Video summary
The outer planets, also known as Jovian or gas giants, represent a stark contrast to the inner terrestrial worlds previously discussed. Unlike Earth-like planets with solid surfaces that can support landings, these massive bodies are composed primarily of hydrogen and helium gases, along with traces of ices such as water, ammonia, and methane. While they are significantly larger than any rocky planet—with Neptune being large enough to contain all four terrestrial planets inside it—they remain incredibly small relative to the Sun. Their composition is dominated by light elements; although rock and metal exist deep within their interiors, they make up a negligible percentage of the total mass because hydrogen contributes most heavily due to its sheer abundance in number compared to heavier atoms like helium.
Each giant planet possesses unique physical characteristics that distinguish it from the others. Jupiter stands as the largest with an orbital period of nearly 12 years and a rapid rotation speed of just under ten hours, causing day-night cycles lasting only five hours each. Saturn is slightly smaller but famous for its rings and has such low density that it would theoretically float in water if a large enough body existed to hold it. Uranus rotates on its side with an orbital period of 84 years, while Neptune, the most distant planet discovered mathematically before observation, takes approximately 165 Earth-years to complete one orbit around the Sun. Despite their vast differences in size and density, all four share a lack of solid surfaces and are dominated by fluid dynamics rather than geological crusts found on inner planets.
Human exploration of these worlds has been achieved through various spacecraft missions that have visited each planet at least once. Jupiter and Saturn have hosted orbiting probes like Galileo, Juno, and Cassini, allowing for detailed study over extended periods; notably, the Galileo probe even descended into Jupiter's atmosphere before being destroyed by heat and pressure. In contrast, Uranus and Neptune were only visited briefly during flybys, primarily by Voyager 2 in a historic grand tour of the solar system. Modern missions like Juno utilize highly elliptical polar orbits to avoid intense radiation belts while capturing unique images of storm patterns at Jupiter's poles that differ from the equatorial views provided by earlier spacecraft.
Beyond simple observation, these missions have revealed surprising phenomena such as Saturn's mysterious hexagonal jet stream around its north pole and complex atmospheric dynamics in Uranus and Neptune. The Galileo probe also succeeded in measuring conditions deep within Jupiter's atmosphere before vaporizing, providing crucial data on pressure and temperature gradients that surface landings would never reveal. While our knowledge of these distant worlds is less extensive than what we possess for Mars or Venus due to the difficulty and cost of reaching them, every outer planet has been explored by at least one mission. As future lectures will delve deeper into their moons and specific atmospheric features, it becomes clear that while Jovian planets share fundamental similarities in composition and structure, each retains distinct characteristics shaped by its unique position and history within our solar system.
Read the full video transcript
Greetings and welcome to the
introduction to astronomy. In this
lecture, we are going to begin our
discussion of the outer planets. We've
looked previous lectures looked at the
inner terrestrial type planets, those
similar to Earth. Now we're going to
look at planets that are very different
than our own.
So, let's look at these in an overview
here. What do we know about the Jovian
planets? Well, we know that they're
larger.
We know that they're composed of ice and
gas primarily.
And those ices that include things like
water, ammonia, and methane. So, it does
not mean just water ice.
And while they are much larger than the
terrestrial planets, you could fit all
of the terrestrial planets within side
Neptune.
Still, they are incredibly tiny as you
see here relative to the Sun.
These have no solid surface. There is no
way we will ever land on these. These
will never be landed on because there is
no place to land.
However, we can land on their moons and
we will look at the planets and their
moons in coming lectures.
So, let's look a little bit about what
these planets are made up of and it's
different than what we looked at in the
inner solar system.
If we're looking by mass, then hydrogen
makes up 75%
and helium 24%.
If we go by number, it's more like 90%
and 10%.
So, because the helium atom is more
massive, it contributes more to the
mass, but the hydrogen atom by number,
if you pick an atom at random out of
Jupiter, for example, you got a nine in
10 chance that it will be a hydrogen
atom.
So, we also again have traces the ices,
water, methane, and ammonia. And yes,
there is a small amount of rock and
metal, but it's very small uh percentage
that we will see of those. And the only
reason the percentage is so high is
again because they're heavier. They are
much more massive, so they're get
weighted a lot higher when we look at
the measurements by mass.
Now, let's look briefly at each of these
planets. Let's start off with Jupiter,
which is the nearest of the giant
planets and is the largest planet in the
solar system. As you recall, we put
these relative to Earth. So, instead of
giving you a diameter in kilometers, we
say that it is 11 times Earth's
diameter, meaning that you could fit 11
Earths across
Jupiter.
Its mass is 318
times Earth's mass. So, if you could
take 318 Earths, you would need that
much to make one Jupiter.
Its rotation period is incredibly short
at 9 hours and 55 minutes. That is the
fastest rotation of any planet in the
solar system, and it's less than 10
hours. So, if you could somehow sit in
the atmosphere of Jupiter and rotate
around with it, the sun would rise and 5
hours later it would set and 5 hours
later it would rise again.
The orbital period is nearly 12 years,
and the semi-major axis 5 astronomical
units, five times further away from the
sun than Earth.
Its density a little greater than the
density of water. So, it has some very
dense interior, but it has very low
density gases on the exterior that
average it out to be a very low density
overall.
Now, we're going to see that Saturn is a
little bit smaller, but has some
similarities here. Saturn being the
ringed planet, its diameter about a
little less than 10 times Earth's
diameter and its mass about 95 times
Earth's mass.
