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9. Biomarkers
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天文学：探索时间和空间
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Exoplanets
In the last few years, thousands of extrasolar planets have been discovered, including some that are Earth-like and possibly habitable.
与讲师见面
Chris ImpeyDistinguished Professor
Astronomy
Many astronomers think that the most likely way we'll find life beyond earth
is through the detection of biomarkers.
That is, tracers of biology from the planet atmospheres of
the exoplanets we're finding.
If we look at the inner solar system we see a stark
difference between the specter of the atmospheres of the three main planets.
Mercury has no atmosphere to speak of.
Venus has a heavy atmosphere of carbon dioxide and
shows the strong imprint of absorption lines from carbon dioxide.
Mars has similarly carbon dioxide as the main imprint and
a much thinner atmosphere.
Earth is special in this comparison.
Because in addition to some carbon dioxide,
it shows the strong imprint of water vapor and absorption, and also oxygen and
ozone, which has an extremely strong absorption transition in the infrared.
If we could inspect the atmospheres of exoplanets, terrestrial exoplanets,
the low mass ones perhaps less than five times the earth mass,
we'd have the prospect of detecting biomarkers, signatures of life.
On earth, this results from the fact that the oxygen we
breathe was produced by microbes in the early earth, 2 to 3 billion years ago.
If life on earth disappeared overnight, the one part in five of oxygen in
the air we breathe would disappear in 5 to 10 million years,
a geologically very short time.
It only exists because it's replenished by photosynthesis.
If life disappeared, that oxygen would react with other materials,
would corrode and rust things, and would dissolve into seawater.
Reversing the logic, if we see the signatures of transient and
reactive molecules like oxygen and ozone, and also methane, in the atmosphere of
an exoplanet, it's a strong indirect indicator that biology is present.
Now that depends, of course, on our form of biology, photosynthetic-based biology.
But biologists think that's quite likely to occur, given the right ingredients.
In practice there's no single, smoking-gun biomarker, that is, one species or
one absorption line that will indicate life on another planet.
Current researchers are making planetary models where they include the geology of
the planet, its interactions, and
vary the planets atmosphere to see what will result.
Because we'll actually need a range a of species or
molecular types in the atmosphere to be convinced that what we're seeing is
not the result of natural geological processes.
Many of the key biomarkers, or
molecular absorption features, are in the near infrared.
So much of this work will have to be done beyond the visible spectrum, and
suitable instrumentation is needed to do this work.
However, that's an advantage too, because exoplanets have a stronger contrast with
their parent stars in the infrared, making their imaging advantageous.
But the goal here is extremely challenging.
We've seen how difficult and rare it is to be able to make an image of an exoplanet.
Now we're talking about going beyond blotting out the star, and
seeing that one part in a few hundred million of reflected light.
We're talking about taking that feeble reflected light and spreading it out,
or dispersing it into a spectrum, and looking for extra information there.
Even more challenging.
Astrobiologists are ambitious.
Some of the missions they're dreaming of will go beyond the simple detection of
biomarkers in an atmosphere, and try and
say what the surface of an exoplanet is made of.
We can look at the spectra of light that reflects off oceans, or
granite, or plant surfaces.
There's a chlorophyll edge, for example, so
green plant life on the earth imprints a spectral feature in the visible range.
So, detailed modeling is currently going on, to show exactly what
the reflected spectrum of an earth-like planet would be under varying conditions.
Biomarkers, as a signature of biology, do not stand still, they evolve with time.
And we're going to see exoplanets and
earth-like planets at different stages of their evolution.
The earth has not always been habitable, and
the earth's biomarkers have not always been the same.
There was a time before there were liquid, stable oceans, and so
the water signature on the earth would have been smaller.
There was also a time before microbial life existed and
put oxygen in the atmosphere.
So 3 or 4 billion years ago,
the earth's reflected spectrum would have looked quite different.
After about 2.5 billion years to about 1.5 billion years,
most of the oxygen had been created by microbial lifeforms, and
the earth attained the strong imprint of oxygen and its cousin ozone.
Even though ozone is a trace ingredient compared to molecular oxygen,
its spectral signature is actually, far stronger, and
easier to detect, especially in the infrared.
Earth's reflected spectrum has not changed substantially in
the last few billion years.
And, of course, if biospheres are transient, or evanescent, or if
life on earth does not last forever, once again, the earth may have a dead spectrum.
How do we detect biomarkers?
It's an enormous challenge in astrobiology, and the instrumentation and
telescopes available now are almost not up to the job.
