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Up until 1781, our ancestors thought that the Solar System comprised of only 6 planets, with Saturn being the furthest away from the sun...

In 1781, however, German astronomer William Herschel first identified the planet Uranus, making it the 7th planet in our solar system.

He was in fact, also a musician, and discovered Uranus while messing around with a homemade telescope in his back garden... so the story goes anyway! He's also responsible for the discovery of several of Uranus' moons, including the 2 largest, Titania (not to be confused with Saturn's Titan!) and Oberon.

Uranus as seen by Voyager 2.... It's amazing how clear of an image we can get from a satellite hurling through space 3 billion kilometres away... not to mention how little light Uranus reflects at that distance.

Picture taken of Uranus by Voyager 2

Being the 7th planet from the sun, Uranus is an extremely cold gas giant (although it's also referred to as an ice giant), sharing many similar properties to that of Neptune.

Although two and a half times smaller than Saturn, and orbiting at more than twice the distance, Uranus can still just about be detected with the naked eye under dark enough skies.

Having said that though, it merely appears as a very faint star in the night sky (under bortle 5-6 skies anyway...), and even through a decently expensive telescope, such as a reflector with a 10 inch aperture and 1200mm focal length, appears as a faint blob of turquoise; Uranus is in fact the last planet that you can see with the naked eye, with Neptune being unobservable without the aid of equipment.

At 2.9 billion kilometres away from the sun (think about that for a second...), Uranus is an extremely cold planet, although its distance from the sun isn't the only factor that contributes to its lack of heat.

Surprisingly, Uranus holds the record for the lowest temperature out of the 8 planets in our solar system, despite being a billion kilometres closer to the sun than Neptune.

So how come it's colder if it's closer to the sun?

It's not completely known, however, what can be said about the planet is that observations from Voyager 2, the only space probe to have come close to the planet (at the time of writing this), tell us that Uranus radiates extremely little heat - far less than any other planet.

Primordial radiation

The emission of primordial radiation from planets comes from their formation - remember that planets form from violent collisions between rock, gases and dust in the early stages of the Solar System.

These collisions create immense quantities of heat that the planets are still radiating from their cores to this day, and will continue to do so for millions of years.

Despite low atmospheric temperatures, the cores of the gas giants in particular are superheated to over 12,000 degrees Celsius, which is even hotter than the surface of the sun!

These scorching temperatures are achieved through the incredibly high pressures that exist at the cores of every planet.

Even if you take the Earth as an example, imagine all of the weight of the crust, the mantle, the atmosphere, the water.... everything on the core of the planet; the pressure would be incredible.

Even so, you still might be wondering how pressure relates to temperature, and unfortunately, it's not the simplest relationship to think about, but I'll give it a shot here.

By the way, I have a separate post that's dedicated to realising the connection between temperature and pressure, so feel free to check it out by clicking here if you're still a bit confused after reading this.

Pressure in and of itself does not increase temperature, rather it is the work done to compress a container to achieve a higher pressure that results in a temperature increase.

When we say "temperature", we're effectively referring to the total kinetic energy of whatever it is that we're talking about - in the case of the planets' cores, the total kinetic energy of the atoms that make up the core.

Brief sidetrack of how pressure works - feel free to skip if you're not interested!

As matter builds up around the core, the gravitational "force" of the core causes the gasses to accelerate towards the core. An increase in acceleration results in an increase in velocity (which is obviously pretty simple to understand), and because of the fact that the kinetic energy of an object is proportional to the square of its velocity, we can say that the kinetic energy of the matter has increased.

When they collide with the core, and their velocity is reduced, energy has been transferred from the form of kinetic energy in said matter, to kinetic energy of the matter that makes up the core.

More kinetic energy means that the atoms that make up the core achieve higher velocities, meaning that the atoms collide with the edge of the "container" (the core in this case) with more force, resulting in an increase in pressure by virtue of pressure resulting from collisions of atoms with the edge of the "container".

It's easier to understand with gases, or liquids, as the atoms in those states of matter are constantly moving around, and it's the collisions with the boundaries that create pressure.

However, since the atoms that make up solids don't move around, rather vibrate or oscillate back and fourth, it's a bit more difficult to understand how they can create pressure. It can however, be briefly summarised by the intermolecular electrostatic forces of repulsion that repel the electrons from each other.

Not an easy concept to understand, but hopefully you now have an idea of how pressure can affect temperature, and more importantly, what that means for the temperature at the core of every planet in our solar system. Note that this relates to now and millions of years in the future, but not indefinitely, as the cores are constantly radiating this primordial heat, and hence becoming cooler with every passing second.

Returning to Uranus....

So it's a dreadfully cold planet, due to this apparent lack of primordial heat.

Speaking of its history, something else is quite peculiar about the planet which is said to have resulted from a past event; the planet's rotation lies almost perfectly in its orbital plane, which is a fancy way of saying that it spins on its side!

Rolling through space

The accepted idea is that Uranus, early on in its formation, was struck by a very large body, something perhaps of the magnitude of Earth or perhaps Mars, and this exchange of momentum caused Uranus to topple over on its side, and has remained as such ever since.

However, it must be stressed that this is merely a supposition and, as of present, we have no way of understanding how Uranus really ended up rotating in its orbital plane.

So to recap, unlike every other planet in the Solar System, Uranus does not rotate perpendicular to its movement (note that the other planets don't perfectly rotate perpendicularly, but they're pretty close to), rather it's completely toppled over on its side, and rotates such that the poles are constantly either facing towards the sun, or completely away.

This might seem like a harmless characteristic, but it's actually bound to have drastic consequences for the motion of its atmosphere....

Uranus takes 84 Earth years to complete a single revolution around the sun, meaning that for 42 years, one pole receives constant light and the other none at all. The roles are then reversed for the following 42 years.

Interestingly though, the temperature of the planet is relatively unaffected by this, as when you're 3 billion kilometres away from the sun, the amount of radiation that you receive is pitiful anyway.

Magnetic field

Like all the other gas giants (plus Mercury and Earth), Uranus has a strong magnetic field that is thought to be the product of the same mechanism as for all the other planets with a strong magnetic field - a dynamo effect at the core of the planet.

Molten metals such as Iron drive a current around the core which induces a magnetic field. The higher the volume of molten material, and the higher the speed at which they circulate around the core, the bigger and stronger the magnetic field.

Hence, in the case of a gas giant such as Uranus, where large amounts of molten material exist due to the high temperatures and pressures at the core, the magnetic field is much stronger and larger than that of a terrestrial planet such as Earth.

In fact, Uranus' magnetic field is 50 times the size of Earth's, although the strength of such at the surface is actually weaker, simply due to the fact that the planet is so much larger than Earth.

Moving on to the constituents of the planet, Uranus is 99% hydrogen and helium, much like the other gas giants, and in fact the sun. What stops Uranus from becoming a star like the Sun though is the inability to fuse atoms together, which can only happen at the extreme scale of pressure and temperature.

The aesthetics

The pale colour of Uranus, which can be seen below, is thought to the be the result of a very concentrated haze that's spread across the planet.

See an image comparing Uranus (middle left) to Neptune (middle right). Here, you can clearly see the paleness of Uranus in comparison, which you can almost imagine being a thick haze that covers the planet. In addition, you can see that Uranus and Neptune are incredibly similar in size, with Uranus being ever so slightly larger (by 900 miles in diameter).

I've also simulated them to show their real axis of rotation, which demonstrates how Uranus is almost completely on its side. All done in Universe Sandbox 2 which you can see more info about here.

Uranus compared to Earth, Neptune and Jupiter

Almost everything that we know about Uranus comes from the albeit brief encounter that Voyager 2 had with the planet while on it's mission to the Solar System and beyond!

Accelerated by the gravity of both Jupiter and Saturn, engineers had to reprogram the probe to achieve a fixed position on the planet to create as long as an exposure as possible (to get the best images possible).

The probe took some amazing pictures of Uranus, many of which can be found freely on the internet (try NASA's website)!

Because it's the only probe to have come closely enough to the planet as of 19/02/2023, our knowledge of Uranus remains relatively limited, but advanced instruments in orbit around the Earth allow us to detect radiation even from as far away as Uranus, and hence perform spectrometry techniques that help us to identify many aspects of the planet.

I like using Universe Sandbox (a cool simulation game that you can check out on my website) to illustrate the scale of the universe.

At the bottom left, you can see Voyager 2 on its path to the beyond. As of 19/02/2023, its 12.93 billion miles away... but still communicating with NASA as its ability to send signals of electromagnetic radiation is unaffected by distance in the vacuum of space, but the time it takes for the radiation to reach us is of course affected.

The Solar System

As mentioned throughout, Uranus is ultimately a very unknown world to us at this point, but there's no doubt that future missions and advancements in technology will enable us to capture more information about our neighbour!

Thanks for reading and, as always, feel free to contact us at any time!

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Imagine hurling through space, 13 billion kilometres away from the Sun, Ryu....

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