We've taken a look at where in
the solar system we might have habitable environments.
It's intriguing to now look across the galaxy, and see where there
might be habitable planets in our galaxy and how we might find them.
I'm going to start out by first stating my opinion.
My opinion is that there is life elsewhere in the universe.
The question, in my mind, is not, is there life in the universe, but
is there abundant life in the universe or is life incredibly rare in the universe?
That question I have no idea what the answer is.
This is one of the reasons for exploring these sorts of things.
So the question is, how do we go about it?
Well we now know enough about what life is composed of, how life works,
that we can try to make guesses and, and guesses are really the right word.
Guesses of where the right places to look for life are.
Now, it may well be that life is abundant in places
like under the ice sheets of Europa or spewing out of Enceladus.
Or in deep thermal vents on Mars, but those make
no global signature that we can see from far away.
And so while those are intriguing ideas, they really
aren't going to help us discover life around other stars.
Places where we can discover life around other stars are presumably planets.
Planets with abundant enough life that it causes enough change on
that planet that we can figure out that there's something going on.
We'll talk about that in the next lecture.
In this lecture we're first just going to think about what
are the right sorts of planets to be looking at.
And thinking about that, the word that is
always thrown around, the words are habitable zone.
Habitable zone is the region around a star where
a planet could have the right conditions to be inhabited.
What does that mean?
Well, then it gets complicated.
People have argued for a long time about exactly what that means.
What the habitable zone is, how big it is around
our star, how big it would be around other stars.
But we have some pretty simple things that we
can work with, and at least get a good approximation.
I don't think arguing too much about exactly what the
habitable zone is, is a good idea because I think we
know so little that it's better to just get an
approximation and then go out and try to learn from that.
Let's think about what we know.
In the solar system, the habitable zone had better include the Earth.
That seems like a good idea.
And it might exclude Mars.
And it might exclude Venus.
It might include Mars.
Certainly might have included Mars in the past.
May even have included Venus in the past.
But you can imagine that that's sort of the,
the boundary that we might think is the important boundary.
What is the important thing that we can
estimate that would be an important thing for habitability?
The main thing that we're going to go off of here is the presence of liquid water.
We've talked about the possibility of other solvents,
we've talked about the necessity of other things.
We're going to assume that if you have a planet that has
liquid water, it will have the other things that are necessary too.
It will have sunlight from which energy can be derived, it
will have chemicals in the rocks from which energy can be derived.
So really what we're looking for is liquid water, and we're
looking for this liquid water at the surface of the planet.
What do you need for liquid water?
Well, let's just say that you need a temperature
something between 0 degrees Celsius and 100 degrees Celsius.
It changes a little bit depending on the pressures of your
of your atmosphere, but let's just, we'll go with those limits.
What do you need to do to be between 0 and 100 degrees Celsius?
Well we know how to do that calculation.
We can calculate what the expected temperature of the surface of
a planet is based on its distance away from the star.
Remember how we do this?
We did this for Mars.
Where we simply said that the energy of the sunlight, incoming energy of
the sunlight has to be equal to the outgoing energy of the black body.
And so that is sigma T to the fourth.
Sigma is the Stefan-Boltzmann constant, T is the
temperature in Kelvin, and this is the incoming sunlight.
We argued back when we talked about Mars that in order to take the average
amount of sunlight that the planet feels over
rotation, we should really divide this by 4.
And the other thing that we argued is that some of the sunlight is reflected.
So you don't actually get all the sunlight in,
you actually get 1 minus the Albedo of the planet.
Albedo tells you the faction that's reflected.
So 1 minus the Albedo tells you the amount that's absorbed.
So this is the amount of, that's on average absorbed by the planet.
This is the amount that's on average reemitted
by the planet as a black body thermal radiation.
And so when this temperature gets to be the right value you have an equilibrium
and the same amount that comes in,
same amount goes out, your temperature doesn't change.
We can easily calculate temperatures using this formula.
And if we calculated the temperature of the surface of the Earth,
for example, by assuming that the Albedo of the Earth is around 0.3.
Sunlight at the top of the Earth's atmosphere is 1,361 watts meters squared.
And we solve for the average temperature of the Earth.
And we get that it is equal to something like minus 18 degrees celsius.
That seems kind of cold I don't think that's actually
the correct volume for the temperature of the Earth, do you?
No so what did we do wrong?
Well, there is one thing that we didn't take into account and this is a crucial
thing for both the Earth and for calculating
habitability zones, and that is the greenhouse effect.
As you remember, the greenhouse effect works
because sunlight coming in comes in in
the visible wavelength ranges and the atmosphere
is clear in the visible wavelength ranges.
But the outgoing thermal emission goes out in the infrared where the atmosphere is
not clear, it absorbs some of that
infrared radiation and it's just like a blanket.
You put a blanket on top of you to absorb the
heat that's coming out of you and re-emit down towards you.
This is what happens, you put this blanket on
top of the earth, it absorbs some of this outgoing
radiation and reradiates this back to the surface and
it heats up the surface and makes less radiation leaving.
You have to have that same amount of radiation
leaving as coming in or your temperature goes up.
So, the greenhouse effect makes your temperature go up.
The actual average surface temperature of the earth is something like 14
degrees celsius, which is a greenhouse warming of about 32 degrees celsius.
The greenhouse effect, it's a good thing for now.
