Okay, so what is the habitable zone?
So, based on our current understanding of life, we believe that the
only way to allow life to form is to have liquid water.
You need to have liquid water on the surface of
your planet in order to allow all the chemistry that that
goes along with life to occur.
Now again this, there could be things that we don't know about.
What could we say, right?
So, so our best understanding of the life on, of our, of everything
we understand about how life formed on Earth is that water is really necessary.
Alright, so we start with the need for liquid water on the surface of the planet.
And then it's very easy to calculate how far away a, you know, here's
your star and here's your planet, how far away from the star does the planet
have to be in order to, for liquid water to exist on the surface.
Because if you're very close up against the star in your orbit, the, the,
the star light will be so strong the the energy that will be absorbed.
Will be strong enough that the temperature of the planet will be so high that
if you dropped some water there it would
just turn into water vapor, would boil away.
And of course if your planet was very far away.
If you put some liquid
water it would very quickly freeze.
So there's gotta be a band of orbits that where
things are just right, for in the, in such that
the temperature on the surface of the planet would allow
water to exist between the being frozen or being boiled away.
So that's what gives us, that, that band
of orbits is what we call the Habitable Zone.
So this is going to depend upon the start that you have.
Because as, as we saw, as we will see next in our next set of lectures.
That some stars have very high temperatures.
Massive stars in particular, stars that are ten times
the mass of the sun or more tend to have
very high temperatures and so that means you're going to
have to move your your habitable zone further outward, ok?
So either you couldn't, placing the earth at the one astronomical unit that at
the same distance from the star that it is as it orbits the sun around
a very massive star would not work because the, the water would all boil away.
But there's other problems with massive stars as well.
Massive stars have they have very short lifetimes.
They will go through their lifetimes in less than,
in around, around a million or ten million years.
And that seems to be a little bit too short to allow
life to evolve all the way up to what we see today.
Perhaps you could get life started on a planet like that,
but once the star reached the end of its life and
blew up, as a supernova, as we'll see, it may
be difficult to imagine how life could be, could form there.
So, we have to keep in mind the fact that different
kinds of stars have different lifetimes have, different conditions on them.
Some stars have a lot of magnetic activity going on and that
leads to explosions off the surface of this, off the star continuously.
And that may make it difficult, for planets to support life.
So all of these things have to be factored
in when we talk about the habitable zone.
We start with a definition that's based on liquid water on the surface.
But then people have begun to become
more, sophisticated in their understanding of that question.
