Why is Uranus on its Side?

It’s impossible to do an article about Uranus without opening up the back door to a spit storm of potty humour. I get it, there’s something just hilarious about talking about your, mine and everyone’s anus. And even if you use the more sanitized and sterile term urine-us, it’s still pretty dirty, in an unwashed New York stairwell kind of way. You’re in us? No.

This is a no-win solution. It’s a Kobayashi Maru scenario here. We’re all doomed.

Can we call a truce? I dare you commentators, to keep the YouTube comments as pure and clean as driven snow, so we can focus on the super interesting science. Think of the children.

Let’s set the stage, I’m going to let planetary astronomer Kevin Grazier give you the proper pronunciation to clear our minds and let us move forward with grace and civility.

Kevin Grazier:
Strictly speaking, it’s pronounced Youranous, is the  pronunciation.

As you probably know, Uranus… I mean Ouranus. No, I can’t do it, my brainwashing is too far along. Save yourselves!. Anyway, Uranus is the 7th planet from the Sun, and the 3rd largest planet in the Solar System. Jupiter and Saturn get all the spacecraft and Hubble space telescopes, but Uranus is an incredibly worthwhile target to visit.

Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA
Diameter comparison of Uranus and Earth. Approximate scale is 90 km/px. Credit: NASA

It’s almost exactly 4 times larger than Earth and has its own set of strange dusty rings – perhaps left over from a shattered moon. It has at least 27 moons, that we know of, and many more interesting features that would fascinate astronomers, if we had a spacecraft there, which we don’t. Which is ridiculous. We’ve only made one close flyby of Uranus by Voyager II back in 1986.

We’ve seen Pluto up close, but there are no plans to visit Uranus? Madness.

Near-infrared views of Uranus reveal its otherwise faint ring system, highlighting the extent to which it is tilted. Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.
Credit: Lawrence Sromovsky, (Univ. Wisconsin-Madison), Keck Observatory.

Anyway, perhaps one of the strangest aspects of Uranus is its tilt. The planet is flipped over on its side, like a Weeble, that wouldn’t unwobble.

Actually, all the planets in the Solar System have some level of axial tilt. The Earth is tilted 23.5 degrees away from the Sun’s equator. Mars is 25 degrees, and even Mercury is 2.1 degrees tilted. These tilts are everywhere.

But Uranus is 97.8 degrees. That’s just 0.2 degrees shy of a 90s boy band.

You might be wondering, why have it be more than 90 degrees. High school geometry tells me that 97.8 degrees is the same as 82.2 degrees. And that’s true. But astronomers define the angle as greater than 90 degrees when you take its direction of rotation into account. When you describe it as turning in the same direction as the rest of the planets in the Solar System, then you have to measure it this way.

What could have done that to Uranus, how could it have happened?

The fact that Uranus is flipped over on its side tells us that the calm clockwork motion of the Solar System hasn’t always been this way. Shortly after the formation of the Sun and planets, our neighborhood was a violent place.

The early planets smashed into each other, pushed one another into new orbits. Some planets could have been spun out of the Solar System entirely, while others might have been driven into the Sun. Our own Moon was likely formed when a Mars-sized object crashed into the Earth. Other moons might have been captured from three body interactions between worlds. It was mayhem.

The Solar System that you see today contains the survivors. Everything that wasn’t delivered a death blow.

And something really tried to deliver a death blow to Uranus, very early after it formed. We know this because the moons of Uranus orbit at the same tilt as the planet’s axis. This means that something smashed into Uranus while it was still surrounded by the disk of gas and dust that its moons formed from.

When the massive collision happened, the planet flipped over, wrenching this disk with it. The moons formed within this new configuration.

Astronomers think it was more complicated than that, however. If it was a single, massive collision, models suggest the planet would just flip over entirely, and end up rotating backwards from the other planets in the Solar System.

It’s more likely that another collision or even a series of collisions put the brakes on Uranus’ end over end roll, putting it into its current configuration. It boggles the mind to think about what must have happened.

Uranus' tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)
Uranus’ tilt drastically affects the amount of sunlight the hemispheres receive during its orbit. Credit: NASA, ESA, and A. Feild (STScI)

Having such a huge axial tilt makes a big different to Uranus. As it travels around the Sun in its 84-year orbit, the planet still has its poles pointed at fixed locations in space. This means that it spends 42 years with its northern hemisphere roughly pointed towards the Sun, and 42 years with its southern hemisphere in sunlight.

