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行星位置变迁 – 译学馆
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行星位置变迁

Were the Planets Always in the Same Order?

二十多年前 天文学家就在其他恒星系中观测到了行星
It’s been more than twenty years since astronomers began detecting planets around other stars.
他们对此并不感到惊讶
They weren’t surprised to find planets in other star systems.
有趣的是这些行星的排布方式使得天文学家们开始
But they were surprised by how those planets were arranged, and they ended up having to
重新思考行星的形成问题
rethink what they thought they knew about how planets form.
通常我们认为太阳系是一个标准的恒星系
We used to think that our Solar System was pretty standard, as star systems go.
较小的四颗固态行星在内 其余四颗气态行星在外
Our arrangement of four smaller inner rocky planets and four bigger outer gaseous ones
这样的分布是有物理学依据的:
makes a lot of physical sense:
在温度极低的远日空间 大量分子聚集起来
When you’re farther away from the Sun, space is colder, which allows more molecules to
便会形成体积庞大的行星
condense and build up larger planets.
然而不久前 我们发现了首颗系外行星——第一颗发现于太阳系外的
But then, we found our first exoplanet — the first planet we’d discovered outside of
行星
our solar system.
它的存在证明了我们过去的理解是错误的
And it revealed a situation we used to think was impossible.
这颗行星大小与土星相当
The first exoplanet was a world the size of Saturn, orbiting more than seven times closer
轨道半径小于水星轨道的七分之一
to its star than Mercury does to the Sun!
这颗行星距恒星太近 应该无法存在 因为较轻的气体
There’s almost no way it could have formed so close to its star — all the light gases
还未来得及形成行星便已被蒸发
would have boiled off before they could’ve formed a planet.
随着逐渐获取到的数据天文学家开始不断发现这种临近恒星的
As astronomers collected more data, they started to find more and more of these big, gas planets,
大型气态行星 将它们命名为热木星
which we now call Hot Jupiters, close to their stars.
他们意识到热木星是从某处迁移而来的
They realized that Hot Jupiters migrated from somewhere else, and they started trying to
继而致力于弄清缘由
figure out how.
自此以后 天文学家认识到在行星形成早期
Since then, they’ve come to realize that planets can actually move around a lot in
其轨道是会产生较大变动的
their early lives.
原行星盘是行星的形成区域
Planets form in regions called protoplanetary disks, made up of the leftover gas and dust
主要由恒星生成之后残余的气体和灰烬构成
after a star forms.
银河系中所有新生恒星的周围都有原行星盘
We see these disks surrounding young stars all over our galaxy.
它们为固态物质的聚集创造了条件
They provide a place for solid matter to clump together, and eventually, the largest clumps
最终 最大的物质团会重到足以吸引气体形成类似木星和土星一样的行星
become massive enough to pull in gas, giving rise to worlds like Jupiter and Saturn.
一旦形成 这些新生行星仍暂时留在原行星盘中
Once they form, these new planets are still embedded in the protoplanetary disk.
原行星盘是它们变轨的关键
And that disk is the key to how they move around.
新生行星的引力将使周围的物质产生巨大波动
A new planet’s gravity affects the nearby disk material, bunching it up into giant waves
——螺旋密度波
called spiral density waves.
同样是这种波动使银河系形成悬臂 土星拥有了星环
They’re the same kinds of structures that give galaxies their arms and Saturn’s rings
不仅如此
there’re details.
行星轨道内部距恒星较近的区域有密度波 轨道外部
There’s one density wave inside the planet’s orbit, closer to the star, and one outside
距恒星较远的区域也有密度波
the planet’s orbit, on the side farther from the star.
距离恒星越近 引力便越强 行星运行速度便越快
Now, the closer you are to a star, the harder its gravity pulls you and the faster you travel.
因此 随着时间推移 朝向恒星的密度波开始处于行星前方
So, over time, the wave towards the star starts to creep ahead, while the one outside the
同时行星轨道外的密度波开始处于行星后方
planet’s orbit starts to fall behind.
这些密度波是由物质构成的
But these density waves are made up of actual, physical stuff and therefore exert gravity
因此也会影响自身的引力
of their own.
因此前方的密度波会向前拖曳行星
So that wave ahead of the planet is pulling it forward in its orbit, which gives the planet
给了它一些推动力 将其逐渐推离恒星
a little boost in energy, sending it a bit farther from the star.
同时 处于行星后方的密度波正做着相反的事情
