Every now and then, you might see a headline about a newly discovered “Earth twin”:

an exoplanet roughly the size and mass of Earth that could have liquid water on its surface.

Astronomers are searching for these kinds of planets because they could potentially harbor life,

or, maybe more realistically, could teach us more about our home.

But there's also a problem:

Just because a planet checks these boxes doesn't mean it's anything like Earth.

Because by those standards, we already have an Earth twin in our own solar system.

It's a planet a little less massive and less dense, but made out of the same building blocks,

and just at the edge of the liquid water Goldilocks zone.

But there isn't liquid water on the surface.

There's liquid metal.

And sulfuric acid clouds.

I'm talking about Venus.

And that means we likely need to reconsider the conditions for a habitable planet.

Thankfully, scientists have proposed a solution.

Some research suggests that, for its first few billion years,

Venus was once a true, habitable Earth twin.

Then, around a billion years ago, it underwent a massive global warming that spiraled out of control,

boiling away its oceans and heating its surface to over 460°C.

Its atmosphere is now 92 times thicker than Earth's, and nearly 97% carbon dioxide.

So, no matter how close in size it is to our planet, it's not a place you'd want to visit.

And today, it's definitely not one that can support life as we know it.

No one knows for sure what caused all of this,

but one team has argued that Venus's fate may have come down to its distance from the Sun.

So, to help us understand exoplanets like it,

they've suggested a new way to think about orbits.

Today, most astronomers think about them in terms of the habitable zone:

the area around a star where a terrestrial planet could have liquid water on its surface.

But this team proposed a parallel to that called the Venus zone.

It's the region where an Earth-like planet could start out habitable,

but be doomed to a runaway greenhouse effect.

And better understanding it would help our hunt for real habitable worlds in the final frontier.

The exact distances of the Venus zone are based on how much radiation is being emitted from a system's star.

More massive stars emit more energy per second, so the zone there would start farther away.

And for less massive stars, the opposite is true.

But regardless of exactly how far it is,

the inner edge of the Venus zone is how close a Venus-like planet can get

before its star's radiation completely strips away its atmosphere.

It means that, anywhere in the zone, a planet like this could maintain some kind of air,

which, you know, is good for anything that wants to live there.

The outer edge of the Venus zone, though, is really key for studying other worlds.

It's the same thing as the inner edge of the habitable zone.

And a planet's fate looks dramatically different depending on which side of that line it's on.

On the habitable side, a planet with surface oceans could keep them.

But on the Venus side, they would eventually boil away.

In other words, an exoplanet in its star's Venus zone could have water on its surface right now,

but someday, it won't.

Because at that distance, the planet's oceans would undergo runaway evaporation.

Oceans are a crucial part of a planet's carbon cycle,

or how carbon gets stored and released, and where.

When there's less surface water,

more carbon gets stored in the atmosphere as greenhouse gases, like CO2.

But those extra gases also cause the temperatures to rise, so more of the planet's oceans evaporate.

That creates a positive feedback loop that ends with a super hot world and no oceans.

And it's all sparked by being a little too close to the star.

As scientists find more Earth-sized planets,

the Venus zone will be really valuable for understanding whether or not they're actually like our home planet.

And it could also help scientists pin down exactly how important a planet's distance to its star is for habitability.

Still, this system isn't perfect yet.

Currently, the boundaries of the Venus zone are based entirely on climate models,

and theoretical ones at that.

But that makes sense.

It's only been seven years since we found our first rocky exoplanet,

so our sample size is pretty small.

It's also worth noting that these models might not be bulletproof.

After all, Venus does have other properties different than Earth's that might have contributed to its fate.

So, to really figure out which planets are exo-Earths and exo-Venuses,

we'll need to learn more about the worlds themselves, too.

Some scientists have already started to work on this,

but we won't be able to confirm many key details, like, about the planets' atmospheres,

until the James Webb Space Telescope launches.

As a next-gen telescope, it will have sensitive equipment dedicated to detecting signals from planets all those light-years away.

Still, one day, thanks to Venus, astronomers will be able to study how habitability could evolve on seemingly Earth-like worlds.

And in the meantime, we'll need to study the original Venus, more, too.

The better we know our neighbor, the better we'll be able to pin down why we, and exoplanets,

ended up so differently.

Thanks for watching this episode of SciShow Space!

If you'd like to keep learning about the universe with us, and exploring everything from exoplanets to black holes,

you can go to youtube.com/scishowspace and subscribe.

[ ♪ Outro ]