Thanks to Mova Globes for sponsoring today's video.
For 76 years astronomy textbooks described our solar system as having nine major planets,
rather than the eight that we see on diagrams today.
And that's because in 2006 everyone's favourite icy world,
Pluto, was demoted from major planet to dwarf planet.
The reclassification of Pluto caused much controversy at the time and even
resulted in the invention of a new word, "plutoed" which means to be downgraded.
Still today, many believe that this fascinating frozen world
should be the ninth major planet from the Sun.
But there is also another debate to be had over Pluto, one that seems to have been forgotten,
and that is whether its largest moon Charon is actually a moon at all.
Pluto's history is one of discovery, debate and a momentous decision that
explains how we found a new class of objects.
But are we looking at this distant world and its so called large moon in the wrong way,
is Pluto and Charon instead our solar system's first double planet system?
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Pluto is classed as a dwarf planet that is located in a region called the Kuiper Belt. Its features
include red snow, blue skies, incredibly tall mountains and heart-shaped glaciers.
It is considered one the most interesting objects in the solar system, but its largest Moon, Charon,
which is often forgotten about, makes the frozen dwarf planet even more fascinating.
Although Pluto was discovered in 1930 by Clyde Tombaugh,
Charon wasn't spotted until 1978, by astronomer James Christy.
Originally it was going to be named, Persephone, after the daughter of Zeus,
who in Greek mythology was kidnapped by Pluto and made queen of the underworld.
Christy, proposed it was instead called Charon, however, after the ferryman who
took souls across the river Styx to the underworld. But as he wanted to dedicate
the discovery of the moon to his wife, Charleen, he pronounced it as Shar-Ron rather than Kha-Ron.
Not much was known about Charon until 2015 when NASA's New Horizons flew past,
revealing an unusual surface covered in surprising features.
An enormous red formation blankets its North Pole, which was later discovered to be a
result of Pluto's thin atmosphere falling upon its frozen terrain.
A spectacular canyon stretches more than 1,600 kilometres (994 miles) across its surface,
it is at least four times longer than the Grand Canyon and twice as deep in some places.
Some bizarre features were also discovered,
such as a single mound nicknamed the "mountain in the moat" as it sits inside a deep hole.
At around 1200 kilometres wide (746 miles), it is about half as wide as Pluto, in fact,
it is the largest known moon relative to its parent body in the solar system.
So is Charon actually closer to the classification of a dwarf planet than a moon? And if so,
does that make Pluto and Charon our solar system's first double planet system?
Binary objects are not rare across the universe, an estimated one third of
the star systems throughout the Milky Way galaxy are thought to be binary,
However, double planets are expected to be much rarer.
Unfortunately, the criteria that distinguish
between a binary planet and a planet moon system are not well defined.
But the most commonly used criteria has to do with how each object orbits one another.
Basically, the centre of mass between the objects has to lie outside the bodies.
When you think of a binary system, you imagine two objects orbiting each other, but what they
are actually doing is orbiting the common centre of mass. This position is called the Barycentre.
Every object has a centre of mass. It is the exact center of all the material an object
is made of and an object's center of mass is the point at which it can be balanced.
An easy example of this is when you balance a ruler on your finger.
But sometimes the centre of mass is not in the middle,
like a sledgehammer can only be balanced much closer to its heavy end for example.
In space, two or more objects orbiting each other also have a centre of mass.
It's like an imaginary point around which all the objects orbit and the
barycenter is usually closest to the object with the most mass.
In the Earth-Moon system, the barycentric coordinates are
about 4,671 kilometres (2902 miles) above Earth’s centre.
Which means these coordinates appear within Earth,
they lie below the surface and so the moon effectively orbits around the Earth.
Meaning that it cannot be classed as a double
planet system. But this is not the case with Pluto and Charon.
Here, the barycentric coordinates are about 2,126 kilometres (1321 miles) above
Pluto's centre. Meaning each object orbits a point outside of Pluto.
It is this point that orbits the Sun and it is this point that Pluto's four other moons,
Nix, Styx, Kerberos and Hydra also orbit. Pluto is just the
largest object in a system of objects orbiting a common centre of mass.
Because of Charon's size compared to Pluto, they were actually considered
by many as a double planet before Pluto’s reclassification in 2006.
But I think there is a strong argument that these distant frozen worlds should be
reconsidered as our solar system's first official double dwarf planet system.
According to the International Astronomical Union, which sets the definitions for planetary science,
a dwarf planet is a celestial body that orbits the Sun, has enough mass to assume a nearly
spherical shape, but unlike a major planet, has not cleared the neighbourhood around its orbit.
If Charon does not orbit Pluto, then it is not Pluto's moon, and if it meets the first two of
the IAU's dwarf planet classification then it must also be a dwarf planet.
And if both Charon and Pluto orbit a centre of mass outside of Pluto,
then they are according to these definitions a binary dwarf planet system.
And they may not be the only ones, It is also possible that the distant
worlds Eris and its moon Dysnomia are a double dwarf planet. Orcus
and its moon Vanth could be the same, and also Varda and Ilmare.
How these double dwarf planets or even double planets form is a mystery,
but it is thought that much like how our own moon was created in a giant impact event,
Charon could have also been created in the same way.
Some large object may have struck Pluto long ago
and tore out a blob which eventually became Charon.
If the galaxy is full of binary stars, then there are possibly binary planets.
Within our solar system, dozens of binary asteroids have been discovered,
so why not have binary dwarf planets as a category as well?
Pluto's reclassification in 2006 caused a lot of uproar at the time,
but the truth is the astronomers who voted to make it a dwarf planet were simply trying
to fix a misclassification that occurred decades before.
But could that still be the case? Is there a solid argument that it
is still in the wrong category? Should Pluto and Charon be in a new category,
as our solar system's first officially recognised double dwarf planet?
In this video, we discovered the bizarre features of Pluto and Charon,
but there are other ways to explore these fascinating worlds, such as on a freely
rotating globe and thanks to the sponsor of today's video, Mova Globes, we can do just that.
When I'm creating my videos about faraway places,
next to me on my desk is a Mova globe of my favourite small world, Pluto.
Pluto is covered in magnificent features and becuase Mova Globes use realistic,
highly detailed graphics provided by NASA and JPL, when I need a break from the screen I can
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because it rotations on its own, using the power of light.
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because inside is a First-of-its-kind technology, which uses solar cells to
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And it's not just Pluto, Mova Globes offer over forty different designs,
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I really enjoy seeing my Mova Globe on my desk,
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For me, the seamless light-powered rotation is an amazing feature that almost brings
the globe to life and makes it feel even more like the worlds they are based on.
So if you want to bring a little piece of the solar system into your home or office, check
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