It seems like every week we hear about new planets being discovered outside of our solar system and scientists are even beginning to describe a select few of those planets as potentially Earth-like.

What does that even mean?  What does it really take for a planet to be habitable and how common are habitable planets likely to be in our galaxy or anywhere else?  Well, there are lots of factors that work together to make certain parts of a star system or galaxy or even the universe as we know it, more habitable than others.

It's like Panem, or like Michigan.  There are some parts that are lovely and quite comfortable and others that you wouldn't want to be caught dead in because you'd be dead

[SciShow intro] (00:36)

Let's start our tour of the universe's nicest neighborhoods at the level of a single exoplanet.  It takes a lot more than air and water for a planet to be a likely candidate for life.  As of March 24th, 2014, the most Earth-like planet confirmed in the galaxy, other than Earth, is Kepler 62E.  It's warm, it's wet, it's probably covered in friendly puffy clouds and there's little chance that civilization would develop on it because Kepler 62E is a water world.

Totally covered in ocean, no land in sight.  Why?  Because Kepler 62E is about 60% bigger than Earth, and bigger planets have a stronger gravitational pull, which means they hold on to more of the water that was around them when they formed.

See, most terrestrial planets actually start out with a fair amount of water, mostly delivered by asteroid impacts during the early days of formation.  But then most of that water gets lost back into space, freezes underground, gets absorbed by the rocks, or boils off.  Fact is, getting water isn't that hard, astronomically speaking.  Keeping water in the liquid form, that's the trick.

So the early days of Earth, for example, we lost a lot of water back into space.  If Earth had been a little bigger, it would have held on to more of its water and it would be a water world too.  So if life had never developed here at all we would be living in pineapples under the sea.  But, by the same principle, if Earth had been a little smaller, it wouldn't have held on to any of its water.

Scientists believe that's what happened to Mars, which is just over half Earth's size and, you know, Mars is great and everything but you don't want to end up like that.  So the size of the planet is important, but so is the size of its star.

(02:15)  If you're old enough have had any schooling in the 20th century, you probably were taught that the sun is a typical star.  FALSE.

The sun is actually larger than 90% of all other known stars, and most stars are red dwarfs, which are about one-tenth of the size of the sun.  To get a sense of what most of our stellar neighbors look like, look no further than proxima centauri, it's the closest star to Earth other than the sun, only 4.2 light-years away, and it's still too small to be seen with the naked eye.

To stay warm around a star like that you'd have to be orbiting very close to it.  And in addition to putting you at greater risk from nasty surprises like solar flares and X-ray bursts, being that close to your star makes it way more likely that you'd end up tidally locked.  A tidally locked planet is one that has one side always facing the star and one side always facing away.  And of course it's not just planets that do this, the moon is tidally locked with Earth, that's why you only ever see one side of it.

Tidal locking occurs when a planet orbits too close to its star and the star's gravity pulls harder on the near side of the planet than the far side.  Over time, this effect drags on the planet's spin, fixing one side to orbit the star.  And just imagine what would happen to a planet like Earth if it became tidally locked.  The water on the side facing the star would boil away and the water on the cold side would freeze.  In terms of habitability, on the hot side everything would be cooked, and on the cold side not only would everything freeze but there would be no light energy to allow for things like photosynthesis.

There'd be other unpleasant effects too, like the entire atmosphere turning into one giant never-ending storm.  The far side of the planet would get so cold that the air would actually condense and fall to the ground, creating a vacuum in the atmosphere, and hot air expands so the cooked air from the hot side would flood in to fill that vacuum.  The result would be like living in the biggest, hottest, fiercest hurricane ever.  And the odds of anything surviving that would be pretty bad.

(04:03)  But it's not just the size of the star that matters, it's how old it is as well.  The Sun is what astronomers call a Population I Star, a relatively young star that formed along with its plants in a cloud of space dust that was unusually rich in heavy elements.  Those heavy elements were created by older stars that formed and collapsed before our sun came to be.

And without all that heavy material created by those previous generations of stars, our solar system wouldn't have started out with things like iron and silica to make rocky planets and carbon to make people and cheetahs and sunflowers to live on them.

(04:34)  The next thing to consider is that where a star system is in its galaxy also affects how habitable its planets are likely to be.  In our case, Milky Way is made up of a giant bulge in the middle and two major arms on the side.  The central bulge is where the Milky Way keeps its super massive black hole as well as lots and lots and lots and lots of stars.

I'm talkin' about, like, 10 million stars packed within the space of a single light year.  Remember, the nearest star to us is more than 4 light years away.  As cool as this sounds, these cramped quarters are not a nice place to be.  In addition to the deadly radiation emitted by the black hole, you also have to worry about any of the stars nearby going supernova, which would kill anything trying to live nearby.

The major arms are also not particularly safe neighborhoods.  They're densely packed with stuff, too, and they are where most of the new star formation is taking place.  And new stars are extremely volatile.  They're screaming toddlers of the stellar population.  They emit jets of intense radiation that can be trillions of miles long and travel at the speed of light and those jets will kill you. Thankfully, we are in a quiet neighborhood of the Milky Way, the Orion arm, which is a minor spur between the two major arms.

(05:41)  There aren't many stars out here.  We're about 26,000 light-years from the central bulge and exciting things don't happen very often, which is great.  Space excitement is terrible.  And you'll be happy to hear that our galaxy is more likely to have habitable planets than most.  That's because the Milky Way is a spiral galaxy, not an elliptical galaxy, which is all tired and flat and stretched out, or a globular cluster which is just like a big star blob.

But that's important because the stars in those types of galaxies tend to form all at once.  Remember how I said that you only get heavy elements from stars dying.  Well, if no new stars form to scoop them up, those new elements just float around in space, which mean no stars with rocky plants, or cheetahs or sunflowers.

But in spiral galaxies, new stars are forming all the time, putting those heavy elements to good use.

(06:23)  So the Milky Way is about 13 billion years old but our Population I sun is a fresh-faced 5 billion.  Meanwhile, galaxies that are a bit younger than ours tend to have Quasars.  They are powered by those enormous black holes you find at the center of most galaxies, and they put out incredible amounts of energy.  Like, a thousand times more energy than every star in our galaxy.  Combined.

That energy blasts out of the galactic center and reaches all the way to the furthest edges of the galaxy and it covers the entire electromagnetic spectrum, including the really deadly pars of it, like X-rays and gamma rays.

If the Milky Way had a Quasar not only would we be dead, but we would never have existed in the first place.

We don't know if the Milky Way used to have a Quasar, but if it did it burned out long ago and turned into a nice, save, super massive black hole.  And it's really sayin' something when you're calling a super massive black hole nice and safe.

(07:14)  So the Milky Way is not too young, not too old and our sun is not too bright, not too dim.  Our planet isn't too big or too small.  There are a lot of things that go in to making Earth or anywhere else habitable.  But, we're getting better and better at picking those things out so finding Earth's twin may only be a matter of time.

Thank you for watching this Scishow Infusion and thanks to our latest president of space Kevin Bradley who says, "Be good and keep learning."

If you'd like to be SciShow's next president of space or score other cool swag go to subbable.com to learn more.  If you have any questions or ideas for episodes you'd like to see you can find us on Facebook or Twitter.  And if you want to keep getting smarter with us you can go to youtube.com/scishow and subscribe.