How Can Jupiter Have 79 Moons While Other Planets Have None?

If you guessed 'gravity' you're right -- but a few specific factors make Jupiter so much moonier than planets like Earth. Learn more in this episode of BrainStuff. 

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Episode Transcript

Speaker 1 (00:01):
Welcome to brain Stuff production of I Heart Radio, Hey
brain Stuff Lauren vogebom Here. Earth has only one moon,
but dozens of natural satellites revolve around Jupiter, the biggest
planet in our Solar System, and new members in the
Jupiter posse are still being discovered. In July, a team

(00:22):
of astronomers announced that they found twelve previously unknown moons
around Jupiter. Scott S. Shepherd of the Carnegie Institute for
Sciences was leading a search for new objects in the
distant Kuiper Belt, an enormous ring of debris that lies
beyond Neptune. Shepherd and his colleagues decided to take a
break from their primary research goal and observed Jupiter for

(00:43):
a while. That's how they wound up standing on Galileo's shoulders.
In sixteen ten, the great astronomer Galileo Galilei noticed four
heavenly bodies that appeared to revolve around Jupiter, named Io, Europa, Ganymede,
and Callisto. These are Jupiter's biggest moons by far, and
they were the first to be discovered. As stargazing technology

(01:05):
grew more sophisticated, it became clear that the quartet had
lots of company. Shepherd's team just brought the total number
of identified Jovian moons, that is, moons that revolve around Jupiter,
up to seventy nine. No other planet in the Solar
System comes close to that figure. Saturn is the runner
up with its sixty two verified moons, while Urinas boasts

(01:26):
twenty seven and Neptune has fourteen. Mars, our beloved next
door neighbor, possesses two satellites, Demos and Phobos. And if
that's making you feel insecure about Earth's lonely single moon,
at least you could take solace in the fact that
Mercury and Venus are totally moonless. According to the astronomer
Neil F. Corman's, if planet Earth had two moons instead

(01:47):
of one, our knights would get brighter because there would
be twice as much sunlight reflecting off of lunar surfaces.
Also would see significantly higher tides, rendering many coastal areas uninhabitable.
There's a reason why Jupiter has so many satellites while
other planets, Ours, for instance, have so few, or a
few reasons really, but it all comes down to gravity.

(02:10):
Astronomers divide the planets within our Solar System into two categories. Mercury, venous,
Earth and Mars are so called terrestrial or inner planets,
while Jupiter, Saturn, Uranus, and Neptune have been classified as
gas giants, also known as outer planets. The size gap
between those factions is quite considerable. Although Uranus is the

(02:30):
smallest outer planet, it's still fifteen times more massive than Earth,
which is the largest of the inner planets. But none
of the other planets can compete with Jupiter in terms
of sheer bulk. You'd need more than three hundred duplicates
of our puny home world to equal Jupiter's colossal mass. Now,
as Isaac Newton observed, there's a positive correlation between the

(02:51):
mass of an object and the strength of its gravitational field.
Because the gas giants are so massive, they're able to
attract more satellites. That's not the only reason why planets
like Jupiter have such large moon collections. Our Solar systems
gas giants are relatively far away from the Sun. In contrast,
some stars have massive Jupiter like planets called hot jupiters. Basically,

(03:14):
these are gas giants that orbit in close proximity to
their stars. Imagine if Saturn switched places with mercury. A
twenty ten paper by French astronomer effect He in the
Money argues that hot jupiters have few, if any moons.
These planets are thought to originate in distant parts of
their solar systems, and then migrate inwards. Along the way,

(03:35):
their moons get caught in a game of celestial tug
of war. Gas Giants may be big, but stars are
much bigger. As such, they've got far stronger gravitational fields,
so when a hot jupiter gets too close to its star,
the star will eventually steal its moons. Distance offsets this ability.
The further you travel from the Sun, the weaker its

(03:57):
gravitational pull on you becomes. Are For, if Naumali is correct,
then our Jupiter has seventy nine moons and counting. Because
it's a ridiculously massive planet that's far enough away from
the Sun to avoid lunar theft. Jupiter's moons are hardly
a monolithic family. A few of them have quirks that
are well known to astronomy enthusiasts. Io is loaded with

(04:19):
active volcanoes, there's a hidden ocean on Europa that might
harbor alien life, and at two thirds the size of Mars.
Ganymede is the biggest satellite in the entire Solar System.
These three moons, along with Kalisto, probably formed in tandem
with Jupiter itself. The big planet likely started out as
a disc of gases and dust that eventually became the

(04:40):
gas giant we know today. While Jupiter took shape, some
of the materials swirling around it coalesced into the four moons.
Galileo spied in sixteen Saturn may have helped move the
process along. It's also been hypothesized that early Jupiter had
a number of failed moons that were pulled into and
absorbed by the huge planet. It's other satellites weren't necessarily homegrown.

(05:03):
Scientists think that many of Jupiter's moons started out as
drifting chunks of rock that became ensnared by the planet's
gravitational pull, And we also have to talk about lunar behavior.
Many of the Jovian moons orbit in the same direction
in which Jupiter spins, but there are those that go
the opposite way, including nine of the new moons discovered
by Shepherd and his colleagues. With so many bodies revolving

(05:26):
in different directions. Collisions are inevitable. Moons that crash into
one another might well be destroyed in the process. Just
as Jupiter acquires new moons, it's finding ways to lose
some of the older ones. Today's episode was written by
Mark Mancini and produced by Tyler Clang. Brain Stuff is

(05:47):
production of I Heart Radio's How Stuff Works. For more
on this and lots of other far from monolithic topics,
visit our home planet, how stuff Works dot com. And
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