Learn about the origin and evolution of life and the search for life beyond the Earth.
What Makes a Planet Habitable?
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來自 The University of Edinburgh 的課程
天体生物学和寻找外星生命
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從本節課中
Habitable Environments Elsewhere in our Solar System?
What are the prospects for life on other planetary bodies in our Solar System and how do we go about searching for it? What conditions are required for a planet to be habitable?
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Charles CockellProfessor
Astrobiology
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In astrobiology we frequently hear the term habitability.
People talk about planets being habitiable.
People talk about planets orbiting distant stars potentially being habitable.
What does this really mean?
Let's take a brief look at this word, habitability.
First of all, what makes a planet habitable?
We've learned something about the requirements for life, and given
those requirements, we can define some criteria that might make
a planet habitable.
For instance we would expect a habitable planet to have a source of liquid water.
At least as far as we know concerning the requirements of life.
Life needs liquid water as a solvent for biochemistry.
We also need a planet where there's a source of energy.
Energy for life to harness in order
to carry out these particular biochemical functions.
We also need a planet where there's a source of elements or
the nutrients to sustain life.
And finally, of course, we need physical
conditions that are suitable life, physical conditions that
are within the boundaries for life to be able to persist on the planetary surface.
One of the most enduring concepts in astrobiology is the habitable zone.
This is the zone around a star where conditions are suitable
for liquid water to form on the surface of a planet.
This is just an image depicting the habitable
zone in our own solar system that lies
between Venus and Mars, and of course, our
own planet, planet Earth is within the habitable zone.
Allowing bodies of liquid water to
persist on the surface of our planet.
It's that zone where the solar heating,
perhaps combined with the greenhouse effect of a
planet is sufficient to create bodies of liquid water on the surface of a planet.
The habitable zone is a very useful concept, because we can use it to assess
the habitability, for example, of planets orbiting other
stars, and to see where in the star
system that planet resides, and whether it is far
enough away from the sun not to be to
hot, but close enough to have liquid water on
its surface, and for that liquid water not to freeze.
The habitable zone of course will vary depending upon the star.
For very hot stars that give out more
energy, the habitable zone will be further away.
And, for very cool stars, the habitable zone will be much further in, much closer
into the star.
And so the habitable zone will vary depending on the
temperature and, of course, on the age of the star.
One has to be a little careful with this concept of the habitable
zone because liquid water can exist
outside the habitable zone in particular environments.
This is the moon of Jupiter, Europa. About the same size as our own moon.
And it has any icy surface,
a crust of ice under which there seems to be a liquid water ocean.
How does that liquid water ocean get there when it's far outside the habitable zone,
very far away from the star where it's very cold in the outer solar system.
Well, this liquid water ocean seems to be
formed by tidal buckling, the huge gravitational force on
Jupiter buckles Europa and creates heat in the center
of that moon that melts the ice and creates
the liquid water ocean.
So here's an example, the moon that seems to have liquid
water despite that fact that it's outside of the classical habitable zone.
But nevertheless, that doesn't detract from the fact that
the habitable zone is still a useful concept in astrobiology.
We just have to be careful about the way we use it and the possible exceptions.
There are other things that people
have discussed that are required for habitability.
Well, one of them is active geochemical turnover.
If a planet is not geologically active, eventually, nutrients and energy sources
will be used for life and the planet will essentially will run down.
There'll be no new energy supplies.
No new nutrients. We need active geo-chemical turnover.
First of all to recycle elements and nutrients in the crafts
that are needed by life, but also to generate chemical reactions
that can create new sources of energy, chemical disequilibrium that's
necessary for life to harness those energy supplies to grow.
There are a couple ways in which that might happen.
We might have, for example, plate tectonics where one plate subducts
under another plate and becomes heated and melts and these movements
of plates over the surface of a planet within its crust
create this turnover that's necessary to create the elements and nutrients
for life. Another way is active volcanism.
Volcanoes spewing lava and magma onto the surface of a planet also creates
turnover within the crust and generates new energy and nutrient supplies for life.
Of course, it's also important that the physical conditions are
not too extreme for life, for long periods of time.
It's no good having a planet where conditions are suitable for life for
a short period and then it becomes too extreme and then suitable again.
We need conditions for life that persist possibly over billions of
years and this leads to another concept called the continuously habitable zone.
The continuously habitable zone is that zone around a
star where not only is there liquid water, but
there are conditions suitable for life for many billions
of years to allow life to evolve and to
proliferate and maybe to eventually evolve into multicellular life and intelligence.
So we need conditions that are suitable for life, for billions of years.
People have also speculated on other requirements for habitability.
For example, some people think that a moon
is very important for a planet to be habitable.
In our own case, our own moon is
responsible for stabilizing the tilt of our planet,
or the obliquity.
If we didn't have a moon the tilt of
our planet would vary wildly over tens of thousands
of years, and this would change the climate very
dramatically over relatively short periods, at least geologically speaking.
Some people say that if we had no moon, the climate would vary so
wildly, it would really be difficult for
life to become sustained on the planetary surface.
But, we have to be careful about these sorts of assumptions.
One could argue, for example, if you didn't
have a moon and the climate varied wildly, it
would lead to a biosphere full of organisms who
are very good at dealing with rapid climatic changes.
Maybe such rapid climatic changes would make them better at evolving to deal
with catastrophic changes, such as asteroid
or comet impacts, or giant volcanic eruptions.
Perhaps being a generalist organism that could survive wildly
varying climatic conditions would be good for life over long periods of time.
So, you can see that there are controversies and complications.
In really finding out what the true
criteria of habitability are, what is really
needed on the surface of a planet for life to be able to persist.
What have we learned in this lecture?
Well, we learned that the habitable zone is the
zone around a star where liquid water is stable
on the planetary surface.
We've learned that the habitable zone will of
course vary according to the temperature of the star.
And although it's a useful concept, water can exist outside the
classic habitable zone, for example on small moons, such as Europa.
We've also learned that what makes a
planet habitable is a matter of great discussion.
But at the very least, we would expect to
need conditions where life has water, an energy source and
where nutrients and elements that it needs
to grow and replicate are being replenished.
These conditions allow us to asses other planets as it bodes for life and
also, use the criteria for habitability for
life to assess planets orbiting other stars.
