Life, an unbelievably perfect combination of elements
that is able to create living matter,
may be the biggest mystery of nature.
Is life an amazing phenomenon exclusive to our planet?
So where might this second earth be?
Second Genesis within the same solar system implies that life,
the origin of life, is a likely event.
If it happens twice in the same solar system,
it's likely happening everywhere in the universe.
Now scientists are searching for planets
far beyond the boundaries of our solar system
where we might detect life in the near future.
Thanks to NASA's Kepler space telescope,
launched in 2009,
we know that, in our galaxy alone,
there are billions of earth-like exoplanets orbiting their stars.
Given the vastness of the universe,
with more than 100 billion galaxies,
it is hard to conceive that somewhere,
there is no earth-like planet that can harbor life.
Unless something very unusual happened here on earth,
then life has developed on thousands of millions of planets
just in our galaxy, right?
If it's not a miracle, then it's all over the place.
That's the bottom line.
When we take a look at what we know
about the origins of life on this planet,
it leads us to think
that the same things that happened here
might well have happened elsewhere.
And so life beyond this planet is quite plausible
in terms of the science that we know today.
Could there be life beyond our solar system?
Captions by vitac... www.vitac.com
captions paid for by discovery communications
Is there life beyond earth?
Humankind has always asked itself this question,
but no answer has yet been found.
However, thanks to the latest advances in space technology,
it appears that we are closer to solving this mystery.
Recent discoveries have uncovered planets
beyond our solar system
that are believed to be similar in many ways to earth.
If I got to ride in a spaceship to one planet
that we've found with Kepler,
the one that I would go to is Kepler-186f.
It's one of the smallest ones.
It's at the right temperature that liquid water
could exist on its surface.
Kepler-186f is the first validated earth-sized planet
to orbit a distant star in the habitable zone
where liquid water might pool on a planet's surface.
The discovery of Kepler-186f
confirms that earth-sized planets exist
in the habitable zones of other stars
and signals a significant step closer
to finding a world similar to earth.
Kepler-186f orbits its star
once every 130 days
and receives 1/3 the energy
that earth does from the sun,
placing it near the outer edge of the habitable zone.
If you could stand on the surface of Kepler-186f,
the brightness of its star at high noon
would appear as bright as our sun is
about an hour before sunset on earth.
Before Kepler was launched, there was hundreds of planets
that we knew of in systems around other stars,
and now we know of thousands.
And that's why Kepler was so revolutionary.
Kepler is a space telescope specifically designed
to survey our region of the milky way galaxy
to discover earth-sized and smaller planets
in or near the habitable zone of their respective stars
and determine the fraction
of the hundreds of billions of stars in our galaxy
that might have such planets.
It works very simply. I mean, anybody can understand this.
It's just staring at one spot on the sky all the time.
Never blinks.
And it's looking at 150,000 stars.
And it just monitors how bright they are,
kind of like a camera light meter, really.
And occasionally, they'll see this star will be here,
for example. It'll get a little bit dimmer,
a very fraction of a percent dimmer for a few hours,
and then it'll get bright again.
Well, that happens if a planet passes in front of that star.
We've gone from finding 100 planets
to over 1,000 planets with Kepler,
those that have been confirmed.
And there's about 3,000 or 4,000 more
which we have strong evidence for,
but we wouldn't consider confirmed planets just yet.
Kepler is on the hunt for planets.
Kepler has found literally thousands of planets
or planetary candidates.
It's a planet-finding machine.
In 2011, for the first time ever,
Kepler provided scientists with an astonishing calculation
of the number of stars in our galaxy
that could have a planet like ours...
Around a billion.
Maybe there are a million. Maybe there are a billion.
Maybe there are 100 billion planets
in the milky way galaxy
that could support life, the kind of planets
that earthly life could survive on.
How many of them have cooked up their own life?
And we don't know the answer to that, okay?
Because that depends on how hard it is to get life started.
Just 'cause I give you all these, you know, these worlds,
doesn't mean that life will get started.
But, on the other hand, those planets are all made
out of the same stuff that earth is.
So, again, unless something very unusual happened here
and nowhere else,
there's going to be biology all over the place.
On April 2013, just four years after its launch,
the Kepler team reported one of their first great triumphs...
The discovery, for the first time ever, of two exoplanets
very similar to the earth,
Kepler-62e and Kepler-62f.
