • The sun would wobble, or dance about, due to the gravitational tug of planets orbiting

  • it.

  • The technique for detecting this wobble is so exquisitely developed that astronomers

  • can see movements down to a meter per second: human walking speed.

  • But there's still a ways to go... Earth causes our Sun to move at less than a tenth that

  • speed.

  • By analyzing the complexity of this movement...

  • They have been able to tease out the sizes and distances of the different planets tugging

  • on the star.

  • Initially, they detected huge gas giants, many of which have wildly elliptical orbits

  • that bring them in close to their stars before driving them way out.

  • Unfortunately, the giant's gravity would eventually fling smaller Earth-sized planets out of the

  • solar system.

  • An exception is the star 55 Cancri, where astronomers detected a Jupiter-sized planet

  • with a nearly circular orbit.

  • That's a good sign. Our Jupiter is thought to have afforded Earth some protection by

  • scooping up destructive asteroids.

  • Beyond occupying a desirable location in the Habitable Zone of its parent star... it helps

  • if a planet has the right chemistry.

  • Our Earth and solar system were likely born within a crowded scene... like the Trifid

  • nebula here... lashed by winds from hot giant stars... and blasted by a violent explosion:

  • a supernova.

  • Nuclear burning within the core of large stars is where most of the heavy elements - from

  • iron and oxygen, to carbon and calcium - are generated. As our Sun formed at the center

  • of an immense dusty disk, these elements became concentrated in planets.

  • Some of the most important elements in the search for life have now been detected, because

  • of the chance alignment of Earth and a solar system 63 light years away that astronomers

  • refer to as HD 189733.

  • The Hubble and Spitzer space telescopes together caught a planet whipping around this star

  • every two Earth days.

  • This is one sizzling planet... with surface temperatures up around 900 degrees Celsius.

  • The planet, tagged HD 189733-B, is so hot that a vapor trail follows in its wake. Starlight

  • passing through this trail allowed scientists to detect carbon dioxide, methane gas... and

  • water in its atmosphere.

  • Then there's the sun-like star HD 209458, visible with binoculars, 150 light years away.

  • It has a planet, known as a "hot Jupiter."

  • It's hot because it's close-in: as near as 7 million kilometers from the star... Compared

  • to our planet Mercury, 46 million kilometers out from our Sun...or the Earth, 146 million.

  • Scientists estimate that stellar winds are blowing some 10,000 tons of material every

  • second off the surface of this hot gas giant. How long before it gets reduced to a shriveled

  • core?

  • That's just what happened to the star called CoRot-7. Astronomers spotted a planet slightly

  • larger than Earth... and just about as dense. But because little CoRoT-7b is so close to

  • its parent star, it's most likely covered in molten lava.

  • If these planets are too close to their stars to harbor life... many of the others discovered

  • so far are too far away and too large.

  • Like the one orbiting a star the ancient Arabs called Fomulhaut - - meaning: "the mouth of

  • the fish." It's just 25 light years away and was the first "exoplanet" to be seen directly.

  • Using Hubble, scientists blocked out Fomulhaut's glare and found a planet orbiting about 10

  • times farther out than Saturn. It would take 872 Earth years to make just one orbit.

  • The brightness of this planet suggests that it's huge and probably surrounded by a dusty

  • disk.

  • Astronomers are seeing hints of the diversity of worlds imagined by the ancients... but

  • still only scant evidence of anything resembling Earth.

  • Still, there's one class of stars that had, until recently, been overlooked.

  • The M-stars are the most numerous in our galaxy... around 76% of the total.

  • They are often called failed stars... because not enough mass fell together as they were

  • forming to fuel the intense nuclear fusion that burns in stars like our sun.

  • To be in the "Life Zone" of one of these dwarf stars, a planet would have to be very close.

  • Unfortunately, because these small stars spin quickly when they're young, they are like

  • electrical dynamos, with intense magnetic fields that generate dangerous flares.

  • If a planet manages to survive these flares, it may have to endure another handicap.

  • In close like this, it's likely to become gravitationally locked to its parent star

  • - just like our Moon is to Earth - with only one side ever facing the star.

  • One side scorched... the other frozen. Is there any hope?

  • On Earth, we know that the unevenness of solar heating, due to the day/night cycle, leads

  • to oceanic and atmospheric circulation patterns that redistribute the heat and equalize temperatures.

  • A computer simulation performed by NASA scientists showed that a tidally locked planet could,

  • in fact, develop circulation patterns like this.

