to have proof of the existence of planets orbiting other stars.
That question was around for two thousand years.
say definitively that there are other planets orbiting other stars.
in are stars are planets that can support life.
system, of any solar system, are conducive for life as we know it.
system can harbor life and that's, the topic of the habitable zone.
the fraction of stars that have planets in their habitable zone.
Okay, so what is the habitable zone?
only way to allow life to form is to have liquid water.
goes along with life to occur.
Now again this, there could be things that we don't know about.
What could we say, right?
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.
have to be in order to, for liquid water to exist on the surface.
the star light will be so strong the the energy that will be absorbed.
just turn into water vapor, would boil away.
And of course if your planet was very far away.
water it would very quickly freeze.
water to exist between the being frozen or being boiled away.
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.
have to move your your habitable zone further outward, ok?
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.
in around, around a million or ten million years.
life to evolve all the way up to what we see today.
be difficult to imagine how life could be, could form there.
kinds of stars have different lifetimes have, different conditions on them.
leads to explosions off the surface of this, off the star continuously.
And that may make it difficult, for planets to support life.
in when we talk about the habitable zone.
We start with a definition that's based on liquid water on the surface.
more, sophisticated in their understanding of that question.
of planets in the habitable zone is of order one.
At which is pretty remarkable. So, let's just talk about this.
the fact that we can go out and measure now.
You know, we can measure exoplanets.
to be able to tell things about the kind of planet that, that's there.
estimated that 64 percent of stars have planets in the habitable zone.
Which is a remarkable, remarkable conclusion.
in the right place for, too have life forms.
in the sky has a planet just in the right place.
designed to find planets via the transiting method.
not only had had one exoplanet, it had two exoplanets in the habitable zone.
you know, a multiple planets in its system that might have life.
the habitable zone if possible you'd like to have earth like planets.
you know, moons are great places to put, rebel bases.
but, the question really is, we know, we'd like to know about Earth-like planets.
So the question is how common are Earth-like planets?
Well, as of, January 2014, scientific report, stated that there about 22%.
The estimate is about 22% of sun like stars have an Earth sized planet.
You know, around, the right sort of size in it's habitable zone.
proba, you're using probability arguments about 12 light years away.
neighborhood in some sense, speaking in a galactic sense.
So it's certainly a star that can be seen with the naked eye.
like planet, orbiting a sun like star in the habitable zone.
around on that planet so its really its a remarkable thing.
worlds orbiting other stars that may have the conditions appropriate to life.
Carl Sagan program Contact where he would talk about billions of stars and planets.
2000's is that we know, we absolutely know the answer to this question.
So with that let's go on.