This discovery created great enthusiasm,
as it implied the confirmation of earth-like planets
where life might be possible.
These planets have a radius
1.6 and 1.4 times that of earth
and orbit Kepler-62,
an orange dwarf star,
in its circumstellar habitable zone.
A modeling study also concluded
that Kepler-62e and Kepler-62f
are likely covered, perhaps completely, in water.
Kepler-62e probably has a very cloudy sky
and is warm and humid all the way to the polar regions.
Kepler-62f would be cooler
but still potentially life-friendly.
Unfortunately, they are at a huge distance
of 1,200 light-years away
in the constellation of Lyra.
Thanks to the Kepler mission,
we now know that there are tens of billions of planets
orbiting stars in our galaxy alone,
and we know that there are billions of galaxies
across the universe.
So if, in just one planetary system like ours,
life arose on one planet,
and there are at least four more candidates,
the likelihood of finding a planet in outer space
that could harbor life should be very high.
But Kepler has discovered for us
not only the existence of earth-like planets
but also has provided amazing data about the universe,
such as the confirmation of the existence of planets
that orbit around not only one but two stars,
like Kepler-16b.
This planet was Kepler’s first discovery of a planet
that orbits two stars,
what is known as a circumbinary planet.
So one of the most exciting discoveries from Kepler
was that we were actually able to find
planets around binary stars.
And the first one that was found was Kepler-16b.
Kepler-16b is many people's favorite planet
that was discovered with Kepler.
It orbits around not one but two stars at the same time.
This was something which was predicted not to exist.
Two stars setting at the same time
was just a piece of science fiction.
But the universe is stranger
than what scientists can imagine,
and it turns out that this sort of thing is true.
Since 1992,
over 2,000 exoplanets have been discovered.
Thanks to future space telescope missions planned for launch,
the number of observed exoplanets
is expected to increase greatly in the coming years.
Despite having discovered just a tiny fraction
of all those billions of exoplanets we think exist,
how could we know how many of them could harbor life?
In astronomy and astrobiology, the region around a star
where a planet with sufficient atmospheric pressure
can maintain liquid water on its surface
is known as the circumstellar habitable zone.
The habitable zone is a place...
It's kind of a way of thinking about the right way
to go look for planets like our own.
The earth is obviously in the circumstellar
habitable zone of our solar system.
A potentially habitable planet implies a terrestrial planet
with conditions roughly comparable to those of earth,
and, thus, potentially favorable to life.
There's a sweet spot, an area where it's not too hot
and not too cold,
and we call that the habitable zone,
or some people call that the Goldilocks zone.
If you've got a planet in that region, even if it's small,
it's rocky enough and it had water,
that water would be in a liquid state more than likely.
So that would be a good place to go looking.
On November 4, 2013, astronomers reported,
based on Kepler space mission data,
that there could be as many as 40 billion
earth-sized planets
orbiting in the habitable zones of sun-like stars
and red dwarfs just in our galaxy,
11 billion of which may be orbiting sun-like stars.
However, now we know that stars very different from our sun
may also be a good place to look for life.
In may 2016, a team of astronomers announced
for the first time ever
the finding of three habitable planets
orbiting
an ultra-cool dwarf star.
It's the first planetary system found around a star like this.
The star, named Trappist-1,
is just 40 light-years away
and is much cooler and redder than the sun
and barely larger than Jupiter.
Stars like this are very common in the milky way,
and they are very long-lived.
The three planets are very similar in size to the earth
and might have habitable regions on their surfaces.
So the answer as to what place in the universe
is best to find life has radically changed.
To date, among all of the more than 1,000 confirmed exoplanets,
there are around 50 that are in the circumstellar habitable zone
of the star they orbit,
and, therefore, they could be potentially considered
earth-like planets,
which does not imply these distant worlds may harbor life.
However, could life be possible
outside those habitable zones?
The discovery of hydrocarbon lakes on Saturn’s moon, titan,
has begun to call into question the carbon chauvinism
that underpins circumstellar habitable zone theory.
Liquid water environments have been found to exist
in the absence of atmospheric pressure
and at temperatures outside the circumstellar
habitable zone temperature range.
Just because a planet is outside the habitable zone
doesn't mean that it couldn't have life.
