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  • This episode of Crash Course is brought to you by Squarespace.

  • You run us. YOU run us. You RAN us. You ran us. Ooo- Ooo ran ose.

  • There are five planets in our solar system you can see without a telescope; well, six

  • if you include the one youre sitting on. But there are two more big ones out there,

  • orbiting the Sun out in the cold depths of the outer solar system: Uranus and Neptune.

  • Theyre a lot alike in many ways, but of course they have their individual quirks.

  • Uranus was the first planet to be discovered, and by that I mean it wasn’t known in ancient

  • times. It took an astronomer with a telescope to find it. In 1781, William Herschel was

  • mapping the skies through his telescope when he spotted a greenish object that was clearly

  • a disk and not a dot, like a star. He noted its position, and moved on. But when he went

  • to observe it again sometime later, he was astonished to discover it had moved!

  • It was quickly determined to be a planet, more distant from the Sun than Saturn. In

  • true brown-nosing style, he named it Georgium Sidus, or George’s star, after the reigning

  • king George III. Yeah, happily, that name didn’t stick, and keeping with the nomenclature

  • of Roman gods, the new planet was dubbed Uranus.

  • Interesting tidbit: It actually is visible to the unaided eye if you have very sharp

  • eyesight and very dark skies. It’s right on the thin hairy edge of visibility. What’s

  • funny is that several people had observed it before Herschel, but none had noticed its

  • movement. Some even catalogued it on their maps as a star!

  • The planet is massiveabout 14.5 times the mass of Earthbut not terribly dense. This

  • means its interior must be made of lightweight stuff. Scientific models of the planet based on physics

  • and chemistry of the outer solar system indicate its interior is probably made up of three general layers.

  • There’s a small rocky core, smaller than Earth, surrounded by a very thick layer of

  • materials like water, ammonia and methane. This mantle makes up the bulk of the planet,

  • in fact, and is dense, hot, and under a lot of pressure. In many ways, it’s more like

  • an ocean than anything else. However, somewhat confusingly I’ll admit, outer solar system

  • planetary scientists refer to water, ammonia, and methane asice,” so even though it’s not

  • ice like we usually think of it here on Earth, we say that the mantle of Uranus isicy.”

  • To distinguish it from the gas giants Jupiter and Saturn, Uranus is called anice giant.”

  • Speaking ofice,” here’s a really weird thing: Studies have shown that the pressure

  • inside Uranus can break up methane molecules, squeezing the carbon in them so tightly that

  • it actually forms diamonds! These would then fall down to the base of the mantle like sparkly

  • hailstones. Except it’s dark. But still, down there in the depths of Uranus there may

  • even be an ocean of liquid diamonds, where solid ones float like, um, diamondbergs.

  • Not that well ever see that. When we observe the planet, were only seeing the top of

  • its atmosphere. Besides hydrogen and helium, the air there is about 2% methane. Methane

  • is really good at absorbing red light, which means the light we see reflected from Uranus is mostly green

  • and blue, making the planet look distinctly cyan or aquamarine. And it’s pretty striking through a telescope.

  • But in visible light the planet looks almost featureless. It doesn’t have that deep banding

  • like Jupiter, or even the pale ones of Saturn, though when you look in the infrared some

  • banding can be seen.

  • There are clouds, but again theyre difficult to see in visible light. The clouds are made

  • of methane, ammonia, and hydrogen sulfidethat last one is what makes rotten eggs smell so

  • bad. I’d avoid breathing through your noise at Uranus.

  • But then, the atmosphere there is negative 220 Celsius, so that might be a better reason not to inhale.

  • In late 2014, a bunch of storms popped up in Uranus’s atmosphere, so big and bright

  • they were easily visible from Earth. The storms may have dredged up very reflective methane

  • ice from lower down in the atmosphere -- and this time, I do mean icy ice -- which is why

  • they were bright. The northern hemisphere of Uranus is approaching summertime, which

  • may be why these storms formed.

  • And that brings us to the weirdest thing about this planet: It’s sideways!

