Why are Inner Planets Rocky and Outer Planets Gaseous?
We all know the solar system contains two
kinds of planets. The first four, Mercury through Mars, are quite small in size, and they are rocky
planets, also called “terrestrial planets”.
The outer four are gaseous giants: from Jupiter to Neptune,
they are called “Jovian planets” and they are immense balls of gas, with a central core.
While
conditions on these planets can be very different from one another, each type of planet shares some
similarities and offers its own set of challenges when it comes to exploration and observation. But
where does this difference come from? Do all solar systems have rocky planets closer to their stars?
Follow me in this video to get to know the answer to these questions, and more!
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The Earth is perhaps the most important rocky planet in our solar system.
On Earth, the core, mantle, and crust are divisions based on composition.
The crust is a really small part of the overall Earth’s mass, accounting for less than 1 percent.
It consists of oceanic crust and continental crust is often more felsic rock. The crust is
where we live, the place we walk on, and the most exposed layer of our planet.
Then,
there is the mantle. The mantle is hot and represents about 68 percent of Earth’s mass.
And if you go deeper, you can reach the core. I mean, you can’t really, because the deeper you go,
the stronger the pressure, and you will reach a point where you won't be able to survive anymore.
But let’s suppose this doesn’t happen. If you could reach the core,
you would see it is made of iron metal. The core makes up about 31% of the Earth.
One may ask: how do we know the core is metallic?
Scientists know that the core is metal for a few reasons.
First of all, the rotation of the Earth gives us some hints
about the average density of our planet. It can be shown that the average density
given by the rotation is much more than the density of Earth’s surface layers, which we
can measure. The only way for this to happen is to have an interior denser than the average.
Basically, if surface layers are less dense than average,
then the interior must be denser than average.
Calculations indicate that the core is about 85 percent iron
metal with nickel metal making up much of the remaining 15 percent.
Also, metallic meteorites are thought to be representative of the core.
Second thing: if Earth’s core were not metal, the planet would not have a magnetic field. But we
know a magnetic field exists around the Earth.
A mechanism that could produce and sustain a magnetic field I the so-called dynamo mechanism.
For this mechanism to act, the outer core of the planet
must be liquid and the inner core must be solid.
The strong magnetic field is caused by convection in the liquid outer core. Convection currents in
the outer core are due to heat from the even hotter inner core. The heat that keeps the outer
core from solidifying is produced by the breakdown of radioactive elements in the inner core.
Speaking of Mercury, it also has a central core,
a rocky mantle, and a solid crust, just like Earth.
It actually is the second densest planet, after Earth. It has a large metallic core with a radius
of about 1,289 miles (2,074 kilometers), about 85 percent of the planet's radius.
There is evidence that it is partly molten or liquid. Mercury's outer shell, comparable to
Earth's outer shell (called the mantle and crust), is only about 400 kilometers (250 miles) thick.
What about Venus?
Interestingly enough, if we could slice Venus and Earth in half, pole to pole, and place them side
by side, they would look remarkably similar. They both have an iron core, surrounded by a
hot mantle. And they both have a crust, which is a quite thin external layer.
The crust is a sort of skin, that changes form over time, causing volcanism.
However, the two planets are also really different.
For instance, the plate tectonics mechanism is active.
It is the slow movement of continents over thousands and
millions of years that reshapes the surface. This is not observed to be happening on Venus.
If it did, it did in the early history of the planet.
But we observe strong volcanic activity on Venus. This could be due to subduction: the sliding of
one continental plate beneath another, which can also trigger volcanoes.
NASA’s Magellan spacecraft saw a land of extreme volcanism.
The orbiter saw a relatively young surface,
one recently reshaped (in geologic terms), and chains of towering mountains.
Last but not least, Mars has a dense core at its center between 930 and 1,300 miles (1,500
to 2,100 kilometers) in radius. It's made of iron, nickel, and sulfur. Surrounding the core
is a rocky mantle between 770 and 1,170 miles (1,240 to 1,880 kilometers) thick,
and above that, a crust made of iron, magnesium, aluminum, calcium, and potassium.
This crust is between 6 and 30 miles (10 to 50 kilometers) deep.
