INTRODUCTION
What is the reason that inner planets are made of rock and outer planets are made of gas? We know that there are two types of planets in the solar system. 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. The planets vary greatly in their conditions, but they all have certain similarities and present unique challenges for exploration and observation. What is the source of this variation? Do all solar systems have rocky planets closer to their stars?
THE EARTH
The Earth is perhaps the most important rocky planet in our solar system. The composition of Earth divides it into the core, mantle, and crust. 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, which makes up about 68 percent of Earth’s mass, is very hot. And if you go deeper, you can reach the core. 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? There are several reasons why scientists believe that the core is made of metal.
- 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. The core is believed to be reflected by metallic meteorites.
- 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 is 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. The outer core experiences convection currents because of the heat generated by the even hotter inner core. The heat necessary to prevent the outer core from solidifying is a result of the decay of radioactive elements in the inner core.
MERCURY
Speaking of Mercury, it is also rocky planet that has a central core, a rocky mantle, and a solid crust, just like Earth. It actually is the second densest planet in our solar system, after Earth. The planet has a substantial metallic core, which is approximately 1,289 miles (2,074 kilometers) in radius, constituting about 85 percent of the planet’s total radius. There are indications suggesting that the core is partially molten or in a liquid state. The outer shell of Mercury, equivalent to Earth’s mantle and crust, is only around 400 kilometers (250 miles) in thickness.
VENUS
Venus, which is also known as Earth’s twin. 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.
MARS
Talking about Mars, it 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. The rocky mantle, ranging from 770 to 1,170 miles (1,240 to 1,880 kilometers) in thickness, surrounds the core. Above the mantle lies a crust composed of iron, magnesium, aluminum, calcium, and potassium. This crust has a depth of 6 to 30 miles (10 to 50 kilometers).
WHAT ARE GAS GIANTS?
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! The rocky or terrestrial planets are not the same as them, as they are composed mostly of 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. The core of gas giants could consist of rock or metal, but most of their mass is made up of gas. Since the pressure is really high, gas on these planets might also be compressed into a liquid state. Jupiter and Saturn probably have liquid metallic hydrogen interiors, which allows them to have a magnetic field since liquid hydrogen conducts electricity. Uranus and Neptune are thought to have interiors comprising a combination of rock, water, methane, and ammonia, possibly arranged in layers. So far, we have given a brief description and overview of the composition of planets in our solar system.
ROCKY PLANETS VS GASEOUS PLANETS
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. Over time, the planetesimals continued to collide and join together, attracted by gravity. These are called protoplanets. Protoplanets, which are similar in size and mass to our Moon, are the larger objects. The process of material gathering to create planets is known as 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. In its early years, the sun released massive amounts of energy and material into the solar system. The rocky planets, which were the nearest to the Sun, bore the brunt of these releases. 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. Is this a pattern that we see everywhere? Is there a similar pattern of rocky planets being closer to the star and massive gas giants being farther away in other solar systems across the universe? No, this is not the case. Researchers have discovered numerous solar systems where gaseous planets, some even more massive than our solar system’s gas giants, are located 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.