If you could stand on the north pole of Uranus, the Sun would be directly overhead in the middle of summer, and then it would make bigger and bigger circles until it dipped below the horizon a few decades later. Then you wouldn’t see it for a few decades until it finally reappeared again. It would be very very strange.

Of course, it’s a gas planet, so you can’t stand on it. If you could stand on it, we’d all be marveling at your ability to stand on planets.

Here we are in our calm, ordered Solar System, everything’s business as usual. But if you look around, you realize it’s pretty amazing that our planet is even here. Poor sideways Uranus is a testament to our good luck.

Why Planets Orbit the Sun

Why Do Planets Orbit the Sun

In ancient times, astronomers thought that all celestial objects – the Sun, Moon, planets and stars – orbited around the Earth in a series of crystal spheres. But as modern science developed, astronomers were better able to understand our place in the cosmos. They discovered that all the planets, including the Earth, actually orbit around the Sun.

Not only did scientists discover that the simple fact that the planets orbit the Sun, they uncovered the underlying reasons for why. What chain of events led us to our current Solar System, with planets orbiting the Sun?

Astronomers Used to Think the Earth was the Center of the Solar System
Ptolemaic systemBecause we live on Earth, and we see objects passing across our view of the skies, it’s natural to assume that the Earth is the center of the Universe. In fact, this perspective – known as geocentrism – was the default for all ancient civilizations. The Sun, the Moon, the planets and the stars appeared to move around the Earth each day. And because the Earth itself didn’t seem to be moving, astronomers like Ptolemy assumed that Earth was the center of the Universe. In fact, they went so far as to create very detailed models for predicting the motions of objects with a high degree of accuracy, using this completely inaccurate model of the Solar System. The predictions made by Ptolemy were used to make astrological predictions for more than 1500 years, until a much better model came along.

Actually, the Sun is the Center of the Solar System
Heliocentric ModelA new, more accurate model of the Solar System didn’t come around until the 16th century, when the Polish astronomer Nicolai Copernicus published his Universe-changing book: On the Revolutions of the Heavenly Bodies. Copernicus accurately reorganized the Solar System, putting the Sun at the center in a heliocentric model. And the Earth took its proper place, as just another planet orbiting the Sun – one of the 6 known to astronomers at the time.

Copernicus’ model helped answer two questions which had troubled astronomers for centuries: why the planets brighten and dim over the course of several months (because they’re getting closer and further away), and why the planets seem to reverse and move in a retrograde direction. Easily explained because of the changing positions of the Earth, planets and the background stars.

But Why Do They Orbit the Sun?
Solar nebulaOnce they could accurately describe the nature of the planetary motion in the Solar System, they were left with a more fundimental question: Why do the planets orbit the Sun? What sequence of events led to the current motions of the planets around the Sun?

To explain this, we need to look back 4.6 billion years ago, before there was even a Solar System. In our place instead, there was a massive cloud of hydrogen gas left over from the Big Bang. Some event, like a nearby supernova explosion triggered a gravitational collapse of the cloud, causing the hydrogen atoms to attach to one another through mutual gravity.

Each individual hydrogen atom had its own momentum, and so when the atoms collected together into larger and larger clumps of gas, the conservation of momentum across all the particles set these clumps of gas spinning. Imagine two spinning skydivers colliding with one another in mid-air; after their collision, they’ll have a new rotation speed and direction based on the addition of their original directions.

Eventually all of this hydrogen gas was collected together into a massive spinning ball of gas that continued to collapse under its own gravity. As it collapsed, it began to spin faster and faster, just like a figure skater pulling in her arms increases her rotation speed.

The spinning cloud of gas and dust flattened out because of the rotational force, with the Sun at the center, and then a pancake-shaped disc of material surrounding it. The planets formed out of this disk of material, collecting together particles of dust into larger and larger rocks until planet-sized objects had accumulated together.

The Planets are in Perfect Balance

The planets orbit the Sun because they’re left over from the formation of the Solar System. Their current motion depends on the gravitational attraction of the Sun at the center of the Solar System. In fact, they’re in perfect balance.

There are two opposing forces acting on the planets: gravity pulling them inward, and the inertia of their orbit driving them outwards. If gravity was dominant, the planets would spiral inward. If their inertia was dominant, the planets would spiral outward into deep space.

The planets are trying to fly out into deep space, but the gravity of the Sun is pulling them into a curved orbit.

Research further:
Cornell Astronomy
The Universe of Aristotle and Ptolemy
Copernical Model: A Sun-Centered Solar System
The Solar Nebula
On the Revolution of the Heavenly Bodies
The Copernican Revolution