For example, Saturn’s moon Enceladus
and Jupiter's Europa,
both outside the habitable zone of our solar system,
may hold large volumes of liquid water in subsurface oceans.
If we are considering the possibility
that life could arise in such harsh environments
that are not in the circumstellar habitable zone,
that implies that we should look for exoplanets
far beyond those zones.
Therefore, the chances of finding an exoplanet
with conditions to support life are much higher.
Just analyzing the planets and moons of our own solar system
is not an easy task,
since they are millions of miles away from earth.
However, exoplanets are not just millions of miles away,
but many light-years away from earth.
How can astronomers manage to study exoplanets
if they are trillions of miles away?
There are several methods used by astronomers
and astrobiologists
to discover and to study
these extremely distant, mysterious worlds.
Before the launching of the Kepler mission in 2009,
the most successful technique for detecting exoplanets
was Doppler spectroscopy,
also known as the radial-velocity method.
The radial-velocity method relies on the fact
that a star does not remain completely stationary
when it's orbited by a planet.
A planet is much smaller than its star,
but it still exerts a tiny gravitational pull
or tug on the star as it orbits.
When a planet is behind the star from our point of view,
it pulls the star slightly away from us.
When it's in front, it pulls the star slightly towards us.
This causes the star to wobble back and forth.
Astronomers look for this wobbling to find planets.
They use something called a spectrograph
and powerful telescopes
to examine the light coming from a star.
A spectrograph, like a prism,
splits the light from the star into its component colors,
producing a spectrum.
Some of the starlight gets absorbed
as it passes through the star's atmosphere,
and introduces small, dark gaps or lines in a spectrum.
As the star moves closer to us,
these lines shift toward the blue end of the spectrum.
As the star moves away, the lines shift back
toward the red end of the spectrum.
So the spectrum appears first slightly blue-shifted
and then slightly red-shifted.
Therefore, astronomers can look for orbiting planets
by looking for these back-and-forth motions
of the lines in a star's spectrum.
And you can work out,
from the speed it's being pulled towards you
and how long it takes to go around in that circle,
you can work out the mass of the planets
or the companion thing which is pulling it around.
If that mass is very small, that thing is a planet.
That method has been very successful.
It found most of the planets which were discovered early on.
However, Kepler’s observations
were based on something entirely different.
The Kepler telescope used the transit method
to detect planets.
It's based on the observation
of a star's small drop in brightness
that occurs when the orbit of one of the star's planets
passes, or transits, in front of the star.
The amount of light lost depends on the size of the star
and the planet. And the duration of the transit
depends on the planet's distance from the star
and the star's mass.
With the Kepler space telescope, when we look at a star,
we don't see the planet directly.
All we see is a tiny dip in the brightness of the star
when the planet passes in front of it.
From the size of the dip,
we can work out the relative size of the planet.
Is it a big planet? Is it a small planet?
The combination of transit methods and Doppler methods
reveals planetary radius, mass, and density,
which are some of the main parameters to evaluate
the potentiality of the exoplanet
as an earth-like candidate
and, thus, able to harbor life.
Once you know the distance away from the star
and you know how hot the star is,
you can estimate what the temperature
on the surface of the planet would be like.
Another of the main parameters
to analyze the earth-like potential of an exoplanet
is its atmospheric compensation.
Atmospheric studies of exoplanets might be performed
with spectroscopy during planetary transits.
During the transit, the stellar light
passes through the atmospheric limb of the planet.
Spectral analysis of this filtered light
reveals the structure and composition of the atmosphere.
Astronomers could identify the most earth-like exoplanets
by detecting the biomarkers,
which are the imprints that life-forms have
on their host planet atmosphere.
For example, the molecular oxygen that we are breathing
results from the presence of life on earth.
Unfortunately, with present-day technology,
it's extremely hard to study the atmosphere composition
of those remote planets.
The main thing that's keeping us from being able
to do that now, of course, is technology.
We don't have the technology, you know,
in functioning instruments at the moment
to be able to do that effectively.
But that's something that will change
with the other missions in the pipeline,
like Jack Webb space telescope
and other missions that are happening.
Kepler continuously monitors over 100,000 stars
similar to our sun
for brightness changes
produced by planetary transits.
Thanks to this ingenious technique,
Kepler has confirmed to date
more than 1,000 exoplanets.
