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开普勒的富矿行星

Kepler's Planetary Bonanza

大家好!我是Reid Reimers欢迎收看太空科学秀。在 2014年2月26日
Greetings! I’m Reid Reimers and welcome to SciShow Space. On February 26th 2014, the
开普勒科学团队宣布发现了715颗太阳系外的新的星球
Kepler science team announced the discovery of 715 new planets outside of our solar system.
这几乎迄今为止发现的是记录的数量的两倍。但是这是怎么做到的呢?
This almost doubles the number of alien planets confirmed to date. But how did this happen?
自从开普勒天文望远镜2009年发射之后,我们每次
Since the Kepler Telescope’s launch in 2009, we’ve been discovering new planets only
发现的星球都很少。所以那715个星球从何而来,并且它们是否真实存在?
a handful at a time. So, where did these 715 new planets come from, and are they actually
或者只是715个美好的想象?当然,大量的发现
there or are they just 715 pretty good guesses? Well, a lot of this discovery has to do with
不仅仅是观测到什么,还有实际观测到的是星球的可能性
not just what’s been observed, but also the probability that what we’re seeing actually
让我们回顾一下。开普勒天文望远镜已经有了光度计设备
are planets. Let me step back. The Kepler space telescope has an instrument called a
这个设备是专门为了测量光强而设计即使是只有被测物体一米宽
photometer. It’s specifically designed to measure the brightness of light with a lens
这个大型的照相机我们已经发射到了太空之中
almost a meter wide, and it’s the largest camera we’ve ever launched into space. And
它可以通过观测星球的阴影发现星球……有点像
it finds planets by watching for their shadows… kind of.
尽管太阳系之外的行星太远太小,但是我们依然能看到星星
Planets outside our solar system are too small and too far away to see, but we can see stars.
突然另一个星星挡在了我们和被测行星之间,它会阻碍被测行星的光强
And when a planet crosses between us and a faraway star, it blocks some of the star’s
开普勒光度计能够侦测光的亮度下降,并且并记录下来
light. The Kepler photometer detects that dip in brightness and it records it. But,
但是这种下降也有可能是由于某种太阳活动或者是太空垃圾
that dip might also be caused by some kind of solar event or a bit of space junk flying
从镜头前飞过,比如一些小行星像讨厌的旅客一样抢了我们的镜
through the shot, like say some asteroid photobombing us like an obnoxious tourist. So, Kepler keeps
所以开普勒保持着观测相同的星球看他们是否会暗下来。如果在几天,几月,几年之后
watching that same star to see if it dims again. If it does days, months, or years later
它又暗下来相同的光强和相同的时间,那时我们就可以知道那不是
and it dims by the same amount for the same amount of time then we know that this isn’t
怪异的偶然现象,而是那有可能有什么东西在沿轨道在转动。
some weird fluke and instead there’s probably something orbiting it.
开普勒望远镜利用这种变化现象观测着成千上万个星星
Kepler watches more than a hundred thousand stars for these kind of fluctuations. When
当它找到一个新的星球的时候,它会标记作为候选行星。开普勒团队的工作就是
it finds one, it flags it as a planetary candidate. The job of the Kepler science team is to verify
审核有多少候选行星是真实的星球,因为其他的
how many of these planetary candidates are actual planets because there is another celestial
太空现象也有可能造成行星的亮度下降。想象下那有两个其他的星球
phenomenon that can cause a star to appear to dim. Imagine that there are two other stars
他们做双星运动,他们可能出现在我们观测的恒星之间
that are orbiting each other and they happen to be directly behind the star that we’re
这叫做双星系统而他们会是开普勒误报的来源
observing. These are called binary stars and they’re Kepler’s biggest source of false
想象下这种情况,当一对双体星与我们观察的恒星之间对齐
positives. Think of it this way. When a pair of binary stars lines up with the star that
双子星的互相旋转,其中一个恒星会造成“月食”现象
we’re looking at, one of those binary stars is going to eclipse the other as they revolve
当这种情况发生的时候,在恒星之前的所有的领域都会变暗。
around each other. When that happens, the whole field including the star in front of
对开普勒团队的科学家来说最失望的是双子星超级常见
it will look less bright. The bummer for Kepler scientists is that binary stars are super
常见到开普勒团队观测到一个恒星周围只有一颗候选行星存在
common, like so common that if Kepler sees a star with only one planetary candidate around
只有25%的概率观测的是真正的行星
it, there’s only a 25% chance that that’s an actual planet.
所以,那么如果是一颗拥有多颗竞争外星行星地位的行星绕行的恒星呢
So, what if a star has more than one candidate running for office of exoplanet? That’s
那即是可能性所在,行星,我的意思是真正的行星,那种开普勒团队正在寻找的
where probability gets involved. Planets, I mean actual planets, the things that Kepler’s
很有可能聚群围绕相同星体的星系,就像我们的太阳系
looking for, tend to cluster around the same star. Just look at our solar system. We’ve
我们已经在这里有八颗真正的行星。但是恒星
got eight of the things right here. But whether or not a star is going to have a pair of binary
拥有伴星是完全随机的。所以误报平均分散在
stars behind it is totally random. So those false positives are evenly distributed among
开普勒发现的候选行星中。那就是为什么从统计学角度来说
all of the planetary candidates that Kepler finds. And that’s why, statistically speaking,
观测一个行星围绕一个恒星(那种确定的行星)
observing one planet around a star, an actual we’re-sure-it’s-a-planet planet, makes
会耗费你三倍的乃至更多时间去观测其他的行星。当你看见两个行星
you thirty times more likely to see another planet. And when you see two planets, you
你会五十倍更可能看到第三个行星所以找到单个候选星体
are fifty times more likely to see a third one. So the odds that a single planetary candidate
并证明是真正的星球的概率是相当低。但是要找到三个行星围绕一个恒星
is a real planet are actually pretty low. But the odds that three planetary candidates
的概率那就相当的高了。像我们
around a single star are planets are really high. Like we can be 99% sure that those are
甚至不用去测定其他的参数就能99%的确认那些星球
planets without even taking any other readings.
这种思维方式,就是为什么开普勒团队能立即宣布找到新的715颗行星
This kind of logic is how the Kepler science team announced finding 715 new planets at
那被称作“从多重性中验证”。通过贝叶斯概率统计
once. It’s called ‘verification by multiplicity.’ Using Bayesian probabilities, astronomers
天文学家能够确定某些情况,一个候选行星很有可能是真正的行星
can determine that in some cases, it’s so likely for a planetary candidate to be an
以至于不这么做都很傻 所以当开普勒团队
actual planet that it’s just silly not to call it one. So when the Kepler science team
计算得出发现的候选行星是行星的概率比其他可能高出的100倍时
calculates that a given planetary candidate is 100 times more likely to be a planet than
那时他们说行星是开普勒望远镜所发现的
something else, at that point they say that planet is officially discovered. So those
所以这715个新的行星作为候选行星被天文望远镜发现并被标记,但是他们
715 new planets were flagged as planetary candidates by the Kepler telescope, but they
是被开普勒团队用数学方法找到的
were discovered by the Kepler scientists using math.
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