Throughout the ages, the number of planets
in our solar system has gone up and down.
No, this is not because planets have suddenly appeared and disappeared,
but rather due to how they
Since antiquity, there were five planets beyond Earth,
which are the ones visible to the naked eye,
水星 金星 火星 木星和土星
Mercury, Venus, Mars, Jupiter and Saturn.
The ancient Greeks added the Sun and the moon to their list of planets,
but that idea never really caught on.
In the late 1700s, Uranus was discovered,
which was revolutionary at the time as no-one
considered there could be more planets beyondthe visible five.
After this discovery, more and more planet candidates were being found,
像谷神星 灶神星 智神星和婚神星
like Ceres, Vesta,Pallas and Juno
which are all found in what we now know
as the asteroid belt, or the large belt of asteroids
between the orbit of Mars and Jupiter.
In science textbooks in the early 1800s, they were all listed as planets.
In the middle of the 1800s, Neptune was discovered,
and within just a few decades,
we had quite a list of planets on our hands.
At that time there wasn’t a clear distinction
between the eight planets we know today and
the large asteroids that were being discovered.
By the 1860s, over one hundred asteroids had been found,
and they were finally given their
own distinct classification as asteroids.
Asteroids were described as objects that couldn’t be distinguished from stars,
bar the fact that they move across the sky.
So even one hundred or so years ago,
we were ruthless and demoted planets.
A lot of people in the world feel
like Pluto has been hard done by.
But Pluto has been treated just as harshly as the large asteroids.
In 1930, Pluto was discovered,
and with the excitement of finding something so large and distant,
it was classified as a planet.
Although,within a few decades,
it was discovered that it didn’t follow the conventions of
The planets we know have circular orbits than align roughly
with the plane of the solar system.
Poluto, on the other hand,
orbits at an angle to the solar system, and its orbit is so elliptical
that at some points during its year,
it’s closer to the Sun than Neptune.
But it was also unique, so no-one minded it being called a planet.
But then, in the 1970s,
Pluto was discovered to be a lot tinier than expected.
It turned out that its mass is only one sixth of our moon.
Trouble continued for Pluto in the 90s,
when telescopic technology had greatly increased,
and other worlds were being discovered in and around the orbit of Pluto.
Just like the discovery of the asteroid belt,
it quickly became apparent that there was another belt beyond
the orbit of Neptune, now known as the Kuiper belt.
These objects are still being discovered,
其中最著名的有厄里斯 塞德娜 妊神星 和鸟神星
the most famous ones being Eris, Sedna, Haumea and Makemake.
But seriously, they have found a lot, just look at this list!
In 2006, the International Astronomical Union knew that something had to be done.
They finally classified a planet as a celestial object that:
(a) is in orbit around the Sun
(b) has sufficient mass for its self-gravityto overcome rigid body forces so that it assumes
a hydrostatic equilibrium or which means a nearly round shape
And (c)has cleared the neighbourhood around its orbit.
So although Pluto fits the bill for the first two categories,
because it lies in the Kuiper Belt,
it has not cleared its orbit and thus is not a planet.
It is instead known as a dwarf planet,
which is the almost the same except it’s not massive
enough to have cleared its orbit.
We shouldn’t see this as a sleight against Pluto,
but rather because of these new discoveries,
we have a whole new category of celestial objects to learn about!
Also because of the classification change,
there is a silver lining for the once-planet Ceres
It got promoted from being an asteroid to become a dwarf planet!
Thanks for watching!
Are you a fan of Pluto?
Then you’ll probably like this video about the New Horizons mission there!
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