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Is Pluto a planet? - Video học tiếng Anh
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Is Pluto a planet?
Is Pluto a planet?
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0:01
Pluto: Planet or not? Before we can answer this question we need
0:05
to know what the word planet is for, and that takes us back to the ancient greeks who called
0:08
Mercury, Venus, Mars, Jupiter, Saturn, the Moon and sun planets. Basically if it moved
0:13
across the sky and was bright, it was a planet. This is a terrible start for the word because,
0:18
1) it excludes Earth from the list and 2) it groups wildly different things together.
0:23
But the greeks couldn't know how different the Moon was from Saturn, because the best
0:26
technology they had to observe the Universe was sadly limited.
0:29
It would take several thousand years until the industrious Dutch made the first telescopes
0:33
and astronomy became much more interesting. Astronomers could now confidently rearrange
0:37
the solar system -- an elegant scientific advance that no one could possibly object
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to -- and reclassify its parts, dropping the Sun and moon from the list of planets and
0:44
adding Earth. Now, if it orbited the Sun, it was a planet.
0:48
As time went on and telescopes got better and better each new century brought with it
0:52
the discovery of a new planet. Which brings us to this familiar solar system:
0:55
nine planets orbiting one star. And looking at this model makes people wonder,
0:59
why do astronomers want to ditch Pluto? The problem is pictures like this in textbooks
1:02
are lies. Well, not lies exactly, but unhelpful. They give the impression that the planets
1:07
are similar-ash in size and evenly-ish spaced, but the reality couldn't be more different.
1:11
Here, dear Terrans, is our home planet Earth, and this is Jupiter next to it at the correct
1:16
scale -- rather bigger than you probably thought. If we take this diagram and adjust for the
1:20
correct sizes of the planets it looks like this. Unless you're watching the video in
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fullscreen HD mode, you might not even be able to see Pluto.
1:27
So size differences are vast, and Pluto is the smallest by far. But it's not just small
1:31
for a planet, it's also smaller than seven moons: Triton, Europa, our own Moon, Io, Callisto,
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Titan, and Ganymede. Even if you show the correct relative sizes
1:39
the distances are still a problem. Think about it, if Jupiter was this close to Earth it
1:43
wouldn't look like a dot in the night's sky but would be rather overwhelming -- so it
1:46
must be really far away, which makes drawing it to scale rather a challenge. If you want
1:51
the length of a piece of paper to represent the distance from Mercury to Pluto, then giant
1:55
Jupiter would be the size of a dust mite on that page, and Pluto a bacterium.
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But excluding Pluto from the plant club just for being tiny and far away isn't reason enough
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and quickly brings out the Pluto defenders. In order to understand what Pluto really is,
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we need to first discuss a planet you've never heard of: Ceres.
2:11
Back in the 1801, astronomers found a new planet in the huge gap between Mars and Jupiter
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-- it was a small planet, but they loved it anyway and named it Ceres.
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The next year astronomers found another small planet in the same area and named it Pallas.
2:22
A few years later they found a third one, Juno, and then, funnily enough, a fourth one,
2:27
Vesta. And for a several decades children learned the 11 planets of the solar system.
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But, astronomers kept finding more and more of these objects and became increasingly uncomfortable
2:35
calling them planets because they were much more like each other than planets the on either
2:39
side, so a new category was born: asteroids in the asteroid belt -- and the tiny planets
2:43
were relabeled which is why you've never heard of them. And it was a good decision too, as
2:47
astronomers have now found hundreds of thousands of asteroids, which would be a lot for a kid
2:52
to memorize if they were all still planets. Back to Pluto. It was discovered in 1930 making
2:57
it the 9th planet. First estimates put Pluto about the size of Neptune, but with more observations
3:02
that was revised down, and down and down. While Pluto shrank astronomers started to
3:07
find other, similar objects orbiting in the same zone.
3:10
Sound familiar? While school kids kept memorizing the nine
3:12
planets, some astronomers grew uneasy about including Pluto because the size estimates
3:16
continued to shrink, they learned that Pluto is made mostly ice, and they continued to
3:20
find lots and lots of icy objects at the edge of the solar system just like Pluto.
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This problem could be ignored as long as no one found an ice ball bigger than Pluto, which
3:29
is exactly what happened in 2006 with the discovery of Eris. Once again, astronomers
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recategorized the solar system and grouped these distant objects, including Pluto, into
3:37
a new area called Kuiper belt. And that's the story of Pluto -- a miscategorized
3:43
planted that finally found its home -- just like Ceres. But this story is really less
3:47
about Pluto than it is about realizing the word 'planet' isn't very helpful.
3:50
The first four planets are nothing at all like the next four, so it's even a little
3:54
weird to group these eight together which is why they often aren't and are separated
3:57
into terrestrial planets and gas giants. And now that we have telescopes that can see
4:01
planets around stars not our own, and we've found rogue planets drifting in empty space
4:05
and brown dwarfs -- objects that blur the very line between planet and star -- the word
4:09
planet becomes even less clear. So as we increase our knowledge of the Universe
4:12
the category of 'planet' will probably continue to evolve, or possibly, fall out of favor
4:17
entirely. But, for the time being the best way to categorize
4:19
the stuff in our solar system is into one star, eight planets, four terrestrial, four
4:24
gas giants, the asteroid belt, and the distant Kuiper belt, home to Pluto�
