Uranus & Neptune: Crash Course Astronomy #19

Summary

This Crash Course episode explores Uranus and Neptune, the two ice giants of our solar system. Discovered by William Herschel, Uranus has a unique sideways tilt and a possible ocean of liquid diamonds. Neptune, discovered through mathematical predictions, boasts the fastest winds and an active moon, Triton. Both planets have mysterious magnetic fields, diverse moons, and faint rings, highlighting the wonders and puzzles of our outer solar system.

Takeaways

• 🌌 Uranus and Neptune are ice giants in our solar system, distinct from the gas giants Jupiter and Saturn due to their composition.
• 🔭 Uranus was the first planet discovered using a telescope by William Herschel in 1781, who initially named it Georgium Sidus, but it was later renamed Uranus.
• 💎 The extreme pressure inside Uranus can break up methane molecules, potentially forming diamonds that could fall like hailstones into the planet's mantle.
• 🌀 Uranus has a unique tilt of about 98°, causing its axis to point almost directly at the Sun during its summer, leading to extreme seasons.
• 🌐 Uranus has a magnetic field that is significantly offset from the planet's spin axis, a mystery that scientists are still trying to solve.
• 🌕 Uranus has over two dozen moons, some of which are named after characters in Shakespeare's plays.
• 🌀 Neptune, discovered mathematically in 1846, is known for its deep blue color and active atmosphere with clouds and storms.
• 🌪 Neptune has the fastest sustained winds in the solar system, reaching over 2000 kph, likely due to low atmospheric friction.
• 🌑 Neptune's largest moon, Triton, is unique for its retrograde orbit and geological activity, including nitrogen geysers.
• 💍 Neptune's ring system is peculiar, with clumpy and incomplete arcs, possibly influenced by small moonlets.
• 📚 The discovery of Neptune demonstrated the importance of mathematics in predicting the existence and position of celestial bodies.

Q & A

Who discovered Uranus and when?

Uranus was discovered by William Herschel in 1781 while he was mapping the skies through his telescope.

What was the original name given to Uranus by Herschel and why was it changed?

Herschel originally named it Georgium Sidus, after King George III. However, the name didn't stick and it was later renamed Uranus, following the nomenclature of Roman gods.

What is the composition of Uranus' interior?

Uranus' interior is believed to consist of three layers: a small rocky core, a thick mantle made up of materials like water, ammonia, and methane, and an atmosphere primarily composed of hydrogen and helium.

Why is Uranus referred to as an 'ice giant'?

Uranus is called an 'ice giant' because its mantle is made up of 'icy' materials like water, ammonia, and methane, which are under a lot of pressure and form an ocean-like layer.

What is the unusual feature of Uranus' magnetic field?

Uranus' magnetic field is unusual because its axis is tipped by over 50° from the planet’s spin axis, and the center of the magnetosphere is about 8000 km from the planet’s center.

How many moons does Uranus have and what are they named after?

Uranus has more than two dozen moons, with the five largest ones named after characters in Shakespeare plays, such as Ariel, Umbriel, Titania, Oberon, and Miranda.

What is unique about Miranda, one of Uranus' moons?

Miranda is unique due to its jumbled terrains, criss-crossed by canyons and grooves, possibly the result of a past giant impact. It also features Verona Rupes, the tallest cliff in the solar system.

How were Uranus' rings discovered and what are they made of?

Uranus' rings were discovered accidentally in 1997 during an observation of Uranus passing in front of a star. The rings are made of dark particles, likely ice and reddish organic molecules.

What is the difference in mass between Neptune and Uranus?

Neptune is more massive than Uranus, with Neptune being 17 times the mass of Earth compared to Uranus' 14.5 times.

Why is Neptune's color different from Uranus'?

While both have methane in their atmospheres, Neptune's deeper blue hue is a mystery and may be related to its more active atmosphere with clouds of methane, ammonia, and hydrogen sulfide.