  • In the computer model, a cloud cover developed on the light side: keeping temperatures down

  • and air pressures high. The result: winds that circled the planet, redistributing enough

  • heat to keep the rate of freezing down.

  • That's one reason these M stars are on planet hunters' radars these days.

  • Another reason is that planet hunting has taken a leap forward.

  • At the European Southern Observatory in the mountains of Chile, an instrument that specializes

  • in looking for solar wobbles has come on line. HARPS - short for "High Accuracy Radial Velocity

  • Planet Searcher" - has been on a planet finding tear.

  • At a recent conference, scientists announced 30 new planets discovered with this instrument.

  • Among its most promising targets...

  • Gliese 581, that inconspicuous red dwarf in Libra.

  • Four planets have been detected so far...

  • Planet E is the lightest exoplanet discovered to date, at 1.9 times the mass of Earth.

  • But it's so close to its parent star that its orbit takes only 3.15 days.

  • It's likely a larger version of Mercury, scorched by its sun and riddled with craters.

  • Planet B is close too, and weighs 16 times what Earth does.

  • Planet C, at 5 Earth masses, lies just inside the Habitable Zone...

  • ... while Planet D, at 7 Earth masses, lies on its outside edge.

  • To find out what direction these planets might have taken, scientists have sought to model

  • their early evolution.

  • Planet C, on the inside margins of the life zone, would have immediately been subjected

  • to a withering barrage of solar radiation.

  • Think Venus. Any water vapor in the atmosphere would have been stripped away. Gases spewed

  • out by volcanic activity would have turned the atmosphere increasingly dense...

  • A strong greenhouse effect would then have turned Planet C into a hothouse world.

  • Planet D is different.

  • Earth-bound instruments showed that its gravitational pull is surprisingly weak - meaning it's not

  • very dense. That suggests that it might have migrated in from the outer solar system, carrying

  • with it significant amounts of hydrogen and oxygen.

  • It could be a water world, with an abundance of vapor in the atmosphere that would strengthen

  • its greenhouse effect.

  • Of course, there are still many unknowns. Is it far enough out to avoid becoming tidally

  • locked? If it is locked, has it developed circulation patterns strong enough to keep

  • the freezing down on the dark side and support a habitable climate?

  • And does it have a magnetic field that protects it against flares that may be bursting from

  • its star?

  • And is oxygen present in its atmosphere - one of the most prominent markers of life?

  • Astronomers will continue to study the planets of Gliese 581 in the years to come... as part

  • of a rapidly expanding list of targets.

  • The next era in planet hunting is now ramping up with the recent launch of the Kepler planet-hunting

  • spacecraft.

  • Kepler amounts to a huge digital camera in space, pointed at a region containing more

  • than 100,000 stars...

  • Its designers are betting that some of those stars have planets...and that a fraction are

  • tilted so that their planets pass between their stars and Kepler's camera.

  • If so, astronomers will detect subtle dips in the brightness of these stars - by as little

  • as one one-hundredth of a percent. That's the signature of a planet passing in front.

  • Another project is breaking new ground by combining data from several ground-based telescopes.

  • Astronomers recently used them to probe a star called 61 Virginis, 28 light years away.

  • It's nearly identical in mass and age to the Sun. What's more, it was here that astronomers

  • found the first Earth-like planets around a Sun-like star, at 5 and 7.5 times the mass

  • of Earth.

  • A simulation of the atmosphere of the larger one - picturing its flow of heat on a global

  • scale - shows how much we can understand, even at this distance. How long before a truly

  • Earth-like planet is discovered?

  • As astronomers' instruments get better - and their techniques more refined - it's likely

  • that they'll discover smaller and smaller planets.

  • For now, though, the "Rare-Earth" hypothesis may well have the edge over "Many Worlds."

  • The conditions that nurture a habitable planet seem hard to come by... such as position in

  • the "life zone"; a nearly circular orbit; and magnetic fields.

  • The threats are many, solar flares... marauding giant planets... tidal locking... and other

  • lurking catastrophes.

  • In fact, Nature has handed us a paradox: Medium-sized stars like our Sun turn out to be particularly

  • hostile to life. They emit barrages of ultraviolet radiation... and solar outbursts powerful

  • enough to ruin a fledgling atmosphere.

  • And yet, somehow, we manage.

  • So if it turns out that the Milky Way is filled with habitable worlds... its not that the

  • galactic real estate is so special... it's that life finds a way.

  • 8

If things don't work out on this planet...

Subtitles and vocabulary

B2 H-INT planet earth star sun habitable solar

The Search For Earth-Like Planets

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    Wonderful   posted on 2013/10/16
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