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
were discovered.
Since antiquity, there were five planets beyond Earth, which are the ones visible to the naked
eye, 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 beyond the visible five.
After this discovery, more and more planet candidates were being found, 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 also 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
traditional planet.
The planets we know have circular orbits than align roughly with the plane of the solar
system.
Pluto, 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, 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 body that: (a) is in orbit around the Sun,
(b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes
a hydrostatic equilibrium (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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