  • If you were above the Earth’s north pole looking down, you’d see our planet spinning

  • counter-clockwise, west to east. The Sun spins that way, and all the planets do as well

  • except Venus and Uranus. While Venus is flipped all the way over, Uranus is tilted by about

  • 98°. That means that in the summer, its axis is pointed almost directly at the Sun, so

  • seasons on Uranus are pretty extreme, by outer solar system standards. Weird.

  • No one knows why Uranus is tipped so much. An obvious thought is that it got whacked,

  • hard, by an impact long ago. If it were a grazing collision by a BIG object, that could

  • have pushed hard enough on the planet to tip it over.

  • Unfortunately, Uranus is very far away, and has only been visited by spacecraft onceVoyager

  • 2, in 1986—and even then it was a quick flyby. Uranus’s weird tilt is just one of

  • those many mysteries that astronomers are trying to solve with limited data.

  • Uranus has a magnetic field, but it’s truly odd: Its axis is tipped by over 50° from

  • the planet’s spin axis, and it’s way off-center; the center of the magnetosphere is about 8000

  • km from the planet’s center. It may be that the magnetic field is generated in the icy

  • mantle, or that the core somehow interferes with the magnetic field, throwing it off.

  • Truthfully, no one really knows why.

  • Uranus has more than two dozen moons; five big ones and a bunch of dinkier ones.

  • Cool fact: The moons are named after characters in Shakespeare plays. So we have Ariel, Umbriel,

  • Titania, Oberon, and Miranda. Even Puck!

  • Of them all, I think the most interesting one is Miranda. When Voyager 2 flew past,

  • it revealed an icy world that looks like it was put together by Dr. Frankenstein: a patchwork

  • of jumbled terrains all crammed together, criss-crossed by canyons and grooves. It’s

  • possible a giant impact in its past actually disrupted the moon somewhat, and it settled

  • back together into this weird mishmash.

  • But the reason I like it so much is a feature called Verona Rupes: It’s the tallest cliff

  • in the solar system, 5 to 10 kilometers high. If you jumped off the top, it would take you

  • six minutes to fall to the surface! That would be a fantastic ride.

  • Like Jupiter and Saturn, Uranus has a ring system, too. They were discovered by accident

  • in 1997; astronomers were observing Uranus pass directly in front of a star. They were

  • hoping to use this to gather information about the planet’s atmosphere as starlight passed

  • through it. But they saw several dips in starlight before the main event, which they realized

  • were from rings around the planet.

  • The rings are made of dark particles, probably ice and reddish organic molecules. There are

  • 13 rings known, most of them are very faint and narrow. They may have been created by

  • an impact completely shattering a small moon orbiting Uranus, but as for now, the ring origins are unclear.

  • And then, finally, we have Neptune, the guardian of the solar system’s nether regions. Neptune

  • is an ice giant, like Uranus, and has a lot of similarities. Like its green brother, it

  • probably has a rocky core surrounded by a thick icy mantle of water, ammonia, and methane.

  • Above that is an atmosphere of hydrogen, helium, and methane.

  • But there are differences, too. Neptune is more massive than Uranus; 17 times Earth’s

  • mass, versus just 14.5 for Uranus. Neptune is slightly smaller than Uranus, which means

  • it’s a lot denser. Also, while Uranus is teal, Neptune is a deep, rich azure—I like

  • to call itthe other blue planet”, the first one being, y’know, Earth. Through

  • a telescope, Neptune’s color is quite lovely. It has roughly the same amount of red-light-absorbing

  • methane in its atmosphere, as Uranus does. So its deeper blue hue is something of a mystery.

  • That may have to do with its active atmosphere. Unlike blander Uranus, Neptune has clouds

  • of methane, ammonia, and hydrogen sulfide lying the skies at different depths, and white

  • streaky clouds were seen during the Voyager 2 flyby in 1989. They looked whipped by wind,

  • and for good reason: Sustained wind speeds in Neptune’s atmosphere have been clocked

  • at over 2000 kph: Faster than the speed of sound on Earth! It’s thought that the low

  • temperatures in the atmosphere reduce friction, allowing the winds to gather to such amazing speeds.

  • Voyager saw a huge storm marring Neptune’s face, calledfor some reasonthe Great

  • Dark Spot. A few years later, when Hubble was used to observe the planet, the spot was

  • gone, but others had appeared. Theyre probably vortices, cyclones, which allow us to see

  • through the upper atmosphere and peer farther into Neptune’s depths.

