Learn about the science behind the current exploration of the solar system in this free class. Use principles from physics, chemistry, biology, and geology to understand the latest from Mars, comprehend the outer solar system, ponder planets outside our solar system, and search for habitability in our neighborhood and beyond. This course is generally taught at an advanced level assuming a prior knowledge of undergraduate math and physics, but the majority of the concepts and lectures can be understood without these prerequisites. The quizzes and final exam are designed to make you think critically about the material you have learned rather than to simply make you memorize facts. The class is expected to be challenging but rewarding.
Lecture 4.17: Habitable zones
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The Science of the Solar System
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Unit 4: Life in the solar system and beyond (week 2)
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Mike BrownProfessor
Planetary Astronomy
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.
What is the main greenhouse gas on the Earth?
For now, the main greenhouse gas is water.
And followed closely by carbon dioxide.
The fact that the greenhouse effect is such an important
component here makes calculating that habitable zone a little more difficult.
But you can imagine you say, okay, look if we have an
atmosphere similar to the Earth's, we add this 32 degrees of heating,
how far away do we have to be before this average temperature
drops down to not 14 degrees, but, zero degrees freezing point of water.
And conversely, how close can we get to the sun before the average temperature
goes up to something like 100 degrees, and all the water starts to boil away?
And then you could imagine that there are other feedbacks.
If you're getting closer to 100 degrees, the water starts
to boil away, there will be more water in the atmosphere.
If there's more water in the atmosphere, you get
more greenhouse warming and you get a runaway greenhouse effect.
In fact, this is the issue with Venus.
So calculating the precise habitable zone is, as I said, a difficult game.
But coming up with reasonable estimates is not bad.
Let me show you a reasonable estimate that comes by a little bit
more complicated than just doing the,
the temperatures, but not much more complicated.
And I'm going to show you this, not just for the
sun, here's now the mass of the star of the mass
of the sun, so at one that is the mass of the sun, and the sun is called a G star.
These are the Types of stars astronomers use these letters to designate stars
from more massive to less massive in a sort of arbitrary order of the letters.
More massive to less massive, hotter to cooler, sun
is in the middle here, is a G star,.
And here the distance away from the sun.
And here are our planets, Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune.
Not drawn to scale because even I can't,
have a hard time drawing them exactly to scale.
And if we were to calculate the habitable zone for the Sun, you know that we're
going to want to come somewhere between around Venus
out to maybe Mars, but certainly including the Earth.
And when we do that calculation we do the same
sort of calculation for more massive stars which are hotter.
Which means their habitable zones will be further out.
Cooler stars will have a habitable zone closer in.
And we get something that looks like this.
And indeed for a sun-like star just touch the, the,
bit of Venus there, don't quite make it to Mars.
And as I said, for the hotter stars you would
have habitable zone all the way out to almost Jupiter.
For the cooler stars even something as far away of Mercury would
be too cold to be habitable for a lot of these M dwarfs.
They're called cold red small stars, which are very abundant
in the galaxy and about which we'll talk in detail.
As I mentioned there are a lot of complications that we have to
deal with there is, this is the habitable zone, we'll call it the HZ.
There's something that people like to call the continuously habitable zone.
Continuously, because, you know, for, for Europa,
and for Enceladus, and even for Titan
we talked about the idea that you would like life to have time to evolve.
One of the things that's known is that stars get hotter as they age.
The sun itself started out probably 70% as bright
as it is today, which means that this entire
habitable zone at the beginning of the solar system,
this entire habitable zone would have to be moved inward.
Was the Earth in the habitable zone at the beginning of the solar system?
Probably.
We think it became habitable at some point, but
you can imagine this whole curve sliding back and forth.
And only a very thin sliver of that sliding
back and forth will be habitable the entire time.
It starts out colder, the slice goes this way and slowly moves outward this way.
The continuous habitable zone is much smaller than the actual habitable zone.
There are things that planets can do to help themselves stay
habitable over a wider range of temperature, there are feedback effects.
On the earth there's actually a, a very beautiful feedback effect that occurs
when carbone dioxide in the earth's atmosphere is drawn down by weathering.
Water and carbon dioxide combine together to make something like
that carbonic acid, that carbonic acid weathers the rocks and the,
the calcium in the rocks, the carbon dioxide, the water combine
in, in the ocean to make things like calcium carbonates limestones.
If you remember, we talked about this when we talked about Mars because one of
the interesting issues on Mars is that
there is not much evidence for calcium carbonates.
There's not much evidence that there was CO2
being drawn down by the action of this weather.
Formation of calcium carbonates is great.
It pulls carbon dioxide out of the atmosphere.
What happens to that carbon dioxide?
Well, it's now on the bottom of the ocean floor.
Ocean floors subduct into the interior of the earth.
When they subduct into the interior of the earth and heat
up, they eventually out-gassed in volcanos, and you get the CO2 Back.
So there's this whole cycle where CO2 comes out of the atmosphere.
It turns into the carbonate, it comes back into the atmosphere with volcanos.
Why does this help?
We can imagine if this process is going on, what
happens if it suddenly starts to get too cold and
you have less and less rocky surface where you can
have this process going on because you're covered by ice?
There's less CO2 drawn down into the atmosphere, but because this cycle
takes so long there's still plenty being pumped out by the volcanos.
More greenhouse gas is pumping in, planet heats up,
ice melts, you're back to having a habitable planet.
What happens the other way around?
Your planet starts to get a little bit too warm this can be less good.
When your planet starts to get a little too warm, water goes into the atmosphere
it makes more of a greenhouse and you can have runaway greenhouse effect like Venus.
So, that's, that's harder on that end of the scale.
But, there are important things it can regulate.
So again, these are reasons why people still debate a
lot about the, the habitable zone, the continuously habitable zone.
In the end, as I said at the beginning, I
don't think we're going to learn very much from these debates.
What we're, what we should be doing is thinking broadly about where to look.
And then we should go find some planets that might be in these broad regions.
Go look at them.
Find out what's going on.
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