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Every now and then, you might see a headline about a newly discovered Earth twin:
有时候，我们可能看到一些头条说发现了地球的姊妹星体：
an exoplanet roughly the size and mass of Earth that could have liquid water on its surface.
某系外行星跟地球的大小和质量相当，表面可能有液态水。
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.
天文学家正在寻找这类系外行星，因为这类行星可能可以孕育生命；更现实的收益是可以让我们更了解自己生活的星球。
But there's also a problem: Just because a planet checks these boxes doesn't mean it's anything like Earth.
但还是存在问题的：就算一颗行星符合上述标准，但依然不能证明它跟地球差不多。
Because by those standards, we already have an Earth twin in our own solar system.
因为从这些标准来看，我们在太阳系里已经有一个地球的姊妹星体了。
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.
其质量和密度都比地球要低，但组成元素相同，而且十分接近于液态宜居区。
But there isn't liquid water on the surface. There's liquid metal. And sulfuric acid clouds. I'm talking about Venus.
只是其表面没有液态水，只有液态金属和硫酸组成的云体。这个星体就是金星。
And that means we likely need to reconsider the conditions for a habitable planet.
也就是说，我们很有可能需要考虑宜居行星的环境问题。
Thankfully, scientists have proposed a solution.
谢天谢地的是，科学家已经提出了一个解决方案。
Some research suggests that, for its first few billion years, Venus was once a true, habitable Earth twin.
一些研究表明，在金星形成后的前几十亿年里，这个星体是真正意义上与地球类似的宜居星体。
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.
后来，过了大概10亿年，金星经历了一次大规模的全球变暖，来势汹汹，难以控制。这次变化导致其表面海洋枯竭，表面温度超过了460摄氏度。
Its atmosphere is now 92 times thicker than Earth's, and nearly 97% carbon dioxide.
其大气层厚度是地球的92倍，而且其中97%是二氧化碳。
So, no matter how close in size it is to our planet, it's not a place you'd want to visit.
所以无论其大小跟地球有多接近，没有人会想去这里待着。
And today, it's definitely not one that can support life as we know it.
而且，今天的我们也知道这个地方根本无法支持生命的存在
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.
没有人知道事情的起因，但有一个团队认为，金星之所以如此可能是因为它跟太阳距离的原因。
So, to help us understand exoplanets like it, they've suggested a new way to think about orbits.
所以，为了帮助我们理解与金星类似的系外行星，他们提出了一个思考轨道的新方法。
Today, most astronomers think about them in terms of the habitable zone:
如今，大多数天文学家都是从可居住区域的角度来进行思考的：
the area around a star where a terrestrial planet could have liquid water on its surface.
类地行星在某颗恒星附近的区域，其表面有可能有液态水。
But this team proposed a parallel to that called the Venus zone.
但该团队提出了一个平行概念，叫金星地带。
It's the region where an Earth-like planet could start out habitable, but be doomed to a runaway greenhouse effect.
在这个地带，类地行星一开始是宜居的，但最后，其一定会产生一发不可收拾的温室效应。
And better understanding it would help our hunt for real habitable worlds in the final frontier.
如果能更好地了解这个地带，就有助于我们在宇宙中搜寻真正宜居的星球。
The exact distances of the Venus zone are based on how much radiation is being emitted from a system's star.
金星地带的准确距离是基于某恒星系统发射出的辐射有多少。
More massive stars emit more energy per second, so the zone there would start farther away.
恒星的质量越大，每秒放出的能量就越多，其一开始形成的金星区域就越广阔。
And for less massive stars, the opposite is true.
而小质量恒星的情况则恰好相反。
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.
但不管金星地带有多宽广，其内缘都等于类金星行星在恒星辐射彻底脱离大气层之前能离恒星有多近。
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.
也就是说，在金星地带的任何地方，与此类似的行星都可能包含一些空气，这就有助于生命的存在。
The outer edge of the Venus zone, though, is really key for studying other worlds.