  • Neptune has a magnetic field, and like Uranus, it’s offset from the planet’s center.

  • Perhaps that icy mantle is at work, somehow interfering with the generation of the magnetic

  • fields in both planets.

  • Neptune has rings, too, but SHOCKER, theyre weird. There are three main rings; two narrow

  • and one broad. Theyre clumpy, and have bright stretches that make the rings look

  • more like incomplete arcs. It’s possible those arcs are being constrained by small

  • moonlets near the rings.

  • Speaking of which, Neptune has over a dozen known moons. Most are quite small, but one,

  • Triton, is by far the largest. At 2700 kilometers across it’s smaller than our own Moon, but

  • the rest of them are really dinky. Triton orbits around Neptune backwards, retrograde.

  • As well learn in a future episode, there’s a repository of giant iceballs out past Neptune,

  • so Triton was probably one of those that got too close to Neptune and was captured by its gravity.

  • Most of what we know about Triton came from a single flyby of Voyager 2 in 1989, and only

  • about 40% of the surface was seen. But this quick glimpse revealed a weird little moon.

  • The surface is covered in nitrogen ice, as well as water and carbon dioxide ice. It’s

  • really flat, and has very few craters, meaning something resurfaced it in geologically recent

  • times. Most likely this was from cryovolcanoes, cold volcanism; that is, volcanoes where water

  • and ammonia take the place of lava there.

  • Also, Triton has been seen to have active geysers of nitrogen erupting from its surface!

  • Theyre probably due to warming from the Sun, and they make Triton one of the few objects

  • in the solar system seen to be geologically active. It also has a very thin atmosphere

  • of nitrogen, probably due to evaporation from the surface.

  • After all this, Neptune is special in another way, too.

  • Neptune is faint, and can only be seen telescopically. It was discovered in 1846, and it wasn’t

  • an accident. Over the decades, astronomers observed Uranus, and found something weird:

  • It wasn’t where it was supposed to be. Over time, its predicted position was off from

  • where it actually was. The French mathematician Urbain Le Verrier concluded that this was

  • due to an unseen planet, and was able to use the mathematics of orbital mechanics to predict

  • where the new planet would be. He sent a letter with the predicted position to the Berlin

  • Observatory. Astronomer Johann Galle read the letter, when right out and found the planet

  • that very night. Neptune was within a degree of the predicted spot.

  • Amazingly, another mathematician, Englishman John Couch Adams, had also worked on the math

  • and had made a similar prediction -- but Le Verrier beat him by two days.

  • Two. Days. Of such tight races are fame made in science.

  • Interestingly, over time, Neptune seemed to wander from its predicted position as well.

  • A ninth massive planet was predicted, leading to a grand search that resulted in the discovery

  • of Pluto. But Pluto was far too small to affect Neptune. When Voyager passed both Uranus and

  • Neptune, it found the masses of the planets were different than what had been measured

  • from Earth. When the new masses were used in the orbital equations, Uranus and Neptune were right

  • where they were supposed to be. It helps to have the right numbers to plug into your equations.

  • Pluto, therefore, was found by accident. That means

  • Neptune is the only planet in the solar system found via math.

  • See? Your algebra teacher was right: Someday this stuff will be important.

  • Today you learned that Uranus and Neptune are ice giants, with small rocky cores, thick

  • mantles of ammonia, water, and methane, and atmospheres that make them look greenish and

  • blue. Uranus has relatively dull weather, while Neptune has clouds and storms whipped

  • by tremendous winds. Both have rings and moons, with Neptune’s Triton probably being a captured

  • iceball that has active geology.

  • Crash Course Astronomy is produced in association with PBS Digital Studios. Head on over to

  • their YouTube channel for even more cool videos. This episode was written by me, Phil Plait.

  • The script was edited by Blake de Pastino, and our consultant is Dr. Michelle Thaller.

  • It was directed by Nicholas Jenkins, and our editor and script supervisor is Nicole Sweeney.

  • The sound designer was Michael Aranda, and the graphics team is Thought Café.