Scientists think that a few dozen of them
can be labeled as earth-like.
Considering the possibilities of finding an earth-like exoplanet
are much higher in the circumstellar habitable zone
of its planetary system,
planet hunting then started to focus on those areas.
One of the first discoveries was 70 Virginis B, an exoplanet
located approximately 60 light-years away
in the constellation of Virgo.
70 Virginis B was located exactly in the middle
of the circumstellar habitable zone
of its planetary system,
so it was supposed to not be too hot or too cold.
Unfortunately, further studies reported that this remote world
was a gas giant with very high temperatures,
which ruled out any potential for liquid water
and, therefore, life.
The early findings were discouraging
in terms of detecting an earth analog,
but this was just the beginning.
In 1998, a discovery made in the star Gliese 876,
a red dwarf located in the constellation of Aquarius
at a distance of 15 light-years away from earth,
really encouraged astronomers.
A gas giant was detected in its habitable zone,
Gliese 876 B.
Three years later, another gas giant
closer to this one was found,
Gliese 876 C.
We know that life as we know it is not possible
on gas giant planets
such as Jupiter or Saturn,
but the big surprise was that both exoplanets
may have habitable moons orbiting around them
as Jupiter and Saturn have.
This was one of the first planets
to be discovered in the habitable zone,
and people theorized that if it had a moon around it,
the moon would be rocky,
and the moon would be at the right temperature
to have liquid water.
Why couldn't any of these hypothetical moons
around Gliese 876 B and C harbor life,
as we hope Jupiter's moon Europa
or Saturn’s moons titan and Enceladus might?
So it's an exciting place to think about
and maybe an exciting place to look for in the future.
After the discovery of these exoplanets
with potential earth-like moons,
several similar exoplanets
with moons orbiting around them were discovered.
Maybe on any of those remote moons,
life arose in the past
or exists in the present
or might appear in the future.
After all these early discoveries,
we started to approach the main goal...
To find the most earth-like world.
An earth analog, also referred to as a twin earth
or earth-like planet,
is a planet or moon with environmental conditions
similar to those found on the planet earth.
If life could arise on earth millions of years ago,
and if we look for exoplanets similar to our planet,
the chances of finding habitable planets skyrocket.
Recent discoveries have uncovered planets
that are believed to be similar in many ways to earth,
with relatively high earth similarity indexes.
The size is often thought to be a significant factor,
as planets of earth's size
are thought more likely to be terrestrial in nature
and be capable of retaining an earth-like atmosphere.
From the point of view of Kepler, a planet is earth-like
if it's small enough that it's probably rocky...
It's not a gas giant like Jupiter or Saturn.
But size alone is a poor measure,
particularly in terms of habitability,
because next to us, there's a planet
with a very similar size and mass, Venus,
where it's almost impossible for life to arise.
There are other criteria to be considered,
like the surface gravity or the star size and type.
A planet is earth-like
if it's the right distance away from a star
that it's in the habitable zone,
that it's not too close and it's too hot
and all the water there would boil away,
and not so cold that if there's any water on it,
it would all freeze to ice.
If we were able to examine all of these parameters
of an exoplanet, we would be able to know
if it is or isn't a real twin earth.
So when we say earth-like for Kepler,
we usually just mean that it's small enough
that we think that it's solid enough
that you could stand on it,
but that doesn't mean that it has an atmosphere
or that it has an ocean.
It's also cited that an earth analog must be terrestrial...
That is, it should possess a planetary surface
composed of materials similar to earth's.
The conclusion would be that extra-solar planets or moons
with substantial atmospheres
may possess oceans and water clouds
like those on earth.
In addition to surface water,
a true earth analog would require a mix of oceans or lakes
and areas not covered by water.
We believe that water is essential,
but just about everything else you can think about
which is important for life,
there seems to be life on the earth that doesn't need it.
There's life that's survives without sunlight.
There is life that survives without oxygen.
There is life that survives deep down underneath the ice
in Antarctica.
Unfortunately, with present technology,
we can't properly evaluate most of the parameters,
like the temperature, the atmospheric composition,
or the surface of the exoplanets.
Nevertheless, considering we have already discovered
hundreds of exoplanets, we can't help but wonder
if a real earth analog has already been discovered.
On April 18, 2013, astronomers from the Kepler team
announced a discovery that created great expectation.