Meanwhile, the wave behind the planet is doing the exact opposite.
它的引力将行星向后方拖曳 减少其动量
Its gravity tugs the planet backwards, reducing its energy and forcing it to fall into a smaller
迫使行星降低轨道
orbit.
如果这些外力正好平衡 行星轨道便不会再变化
If these forces exactly balanced out, the planet wouldn’t move at all, but usually
但是通常情况下 原行星盘的密度分布是不均匀的
the protoplanetary disk is less dense in some places than others.
这使得两密度波的拉力难以抵消
That difference makes the pull of one wave more powerful than the other, so the planet
所以 行星轨道会产生变化
moves.
在模拟环境下 这一推演过程切合行星的运动
In simulations, this process is so effective at moving planets around that some scientists
一些科学家难以解释为何还会有如此多的行星幸存
aren’t really sure why so many survive at all.
最终 他们有了这个解释热木星存在的理论
So, finally an explanation for how those strange Hot Jupiters could exist where they do!
它们可能先在距恒星较远处形成
They probably formed much farther away from their stars before migrating in to where they
然后才迁移到现在的轨道
are now.
甚至我们太阳系的行星也是很久以前迁移过来的
Even the planets in our own Solar System probably migrated long ago.
这个假说在法国提出后 被天文学家称为Nice模型
Astronomers call this idea the Nice Model, after the city in France where it was developed.
Nice模型认为 45亿年前 外部行星刚形成时
The Nice Model says that when the outer planets were forming four and a half billion years
距离非常近
ago, they were much more closely spaced than they are today.
随着时间推移盘内相互作用使它们运动到了现在的位置
Over time, interactions with the disk drove them to where they are now, but they might
但迁移过程可能并不顺利
have made some major detours along the way.
例如 木星可能在我们今天称为太阳系内部的区域
Jupiter, for example, might have spent some time in what today we’d call the inner Solar
花费了一些时间
System.
如果它运行到火星轨道附近此时两个行星都已形成
If it wandered close to the orbit of Mars as both planets were forming, it could have
它可能戏剧性的变轻而成为红行星
dramatically reduced the amount of material left to make the Red Planet.
这就解释了为什么在许多行星构造模型中
This would help explain why many models of planet formation suggest that Mars should
火星本该和地球一样大 但实际却只有地球质量的十分之一
be roughly Earth-sized, when really, it only has about a tenth of our mass!
海王星也许在初始时比天王星距太阳更近
And Neptune may have started its life closer to the Sun than Uranus.
然后 在运动过程中 它们交换了位置
Then, once they began to move, they would have swapped places.
这些行星位置不断变化
It might have taken hundreds of millions of years for things to settle down to how we
至今可能已有上亿年
see them today.
直到原行星盘中的气体和灰尘全部消散
By that time, the disk of gas and dust would have blown away, leaving behind only the scattered,
只剩行星形成后留下的碎石
rocky remains of planet formation.
当那些碎石遇到像木星样的行星时行星的引力会将它们推向太阳
As those rocks encountered a world like Jupiter, the planet’s gravity could have flung them
同时 碎石也会推动木星一点点远离太阳
towards the Sun — and in the process, propelled the gas giant just a little bit farther.
慢慢的 我们今天看到的太阳系便形成了——一次一块碎石
Slowly, the Solar System we know today took shape — one rock at a time.
对太阳系外行星的研究使我们明白了
And if our search for planets around other stars has taught us anything, it’s that
这是事物发展的唯一可能途径
this is only one possible way things might have played out.
现在我们已经知道 行星形成后并不是原地不动的
We now know that planets don’t just stay put after they’re formed — there are all
它们总是在变换轨道
kinds of processes that can move them around.
感谢您观看这段SciShow
Thanks for watching this episode of SciShow Space, and thanks especially to our patrons
特别感谢Patreon 是他的帮助让节目持续到现在
on Patreon who help make this show possible.
如果你想帮助我们继续制作这样的短片
If you want to help us keep making episodes like this, just go to patreon.com/scishow
请去patreon.com/scishow了解更多
to learn more.
别忘了去 youtube.com/scishowspace 订阅
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视频概述

行星的位置是否一直如此,不曾改变呢?在它们的形成过程中,又有哪些趣闻呢?

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知易行难

视频来源

https://www.youtube.com/watch?v=UYMsaFcv83Y

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