Its rotational period is a little bit
longer than Jupiter's and its orbital
period is a lot longer going to 30 years
from 12. It is nearly twice as far away
from the Sun as Jupiter is and its
density is even less. Its density is
less than that of water meaning that if
you had some something full of water
large enough and could somehow set
Saturn in it, Saturn would float.
So it is less dense than water.
Next, we look at the two icy planets and
those are Uranus which was discovered in
1781.
So we have the first planets now that
were discovered and this is the planet
that lies on its side. It is four times
Earth's diameter, 14 and a half times
Earth's mass, so much smaller than
Jupiter and Saturn. Its rotational
period is getting back closer to
Earth's. It's 17 hours
and its orbital period of 84 years. So
one year on Uranus would be 84 Earth
years.
>> [snorts]
>> Semi-major axis would be almost 20
astronomical units
and the density has increased a little
bit. We're back to about the density
that Jupiter had.
And then finally, we have Neptune.
Neptune, discovered in 1846
and it is the most distant planet. It is
about the same size and mass as Uranus.
Its rotational period is also similar
but its orbital period of course being
further away, 30 astronomical units
instead of 20, means it takes much much
longer to go around the Sun once, 165
years. So since it was discovered it is
just starting its second orbit. So, it's
[snorts] gotten back to where it started
in its orbit, and it is just starting to
work the first few years of its second
orbit around the sun since it has been
discovered. And its density again is
comparable to Uranus and Jupiter's.
Now, all of these planets have been
explored
by spacecraft. Each has been visited
each by at least once. Jupiter and
Saturn have had orbiting spacecraft, and
around Jupiter we had the Galileo
mission,
and we have the Juno mission, which is
ongoing. These were both uh orbiting
missions, and around Saturn we had the
Cassini mission. So, those are orbiting
missions. The rest of these were all
flybys, and some of them were flybys to
go someplace else. For example, Cassini
flew by Jupiter and used its gravity to
help it on to Saturn.
You'll see that Voyager 2 went from
Jupiter
to Saturn,
to Uranus, and to Neptune. So, the
Voyager 2 craft visited all four planets
and is the only craft to visit uh visit
Uranus and Neptune. So, all of our
detailed images of those come from the
Voyager 2 spacecraft, which was able to
make a grand tour of the solar system
because of the alignment, the
positioning of the planets in the solar
system when that craft was launched in
the late 1970s.
So, each of them has been explored to
varying extent. Jupiter and Saturn
pretty well, Uranus and Neptune just a
little bit.
So, what do we know? What else have we
done with exploration? Well, we've done
a few more things, too, than just those.
The Galileo spacecraft actually had a
probe into Jupiter's atmosphere. So, a
probe that went into the atmosphere
doesn't last very long. It will quickly
heat up and the pressures and
temperatures will vaporize it. But, we
were able to get some measurements of
the atmosphere as it plunged deep into
Jupiter.
Or at least the upper layers of the
atmos-
Then there is Juno, which is studying
the polar regions of Jupiter. This is
currently, as of this recording, an
active satellite around Jupiter. It is
in a polar orbit, meaning that it goes
up and over the poles instead of around
the equatorial regions. So, we can see a
little bit about what it sees here and
it sees a little bit different than what
we're used to seeing in the equatorial
regions of Jupiter. Very interesting
patterns of storms that occur in those
in the polar regions. And this is really
the first time we've gotten to get good
looks at the storms. Most of the other
craft have come in the equatorial plane
and given us great views of the
equatorial regions with its banding
structure,
but not so much a good look at the
poles. Juno is also in a highly
elliptical orbit, meaning that it zips
in close to Jupiter, comes in close,
takes a quick look at it, and then heads
back out. Remember Kepler's second law,
and what that says is that the closer
you are to the object, the faster you're
going to move. So, when it comes in
close to Jupiter, it zips by and gets a
few up-close pictures, then gets out of
there. Jupiter being very intense,
massive, massive and intense, has a lot
of radiation near it, so it doesn't want
to spend a lot of time there. And it
spends most of its time well away from
Jupiter, but passes in uh to be able to
get some close-up shots.
Then we have the Cassini spacecraft,
which discovered a polar hexagon on
Saturn. So, a hexagonal pattern, unusual
when we look at things in the universe.
Usually, we get things that are
spherical, elliptical, ellipsoidal
shaped, but some kind of really smooth
curve like that. To get a hexagon,
we tend not to see that. We don't see
things like triangles or squares very
often. Those would be very unusual.
So, the polar hexagon is something
interesting that Cassini was able to
find and may have something to do with
the fluid mechanics in the atmosphere
near Saturn's pole.
So, we are going to look at all of these
in a little bit more detail in coming
lectures, but let's go ahead and finish
up here with our summary.
And what we looked at is the terrestrial
planets that
were are quite different that we've been
studying are quite different than the
Jovian planets that we are going to look
at now. The properties are very
different, but each of these has its own
unique characteristics. So, there are
similarities between them, but there are
also differences.
Each of these has been visited by
spacecraft, but to a much smaller extent
than the space the number that we talked
about for Mars and Venus.
So, that concludes this lecture on the
overview of the giant planets.
We'll be back again next time for
another topic in astronomy.
So, until then, have a great day,
everyone, and I will see you in class.