How was Neptune discovered and what is unique about its discovery?

Neptune was discovered in 1846 based on mathematical predictions by Urbain Le Verrier and John Couch Adams, who calculated its position due to discrepancies in Uranus' orbit. It is the only planet found via mathematical prediction.

Outlines

00:00

🌌 Discovery and Characteristics of Uranus

The script introduces Uranus as the first planet discovered with a telescope, found by William Herschel in 1781. Herschel initially named it Georgium Sidus, but the name Uranus, following Roman god nomenclature, prevailed. The planet's visibility to the naked eye under optimal conditions and its misidentification as a star in the past are highlighted. The text delves into Uranus's composition, describing its small rocky core, icy mantle of water, ammonia, and methane, and the atmospheric composition that gives it a cyan appearance. The script also discusses the planet's peculiar tilt, extreme seasons, and the mystery of its formation, including the possibility of a massive impact. The magnetic field's peculiarities and Uranus's moons, particularly Miranda with its unique terrain and Verona Rupes, the tallest cliff in the solar system, are also covered.

05:02

🌀 Neptune's Atmosphere, Rings, and Moons

This paragraph explores Neptune, another ice giant with similarities to Uranus, such as its composition of a rocky core, icy mantle, and an atmosphere rich in hydrogen, helium, and methane. The differences are noted, including Neptune's greater mass and density, and its distinct deep blue color, possibly due to its more active atmosphere. The paragraph discusses Neptune's extreme weather, including its high-speed winds and the transient Great Dark Spot. The discovery of Neptune through mathematical predictions by Urbain Le Verrier and Johann Galle is recounted, emphasizing the importance of accurate data in celestial mechanics. The rings of Neptune, described as clumpy and arc-like, and its moons, with a focus on the unique and geologically active Triton, are also detailed. Triton's surface features, potential cryovolcanism, geysers, and thin nitrogen atmosphere are highlighted.

10:04

📚 The Mathematical Discovery of Neptune and Conclusion

The final paragraph wraps up the episode by recounting the historical discovery of Neptune, which was mathematically predicted and then observed, marking a significant achievement in astronomy. It touches upon the work of Urbain Le Verrier and John Couch Adams, noting the close race in predicting Neptune's position. The paragraph also corrects the historical misunderstanding about the discrepancies in Uranus's and Neptune's observed positions, which were resolved with more accurate measurements from the Voyager missions. The script concludes by summarizing the key points about Uranus and Neptune, their classification as ice giants, their weather, rings, and moons, and acknowledges the production team behind Crash Course Astronomy.

Keywords

💡Uranus

Uranus is the first planet that was discovered using a telescope, not known in ancient times. It was named after the king George III by William Herschel but was later renamed following the convention of Roman gods. The script describes Uranus as an 'ice giant' with a unique composition and tilt, which is a central theme of the video, highlighting its distinctive characteristics among the planets.

💡Neptune

Neptune, like Uranus, is categorized as an 'ice giant' and is the more massive of the two. It is known for its deep blue color, which is a result of its atmospheric composition, including methane. The script emphasizes Neptune's discovery as a result of mathematical predictions, making it a unique case in the history of astronomy.

💡Ice giants

The term 'ice giants' refers to the class of planets in our solar system that have a small rocky core surrounded by a thick mantle of water, ammonia, and methane, with an atmosphere of hydrogen and helium. Both Uranus and Neptune are ice giants, and the video discusses their similarities and differences, making this a key concept for understanding these planets.

💡Methane

Methane is a chemical compound that plays a significant role in the atmospheres of both Uranus and Neptune, giving them their distinct colors. In the script, methane's absorption of red light is highlighted as the reason Uranus appears cyan or aquamarine, while Neptune's deeper blue hue is a subject of scientific curiosity.