而金星地带的外缘对于研究其他星体至关重要。
It's the same thing as the inner edge of the habitable zone.
这根宜居地带的内缘是一样的道理。
And a planet's fate looks dramatically different depending on which side of that line it's on.
而一颗行星的命运可能会因为其所在侧的不同而产生巨大的不同。
On the habitable side, a planet with surface oceans could keep them.
在宜居的一面，表面有海洋的行星就能保留住这些海洋。
But on the Venus side, they would eventually boil away.
而在金星的那一面，水分都会蒸发掉。
In other words, an exoplanet in its star's Venus zone could have water on its surface right now, but someday, it won't.
换言之，处于所属恒星的金星地带的系外行星现在可能含有水分，但有一天，总会消失的。
Because at that distance, the planet's oceans would undergo runaway evaporation.
但在这样的距离下，该行星的海洋会经受无法控制的蒸发。
Oceans are a crucial part of a planet's carbon cycle, or how carbon gets stored and released, and where.
海洋是任何行星碳循环的关键要素，决定着碳元素储存和释放的方式以及地点。
When there's less surface water, more carbon gets stored in the atmosphere as greenhouse gases, like CO2.
党行星表面水分更少时，更多的碳会储存在大气层中，以温室气体的形态出现，比如二氧化碳。
But those extra gases also cause the temperatures to rise, so more of the planet's oceans evaporate.
但这些额外的气体也会导致气温升高，所以该行星海洋的更多部分会蒸发掉。
That creates a positive feedback loop that ends with a super hot world and no oceans.
这样就形成了愈发猛烈的闭环，使星体气温极高，海枯石烂。
And it's all sparked by being a little too close to the star.
而这一切的根源是：离恒星太近了。
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.
随着科学家发现的类地行星越来越多，金星地带将变得极有价值，可以帮助我们理解它们跟我们的地球是否类似。
And it could also help scientists pin down exactly how important a planet's distance to its star is for habitability.
也能帮助科学家确切了解行星与其恒星的距离对于宜居性有多重要。
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.
不过，这个系统还不够完善。目前为止，金星地带的边界完全取决于气候模型和理论模型。
But that makes sense.
但“纸上谈兵”也是有意义的。
It's only been seven years since we found our first rocky exoplanet, so our sample size is pretty small.
7年前，我们发现了第一颗布满岩石的戏外行星，所以我们的样本是非常小的。
It's also worth noting that these models might not be bulletproof.
值得注意的是，这些模型可能经不起推敲。
After all, Venus does have other properties different than Earth's that might have contributed to its fate.
毕竟，金星的一些特质与地球不同，也可能是这些特质使现在的金星变得不宜居。
So, to really figure out which planets are exo-Earths and exo-Venuses, we'll need to learn more about the worlds themselves, too.
所以，为了弄清楚哪些行星是系外的类地行星和类金星行星，我们需要对这些地方了解的更多。
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.
有些科学家已经着手研究这个问题，但我们无法确定很多细节，比如这些行星的大气层。除非詹姆斯·韦伯空间望远镜正式使用。
As a next-gen telescope, it will have sensitive equipment dedicated to detecting signals from planets all those light-years away.
作为下一代的望远镜，詹姆斯·韦伯空间望远镜将配置灵敏设备，可以勘测到很多光年外的信号。
Still, one day, thanks to Venus, astronomers will be able to study how habitability could evolve on seemingly Earth-like worlds.
不过，终将有一天，金星将帮助天文学家研究类地行星宜居情况的变化。
And in the meantime, we'll need to study the original Venus, more, too.
与此同时，我们也需要研究金星本身更多一些。
The better we know our neighbor, the better we'll be able to pin down why we, and exoplanets, ended up so differently.
我们对邻近行星了解的越多，就越有助于知道系外行星终结的方式为何如此不同。
Thanks for watching this episode of SciShow Space!
感谢收看《太空科学秀》！
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.
如果大家想和我们一起了解宇宙，探索系外行星、黑洞等奥秘的话，可以订阅youtube.com/scishowspace。
