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