For the first time ever, two very earth-like exoplanets
were found.
They were the Kepler-62e...
and the Kepler-62f...
which orbit Kepler-62, an orange dwarf star,
in its circumstellar habitable zone.
They immediately became prime candidates to host alien life.
A modeling study also concluded that Kepler-62e
and Kepler-62f are likely covered mostly,
perhaps completely, in water.
Kepler-62e probably has a very cloudy sky
and is warm and humid
all the way to the polar regions.
Kepler-62f would be cooler but still potentially life-friendly.
Unfortunately, they are a huge distance
of 1,200 light-years away
in the constellation of Lyra.
Soon after, an exoplanet even more similar to the earth
was discovered,
Kepler-186f.
This finding was a milestone, as it was the first rocky planet
found in the habitable zone of its system.
It is 492 light-years away from the earth.
Kepler-186f is possibly my favorite planet
to come out of the Kepler mission.
It's a small planet.
It's maybe 10% to 20% bigger than the earth.
And based on everything we know about planets,
it's almost certain to be rocky,
and it's the right distance away from its parent star
that if the atmosphere is right,
if the greenhouse effect is right,
it could have liquid water on its surface.
After that discovery,
several more earth-like candidates
started to arise,
like Kepler-438b,
Kepler-442b,
or Kepler-440b.
All of them were very similar to our planet,
but none of them was a real twin earth.
But everything changed on July 23, 2015.
That day, NASA's Kepler space telescope science team
shocked the scientific community with an amazing finding.
Scientists announced an amazing finding.
They had discovered the most earth-like planet ever.
Its name... Kepler-452b.
What made this one different
to the other previous earth analog candidates?
Kepler-452b is the very first apparently rocky planet
that orbits a "G" -type star like our sun.
It's a planet in the habitable zone around a star
which is almost a clone of our own sun.
After this discovery,
the earth is a little less lonely in the universe.
Kepler-452b circles its star,
which is about as hot as our sun,
10% brighter, and 20% larger,
at an orbital radius
just 5% larger than that of the earth.
A year on this planet is 385 earth days long,
just 20 days longer than earth's.
What makes this slightly less exciting from the point of view
if it's going to be habitable is its size.
Our best guess at the size is that it's about 60% bigger
than our own earth.
It is the smallest earth analog planet ever found
in the habitable zone of a "G" -type star like our sun.
Previous research on super earth-sized planets, like 452b,
suggests this one has a good chance of being rocky.
If it is a rocky world, it would weigh in
at about five earth masses,
giving it a surface gravity of roughly 2-g,
which would mean that our weight would be double on its surface.
Kepler-452b could have a thick, cloudy atmosphere
and volcanic activity.
Even more exciting
than Kepler-452b's earth-like demeanor
is the fact that this world has spent around 6 billion years
in the habitable zone of its star.
That's considerable time for life to arise
somewhere on its surface
or in its oceans, should the conditions for life exist.
Kepler-452b is about 1.5 billion years older than the earth.
If it was earth-sized,
the planet and its aging, brightening star
might be at a point in their evolution
where liquid water would be rapidly evaporating
from the surface.
But because of its higher mass, astronomers believe
Kepler-452b could continue to hold liquid water
for the next 500 million years.
So far, it's the only known world in its system,
which lies some 1,400 light-years away
in the Cygnus constellation.
Obviously, we're not going to get there any time soon,
but it's fascinating to imagine
that far off in the distant reaches of space,
a world very much like our own
might already exist.
If this twin earth exists,
why couldn't thousands more like it also exist?
Less than a year after the amazing finding of Kepler-452b,
in may 2016,
a new discovery shocked the scientific community.
Astronomers using telescopes
at European Southern observatory in Chile
discovered three planets around a dim dwarf star
just 40 light-years from earth
in the constellation of Aquarius.
These worlds may be the best targets so far
found in the hunt for life elsewhere in the universe.
They used the Trappist telescope
to monitor the brightness of an ultra-cool dwarf star
in the constellation of Aquarius,
which has been named Trappist-1.
Trappist-1 is much cooler and redder than the sun
and barely larger than Jupiter.
Stars like this are very common in the milky way,
and they are very long-lived.
This was the first time
that planets have been found around one of them.
The three planets are very similar in size to the earth
and might have habitable regions on their surfaces.
