Education and Communications

Jupiter: Crash Course Astronomy #16

As we take our grand tour of the solar system
here on Crash Course Astronomy, we’re going to skip over the asteroids for now—we’ll
get to ‘em, I promise—and instead pay a visit to the King of the Planets, the big
guy, the top dog, the big cheese, the head honcho, the one and only: Jupiter. Jupiter is the largest planet in the solar
system. It’s not even close: All the other planets could fit inside it with room to spare.
It’s a gas giant, which means it’s gassy, and… giant. And I do mean giant. It’s 11 times wider
than Earth—more than a thousand Earths could fit inside it, and it has a mass over 300
times that of our planet. Despite its bulk, it rotates extremely rapidly: One day on Jupiter
is a mere 10 hours long! That’s the fastest spin of any of the planets in the solar system. Not surprisingly, a planet that big can reflect
a lot of sunlight, and even though it orbits the Sun on average at a distance of about
800 million kilometers, it’s one of the brightest objects in the night sky. With binoculars or a small telescope, Jupiter
is a wonder. You can easily see it as a disk, and its four biggest moons are readily visible—if
they weren’t hidden by the planet’s glare, they’d be naked eye objects. Galileo himself
discovered those moons. They’re worlds in their own
right, and so we’ll dive into them—literally—in the next episode. When we look at Jupiter we’re not seeing
its surface. We’re seeing the tops of its clouds, and they’re a strange mix of permanence
and change. The atmosphere of Jupiter is banded, with multiple stripes running parallel to
its equator. The lighter-colored stripes are called zones, and the darker ones belts. They’re
fairly stable, though their shape and coloring change over time. Belts and zones circulate around the planet
in opposite directions. They form due to convection in Jupiter’s atmosphere. Upwelling air cools
and forms white ammonia clouds; that creates the light colored zones. That air flows to
the sides and sinks, and sunlight changes the chemistry in the clouds forming molecules
that color the air yellow, red, and brown. This is what causes the darker belts. In May of 2010, one of Jupiter’s biggest
belts sank so deeply it disappeared from view completely, covered by other clouds! Then,
a few months later, it popped back up and reappeared, none the worse for wear. This
has happened several times in the past, too. I saw one of these events once through a telescope,
and Jupiter looked really weird. Lopsided. Turbulence in the regions between zones and
belts can create storms, gigantic vortices raging in the clouds. Dozens of them dot the
face of Jupiter all the time, but there is one to rule them all: the Great Red Spot,
a fittingly huge storm for a giant planet. It’s actually a colossal hurricane, several
times larger than our entire planet Earth, with sustained wind speeds of 500 kph. And
it’s old; it was first seen in the late 17th century—imagine a storm on Earth lasting
for more than three centuries! And it may be far older; the 1600s is just when we first…
spotted it. So, why is it so stable? It turns out that
a vortex, a local spinning region in a fluid, can persist if the fluid in which it’s embedded
is itself rotating. Jupiter’s rapid spin is what keeps the Red Spot circulating! And
the redness is probably due to cyanide-like molecules that absorb blue light, letting
redder light pass through. Weirdly, the Red Spot appears to be shrinking!
It was substantially bigger and more elongated just a few decades ago. It changes color over
time, too, having gone from deep red to salmon and then back again. No one knows why its
shape, size, and color change, but given how long the Spot’s been around, I doubt it’s
going to evaporate any time soon. Remember, we’re only seeing the tops of
the clouds on Jupiter. Its atmosphere is thick, several hundred kilometers deep! Like the
composition of the Sun, the air on Jupiter is mostly hydrogen and helium, but it’s
also laced with ammonia, methane, and other poisonous gases. As you dive into Jupiter’s atmosphere, the
pressure increases with depth. But you’ll never reach the surface; the planet doesn’t
really have a proper surface. The gas gets thicker and hotter, and eventually just sort
of smoothly changes into a liquid over a several hundred km range below the clouds. Below that is where things get really weird.
Instead of a mantle, like terrestrial planets, Jupiter has a huge region made up of liquid
metallic hydrogen. We think of hydrogen as being a gas, or, if it gets really cold, a
liquid. But under the ridiculous pressures generated deep inside Jupiter, hydrogen undergoes
this strange transformation. Individual atoms don’t hold on to their electrons, but instead
share them. This means the hydrogen can conduct electricity, and has other properties more
like a metal. This substance is hot, too: about 10,000° C, hotter than the surface
of the Sun! If we could see it, it would glow tremendously bright. Finally, at its center is most likely a dense
core of material, probably composed of rock and metal. We really don’t know, because
it’s incredibly difficult to understand the physics and chemistry of material locked
in at those pressures and temperatures. What’s weirder is that we’re not even sure if Jupiter
has a core! If it did, it’s possible it was eroded away by currents of hot metallic
hydrogen early on in Jupiter’s formation process. It’s also possible it never had a core in
the first place. The solar system formed from a flat disk of
gas and dust. The center of this disk is where the Sun was born, and it’s thought that
the planets formed as smaller particles of material stuck together during random collisions
farther out in the disk. As they got bigger—much bigger—these protoplanets eventually started
to grow even faster by drawing in material around them due to their gravity. Jupiter formed where the disk was thick, rich
with material. It’s possible that several large protoplanets were forming, collided,
and stuck together to really kickstart Jupiter’s growth. If that were the case, it started
out with a rocky metallic core, and once it got big enough it drew in that gas that made
it the giant we see today. Another idea is that Jupiter didn’t grow
from the bottom up, but from the top down: The disk itself collapsed in several places
to form huge, distended clumps. These then would have collided, stuck, and created the
planet. If that’s the case, then Jupiter might not have a core at all. These two different mechanisms make different
predictions about Jupiter’s structure, and that means that, hopefully, we can eventually
figure out which is correct by studying Jupiter more carefully. But at the moment we still
don’t know. Either way, Jupiter grew immense, and it’s
mostly liquid under all that atmosphere. Couple that with its rapid rotation, and you can
see it’s noticeably flattened! It’s wider at the equator than through the poles by about
6% due to centrifugal force. So, Jupiter is a big bruiser of a planet.