But we hope in the next five or ten years it will be possible.
This will be done both from the ground and in space.
In space the big hope is the James Webb Space Telescope,
6.5 meter telescope in space, optimized to work in the near infrared.
The James Webb was conceived of before exoplanets have even been discovered.
So it's main scientific mission was never assumed to include exoplanets, and
it's not actually perfectly suited for the job.
People have run simulations about how hard or
easy it would be to get near infrared spectra of earth-like exoplanets.
It turns out to be very difficult.
Getting a single decent spectrum of an exoplanet at a distance of a few
dozen light years will take dozens of hours, perhaps 50 to 100 hours.
Most people expect that as much as a third of the James Webb mission will be
devoted to exoplanet research.
Which is causing some consternation amongst other astronomers who plan to
use James Webb for their research.
It means that time will be even more hard to get and
oversubscribed than previously thought.
Meanwhile, on the ground, using techniques like adaptive optics, already discussed,
astronomers hope to use their large new ground based telescopes, and
adaptive optics to blank out the light from a central star,
retrieve that feeble reflected radiation from the exoplanet,
disperse it into a spectrum, and look for biomarkers that way.
It's a coin toss whether this detection will be done first from the ground or
from space.
But many astrobiologists are taking bets on this being done within the next decade.
And in fact, this may be the way we first find life beyond earth.
Astrobiologists are frustrated.
In the recent Decadal Survey,
which is the way that astronomers orchestrate their priorities for
federal funding for the next decade, astrobiology didn't fair that well.
And some very ambitious missions that NASA's been considering,
such as the terrestrial planet finder, designed purposefully to
characterize earth-like planets, are currently on the shelf and unfunded.
The space missions to follow up exoplanets are going to be quite expensive,
multi-billion dollar missions.
And it's probably a decade or
more till we get a new mission, capable of following up Kepler targets.
What people sometimes forget about Kepler is that it's quite difficult to follow up
the targets it finds.
Kepler was constrained to look at one region of space,
to monitor a sufficient number of stars at one time with its relatively small CCD.
That means that those stars are quite faint, much fainter than
the bright stars that can be followed looking anywhere on the sky.
It also means that the exoplanets it finds are 10, 20, or
even 50 times further away than the nearest exoplanets we might locate.
Typical Kepler exoplanets are hundreds of light years away,
as much as 1,000 light years away.
So people are trying to complement exoplanet surveys with Kepler with ground
based surveys often using very small telescopes that look at the whole sky or
much larger regions of sky.
These are complementary approaches and we really need both.
The corollary of the faintness of Kepler's targets is that it's extremely difficult
to do biomarker research on them because their such feeble light.
What's the bottom line in the current study of exoplanets?
Having come from 0 exoplanets in 1994,
there now over 3,000, mostly from the Kepler mission in the last few years.
It seems that essentially all sun-like stars are likely to have one or
more planets, and perhaps one or more terrestrial planets.
Computer simulations and models, and increasingly, data suggest that
water worlds like the earth will be common, perhaps one per stellar system.
Extrapolating current surveys and numbers, there should be 1 billion
terrestrial planets in the Milky Way, and that's a conservative lower limit.
And perhaps 10 billion habitable worlds,
including the much larger cryogenic biosphere.
The next and final frontier in exoplanet research is characterizing the exoplanets
in increasing detail, and looking for the presence of life through biomarkers.
All of this frames a pivotal question, put by Enrico Fermi in the 1950s.
Fermi went through logic based on no exoplanets having been discovered but
just speculation based on current research and conditions,
that our civilization is young, and a modest extrapolation of our capabilities
say we should have left our planet, maybe in 100 years maybe in 1,000 years.
Fermi also knew the universe was old, and we were unlikely to
be the first civilization to get to this stage of development.
He also knew that the ingredients for life were widespread through the Milky Way, and
he speculated that there were large numbers of
habitable worlds where life had, in fact, started.
Putting all of these pieces together, and the fact that the technology to travel
through space, either with robots or the actual organism's traveling, was quite
close and nearly in our reach already, he came up with a provocative question.
Where are they?
The exciting current frontier in exoplanet research is the detection of biomarkers.
Biomarkers are molecular traces of life such as oxygen or
ozone seen by a reflectance spectroscopy.
This is an extremely challenging technique,
given that imaging exoplanets is hard.
Missions are being planned and ground-based observatories may be able to
do this within five to ten years, and first answer the question of whether we
are alone as biological organisms in the universe.