But the really exciting result
is that these are the first earth-like planets
that are well-suited for the detection of life.
The ultra-cool dwarf stars are the only places
where life could be detected on an earth-sized exoplanet
using our current technology.
The light from much brighter stars,
like the sun, for example,
would swamp vital measurements of the atmospheres
of any candidate planets.
The next step is to make more detailed observations
using the next generation of telescopes,
such as E.S.O.'S European extremely large telescope
and the James Webb space telescope.
That will allow astronomers
to study the atmospheres of planets like this
and to search for molecules
related to biological activity,
like ozone, methane, or water.
Although there is not yet any proof of the existence of life
on all of those exoplanets that we've already found,
even in the most earth-like of them, like Kepler-452b,
we can't help but wonder if any of those potential forms of life
that might have arisen there
were or will be able to evolve into intelligent life.
If any of those extremely remote worlds
were formed billions of years ago,
as earth was, and it became a habitable planet,
then the organic compounds had time enough to be mixed up
and organized into living forms.
Perhaps any of those living forms
might have evolved into complex forms of life,
and some of those complex forms of life
might evolve into intelligent beings with consciousness.
Thanks to Kepler mission research,
it's known that just in our galaxy,
the milky way, there might be millions of earth analogs,
and there are billions of galaxies across the universe.
So the chances are much higher
than we could have ever imagined.
For that reason, science is carrying out an intense search
for intelligent life.
The SETI institute in California is the main world institution
devoted to the search for extraterrestrial intelligence.
It's name, SETI,
stands for "search for extraterrestrial intelligence."
This search is based on the use of radio telescopes.
Radio telescopes receive radio waves.
As we can't go to space aboard spacecrafts
to find that intelligent life,
what we look for are radio signals.
What we're looking for is a signal that's at one spot
on the radio dial,
just like when you're listening to the radio in your car.
You know, you tune across the dial.
You hear static everywhere.
And then at one spot, you hear [hums]
And there's a station.
Okay, that's the signal
that's produced by a transmitter somewhere.
It's not natural static.
It's not like a quasar or a pulsar,
or galaxies or hot gas, cold gas.
All those things in space make radio noise.
But it's all over the dial.
So we look for signals that are at one spot on the dial,
and, of course, the source of the signal
has to be up in the sky.
Those are the kinds of criteria we use
to know that even if we don't know what it means,
we at least know they're there.
They're on the air.
Unfortunately till now, we haven't received a signal
that can be really attributed to extraterrestrial intelligence.
In the coming years, NASA
and E.S.A., the European space agency,
have planned to launch several space telescopes
that will surely help to unveil the mystery of life
beyond the boundaries of our solar system.
The most ambitious one is the James Webb space telescope.
This project is an international collaboration
between NASA, E.S.A.,
and the Canadian space agency, CSA.
It will be the premier observatory of the next decade,
serving thousands of astronomers worldwide.
It will study every phase in the history of our universe,
ranging from the first luminous glows after the big bang
to the formations of solar systems
capable of supporting life on planets like earth,
to the evolution of our own solar system.
This new telescope, three times more powerful than Hubble,
will be able to analyze starlight
passing through the atmosphere
of the closest earth-like worlds,
looking for the telltale signs of life itself,
like, for instance, detecting gases in its atmosphere
usually linked to life processes such as oxygen, methane,
carbon dioxide, or nitrogen.
Just is really going to help us understand
what atmospheres of planets are like under different conditions,
and that's going to be a really exciting result.
There is another mission, the Cheops mission.
The Cheops come from
"characterizing exoplanet satellite."
This is an E.S.A. mission
dedicated to searching for exoplanet transits
by performing ultra-high precision photometry
on bright stars already known to host planets.
With all of these new space telescopes
programmed to be launched in the coming years,
we are sure that finding a habitable exoplanet
and any consistent bio-signature will be just a matter of time.
If we are ever able to find evidence
of the existence of any form of life
among one of those billions of exoplanets
we know are across the observable universe,
it would be undoubtedly shocking.
If we did find life somewhere else in the solar system,
and we were able to do, for example, a genetic analysis
to determine that it was distinct from life on earth,
that would be a really earth-shaking discovery.
Living generations might be witness to a finding
that would undoubtedly be a turning point
in the history of humankind,
the discovery of life in outer space.