[music playing]
In the beginning, there was one planet. At least as far as we knew. And then there were nine.
And then there were eight.
Our solar system was born as a cloud of gas in the corner of a young Milky Way galaxy.
One molecule stuck to another, and then another, spinning around, and a giant burning sun was born.
The leftover rocks and pebbles swung around, colliding into larger rocks, then larger and larger still,
until they had sucked up most of the debris and gas to become the planets we know today.
Now we know that this story has played out around a hundred billion suns.
Some are larger than ours, and some are smaller, some burn hotter and some burn colder,
but in the Milky Way, planets are a dime a dozen.
If you were to count our galaxy's hundred billion solar systems one by one,
it would take you more than 3,000 years! Think about where we were 3,000 years ago.
The ancient Greeks had just noticed that some dots in the sky moved a little differently than others.
They named them "wanderers". But then their explanations got a little weird.
Most Greeks assumed that they lived in the most important place in the universe.
Earth sat in the center of a set of spheres: one for the moon, one for the sun and the stars,
and everything else, like a set of Russian dolls.
Even back then, some Greeks had the right idea. They knew that everything up there was
made of the same stuff as everything down here, a substance they called "atoms".
And that stars were just far-away suns. Some even figured out that our Earth orbited around the sun.
The Platos and Aristotles of the world weren't having any of that.
While they were busy worrying about something called "aether", those ideas were lost to history
just so Earth could maintain its place on a pedestal above the rest of the universe.
Thanks to them, modern astronomy got kind of a late start.
It took more than 1,500 years for Copernicus to put Earth in its place,
or for Galileo to give us the telescope, or for Kepler to fill the sky with planets.
No, not that Kepler. THIS one.
The Kepler telescope is a planet-hunting eye in space that's discovered most of the exoplanets
that we know about today.Now, it can't actually SEE those exoplanets, because the stars that
they surround are so big and bright. Instead it looked for the tiny shadow of a planet
as it passed in front of its parent star.
If that sounds hard, it's because it is. It's like seeing a flea on a light bulb in Los Angeles, from New York City.
Out of the maybe 100 billion planets in our galaxy, maybe 20 billion of them are Earth-like,
depending on how you define Earth-like. Around every star is this "habitable zone" where
it's not too cold, not too hot, not too dim, or too bright, and if a planet in that region
is not too big or too small, and it's made of rock instead of gas, then maybe, just maybe,
there could be water on its surface. And that's really all it takes to be Earth-like.
The thing is that not every planet that passes the test is another Earth.
Venus and Mars pass a lot of the criteria, but one of them has an atmosphere hot enough to melt lead,
and the other is a frozen rock whose atmosphere and water have been almost completely blown away.
And take Earth three billion years ago, almost devoid of oxygen, covered in toxic oceans,
and brimming with sludges of gooey bacteria. That doesn't seem very Earth-like when you think about it.
Here's an artist's rendering of an exoplanet. Pretty cool huh? They don't look like that.
They look like this. When we first discovered exoplanets we were like "Woo!" and now we're like "sigh".
I think that's because the question we're really interested in is not how common are planets,
but how common are we?
From up in space, Earth looks pretty big, but the more we've looked out into the universe,
it seems like we're getting smaller and smaller. If we were to find another living needle in
this haystack of planets, would that make us more, or less significant?
Tell me what you think down in the comments.
As for me, Carl said it best: "We make our world significant by the courage of our questions,
and by the depth of our answers."
In part 2, we'll go beyond the search for other Earths and on to the search for other intelligent life.
What are we looking for exactly? What are we listening for? And are we running out of time?
If you'd like to discover more about other Earths and other intelligent life, check out
Lee Billings' book "Five Billion Years of Solitude". It is awesome. It's proof that
life down here is very intelligent indeed. there's a link down in the description.
And if you'd like to become a more intelligent life form, might I suggest subscribing?
See you next week. Stay curious.