💡Atmosphere

The atmospheres of Uranus and Neptune are composed mainly of hydrogen and helium, with traces of methane that give them their unique colors. The script describes how the atmospheres of these planets are observed from Earth and how they differ in terms of weather phenomena and cloud formations.

💡Magnetic field

The magnetic fields of Uranus and Neptune are peculiarly offset from the planets' centers and are not aligned with their rotational axes. The script discusses the oddities of these magnetic fields and how they might be related to the planets' icy mantles, which is a key aspect of understanding their internal structures.

💡Rings

Both Uranus and Neptune have ring systems, which are composed of dark particles, likely ice and organic molecules. The script explains how these rings were discovered and describes their composition and structure, which is an important feature of these planets.

💡Moons

Uranus and Neptune have multiple moons, some of which are named after characters from Shakespeare's plays. The script mentions specific moons like Miranda and Triton, highlighting their unique geological features, such as Verona Rupes on Miranda and the geological activity on Triton.

💡Astronomy

Astronomy is the scientific study of celestial objects and phenomena outside the Earth's atmosphere. The script is an episode of Crash Course Astronomy, which aims to educate viewers about the planets, their characteristics, and the scientific methods used to study them.

💡Telescope

A telescope is an optical instrument used to observe distant objects by collecting electromagnetic radiation. The script mentions the use of telescopes in the discovery of Uranus and Neptune, as well as in observing their features, making it a key tool in the study of these planets.

💡Orbital mechanics

Orbital mechanics is the study of the motions of celestial objects in response to gravitational forces. The script discusses how the mathematical principles of orbital mechanics were used to predict the existence and location of Neptune, demonstrating the practical application of this scientific field.

Highlights

Uranus and Neptune are ice giants with unique characteristics compared to gas giants like Jupiter and Saturn.

Uranus was the first planet discovered using a telescope by William Herschel in 1781.

The original name 'Georgium Sidus' was not adopted, and Uranus was named after the Roman god of the sky.

Uranus can be seen with the naked eye under very dark skies, despite being on the edge of visibility.

Uranus' interior is likely composed of a small rocky core and a thick mantle of water, ammonia, and methane.

High pressure inside Uranus can create diamonds that may form an ocean of liquid diamonds.

Uranus' atmosphere is primarily hydrogen and helium with a small percentage of methane, giving it a cyan appearance.

Uranus has a unique tilt of about 98°, causing extreme seasonal variations.

Uranus has a magnetic field that is significantly offset from its spin axis.

Uranus has over two dozen moons, including Miranda, which has the tallest cliff in the solar system, Verona Rupes.

Uranus' ring system was accidentally discovered during an observation of the planet passing in front of a star.

Neptune is denser than Uranus, with a deep blue color due to an active atmosphere and unknown factors.

Neptune's atmosphere has the fastest recorded wind speeds in the solar system, over 2000 kph.

Neptune's moon Triton is unique for its retrograde orbit and geological activity, including nitrogen geysers.

Neptune was the first planet to be discovered based on mathematical predictions, not direct observation.

The discovery of Neptune resolved discrepancies in the predicted and actual positions of Uranus.

Pluto was discovered accidentally during the search for a ninth planet to explain Uranus and Neptune's orbital anomalies.

Transcripts

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

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

if you include the one you’re 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.

They’re 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 massive—about 14.5 times the mass of Earth—but 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 as “ice,” so even though it’s not

ice like we usually think of it here on Earth, we say that the mantle of Uranus is “icy.”

To distinguish it from the gas giants Jupiter and Saturn, Uranus is called an “ice giant.”

Speaking of “ice,” 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 we’ll ever see that. When we observe the planet, we’re 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 they’re difficult to see in visible light. The clouds are made

of methane, ammonia, and hydrogen sulfide—that 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 once—Voyager

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 it “the 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, called—for some reason—the Great

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

gone, but others had appeared. They’re 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, they’re weird. There are three main rings; two narrow

and one broad. They’re 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 we’ll 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!

They’re 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.