The four gas giants in our solar system are Jupiter, Saturn, Uranus,
and Neptune. As we’ve already mentioned, they are also called the Jovian planets.
A gas giant is a GIANT planet that is made of gas! They are different from rocky or terrestrial
planets that are made of mostly rock. Unlike rocky planets, gas giants do not have a well-defined
surface – there is no clear boundary between where the atmosphere ends and the surface starts,
and you can’t walk or land on their surface. The gas giants have atmospheres that are
mostly hydrogen and helium, the most common gases in the universe.
Gas giants may have a rocky or metallic core but the majority of their mass is in the form of gas.
Since the pressure is really high, gas on these planets might also
be compressed into a liquid state, just like mercury in a thermometer.
Jupiter and Saturn probably have liquid metallic hydrogen interiors, which allows them to have a
magnetic field since liquid hydrogen conducts electricity. Scientists believe Uranus and
Neptune have interiors that contain a mixture (or layers) of rock, water, methane, and ammonia.
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So far, we have given a brief description and overview of the
composition of planets in our solar system. Now let’s get to the point:
why are they different?
To understand why the rocky planets are closer to
the Sun, we must go back to when the solar system was young and the planets were still being born.
The planets in our Solar System are believed to have formed from the same
spinning disc of dust that formed the Sun. This disc was composed mainly of hydrogen
and helium, but also had other elements in smaller proportions.
Dust particles in the spinning disc began to clump together as gravity attracted them to each other.
You have to think about it as a fight: during this period,
clumps of matter were trying to grow as big as they could,
in order to overcome the others in size, thus being the dominating objects in the disk.-
Over a few million years many of these chunks had merged together and there were about 109
objects called planetesimals, with diameters of about 1000 m.
Over time the planetesimals continued to collide and join together, attracted by gravity.
These larger objects, about the size and mass of our Moon, are called protoplanets.
The accumulation of material to form planets in this way is called accretion.
The solar system in its youth was very chaotic. All the planets, and even our own Sun,
were in exotic and excited states. For example, the Earth was basically a ball of lava. A long
time passed before it cooled down! In the early solar system, there
were many collisions and explosions, and heat waves passing between them.
Once upon a time, the rocky planets we see now also had huge gaseous layers around them,
just like the gas giants and ice giants. However, during its angry young years,
the Sun let out huge streams of energy and matter into the solar system.
The rocky planets, being the closest to the Sun, took the full force of these streams.
This made the gases of these planets part ways with them, leaving just the rocky part behind,
which went on to become the terrestrial planets. The remaining four planets were too far from the
Sun to experience these streams, so they stayed relatively unaffected and formed the gas and ice
giants of today.
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So is this a pattern that we see everywhere? Do other solar systems in other parts of the
universe show the same trend of having the rocky planets closer to the star,
while the massive gas giants are quite far away?
The answer is no. Scientists have found many solar
systems in which gaseous planets (many of which are even more massive than the gas giants of our
solar system) are the ones closest to their stars. For instance, they found a lot of HOT JUPITERS.
Hot Jupiters are Jupiter-like planets orbiting really close to their host star. The first Hot
Jupiter ever discovered is 51 Pegasi b.
It was actually the first exoplanet to be discovered around a star similar to our Sun.
It is the prototype of the hot Jupiter planets and is located in orbit around the star 51 Pegasi,
in the constellation of Pegasus. The discovery of this planet changed our understanding of the
universe forever. When 51 Pegasi b was discovered, planetary formation theories were not compatible
with a giant planet so close to its star and it was considered an anomaly. However, since then,
numerous others have been discovered, such as in the 55 Cancri and Tau Bootis systems.
The authors of the discovery of 51 Pegasi b, Michelle Mayor and Didier Queloz,
had been awarded the Nobel prize in 2019 and new planetary formation theories are being proposed
every day. Most of them take into account the dynamics of the diverse systems. For instance,
astronomers have begun to revise their theories and study the possibility of amazing dynamical
phenomena, such as orbital migrations. But this is a story for another video.
This video ends here! Thanks for watching! I hope you enjoyed it. See you next time
on the channel!