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Cosmic Journeys: The Search For Earth-Like Planets
Poziom:
Temat: Nauka i technologia
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.
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