5.1. ASTROBIO – What makes a Planet Habitable?

By | July 6, 2014
5.1. ASTROBIO - What makes a Planet Habitable?

[BLANK_AUDIO] 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

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