But how close was it to becoming a star? Sometimes, people ask me if Jupiter is a “failed
star”; in other words, as it formed it almost got massive enough that nuclear fusion could
start in its center, turning it into a star. I see this a lot on TV shows and in articles,
and it really burns me up. When a star forms, hydrogen fusion starts
when the star gathers so much mass that its gravity can compress atoms together in its
core hard enough to get them to fuse. This happens when a star has roughly 1/12th of
the Sun’s mass. In fact, the smallest stars we see do have about that mass. What about Jupiter? The mass of Jupiter is
about 1/1000th the mass of the Sun, far too little to undergo fusion in its core. If you
want to turn Jupiter into a star, you’d have a lot of work ahead of you: You’d have
to take Jupiter… and then add about 80 more Jupiters to it! Saying Jupiter is a failed star is really
unfair. It’s not a failed star. It’s a really successful planet. Even though Jupiter isn’t a star, it does
have another funny property: It emits more heat than it receives from the Sun. The Earth and other terrestrial, rocky planets
are in a heat balance with the Sun; we emit pretty much the same amount of heat that we
receive. But Jupiter is different. After it formed, it started to cool by radiating away
heat from its upper atmosphere. A large fraction of the planet is gas, remember, and when you
cool a gas in contracts. So the atmosphere cools and contracts, but this increases the
pressure inside the planet, so it heats up! That heat works its way out of Jupiter, and
gets radiated away as infrared light. In the end, the amount of heat Jupiter gives off
is more than it receives from the Sun. It’s still actively cooling, 4.5 billion years
after it formed! Oh and hey, remember the belts and zones, the stripes we see in Jupiter’s
atmosphere, and all the storms that pop up? Those are driven in large part by Jupiter’s
internal heat. On Earth our weather is powered by heat from the Sun, but on Jupiter they
get their energy from the planet itself! Jupiter has a very strong magnetic field,
no doubt due to all of that metallic hydrogen inside it coupled with its rapid rotation.
Like Earth it has aurorae at its poles as the solar wind is funneled down to the cloud
tops. As we’ll see next week, Jupiter’s moons affect the magnetic field and aurorae
on Jupiter as well. Jupiter also has a ring, though it’s not
nearly as grand as Saturn’s. It wasn’t even discovered until we sent space probes
to the planet. The ring is made of dust, probably thrown into orbit around the planet due to
meteorite impacts on its smaller moons. Speaking of impacts, we know that Earth gets
hit by interplanetary debris all the time: Go outside for an hour and you’re bound
to see a few meteors. Jupiter, being larger and with more gravity, gets hit a lot more.
A lot. A lot more. And sometimes it gets hellaciously whacked. In 1994, the comet Shoemaker-Levy
9 impacted Jupiter. Multiple times: Jupiter’s fierce gravitational tides had ripped the
comet into dozens of pieces, and each slammed into the planet one after the other with the
force of millions of nuclear weapons. The scars left in the upper atmosphere from the plumes
of material that exploded outward lasted for months. Several smaller impacts have been seen in
Jupiter’s atmosphere since then, and it may suffer an impact large enough to see from
Earth every year or so. And while that sounds scary, it might actually
be our savior. There’s an idea that Jupiter’s gravity tends to take comets that fall toward
the inner solar system and fling them away into interstellar space. Over the eons, this
has cleaned out a lot of otherwise dangerous objects that could have eventually hit Earth.
On the other hand, Jupiter has a tendency to warp the orbits of some other comets so
that they do swing by the Earth. It’s hard to say if Jupiter’s influence is a net benefit
or not. But either way, it’s clearly the 2 septillion
ton gorilla in the solar system. Today you learned that Jupiter is really,
really big. It’s the biggest planet in our solar system, a gas giant. It has a dynamic
atmosphere, including belts and zones, and a gigantic red spot that’s actually a persistent
hurricane. Jupiter is still warm from its formation, and has an interior that’s mostly
metallic hydrogen, and it may not even have a core. It has the fastest spin of any planet,
and it’s not a failed star. Crash Course Astronomy is produced in association
with PBS Digital Studios. Head on over to their channel to discover more awesome 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 co-directed by Nicholas Jenkins and
Michael Aranda, edited by Nicole Sweeney, and the graphics team is Thought Café.
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