Tracing The Birth Of a Giant Planet In The Solar System

In Astronomy, you will be faced with enormous numbers unimaginable before. Zero so much, to the extent that we may be difficult to imagine. Even my age and you are nothing compared to Earth's age. The Earth, as well as the Solar System today from our point of view, may be old. At least for humans, the 4.6 billion years old is obviously very old.

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At such an "old" age, human life history records are unable to tell how the Earth and the planets in the Solar System are formed. Human curiosity as to how life on Earth will grow will certainly lead us to the question of how the Earth and other planets are formed.

It is not easy to know how the Solar System is formed because we can not go back to the past and we also have no other Solar System example in comparison. The presence of planets in other stars or new exoplanets began 20 years ago.

The centuries of research led astronomers to construct theories of the formation of the Solar System. Since it is impossible for us to return to the past, the built theory must produce the Solar System we know today. One that must be fulfilled is the mechanism of planet formation in a short time, especially the giant planets.

One accepted theory is the theory of core accretion. This theory has been discussed in the answer to question LS. Briefly, once the star is formed, the planet is formed from the material gas and dust that is on the disk around the star. The material collides with each other and combines to form planets in the Solar System, both Earth and Earth.

Giant Planet Formation Theory

How the giant planet is formed is very important. In the Solar System, giant planets dominate in terms of mass and also angular momentum. How they are formed and can occupy their orbits currently play an important role in the evolution of the Solar System.

There are two theories currently considered in connection with the formation of giant planets. The first is the theory of accretion, and the second is the theory of gravity instability.

According to the theory of core accretion, the process of formation of a giant planet is similar to that of a terrestrial planet. The birth of the giant planet begins with the formation of the planet's core from the dust present in the protoplanet disk. Dust accumulates to form a planet's nucleus that sizes up to several Earth masses. Once formed, the nucleus then captures the gas present in the protoplanet disk before the gas disappears or ceases to exist.

The problem with core accretion theory is how massive planets can form quickly before the gas disappears from the protoplanet disk. Another question, whether in a short time Jupiter's core is formed is massive enough to capture gas in large quantities.

Another alternative theory, the giant planet is formed by instability in the protoplanet disk. In this model, the gas planet will be directly formed from the instability of gravity in the protoplanet disk. However, this model of instability still can not explain the abundance of heavy elements in Jupiter and Saturn. Another problem, this model can not explain the origins of Uranus and Neptune. Nevertheless, the formation of giant planets through protoplanet disk instability is still possible, especially in regions of protoplanet disks away from stars. Unfortunately, the modeling of the birth of a giant planet through gas instability has not been able to answer the question that arises and has not been tested on the condition of the protoplanet disk. The test in question is through modeling with specified parameters and adjusted to actual conditions.

The Birth of the Planet Giant

Of the two models, the core accretion model became the most accepted theory. Various trials were conducted to answer the questions and problems that exist from the theory. The point is how giant planets can form quickly before the gas disappears from the protoplanet disk.

Before answering that question, let us first look at the birth story of the giant planet in general. Just like planet Earth, giant planets are also born from the process of accretion of solid matter in the protoplanet disk or planetary embryo disk. Here, the dust that interacts and binds together form planetesimal. On a terrestrial planet, this process does form a planet. But for the case of giant planets, the process of solid matter accretion takes place to form the planet's core. Planetesimal that continues to grow and then forms a solid object or embryo of the giant planet's core with a low mass gas veil. At this stage, the rate of capture of gas is still very low, resulting in not much gas being captured. Until then there is no more dust that can be accreted or invited to join, then the rate of increase of solid material is reduced. It is at this point that the rate of capture of gas increases and even exceeds the increase of solid matter.

The gas catching process continues and the planet's nucleus continues to grow at a steady pace. This growing nucleus is also heated by the energy of radioactive accretion and decay which also plays a role in transforming the solid matter into liquid and steam. This change of being is apparently having an effect on the planet's atmosphere and its ability to capture more gas. After going through a long process, the nucleus and the envelope of the planet have the same mass. And finally, the rate of gas capture increases and protoplanet grows at a very fast pace. Finally, the formation of giant planets was finally marked by the cessation of gas and planetary capture and then enter the cooling stage. The accretion process stops when a gap is formed on the disk by the planet's tidal effect or because the gas and dust disks have disappeared.

Race against time

For giant planets like Jupiter and Saturn, hydrogen and helium are the components that dominate the composition. Both of these components are also known to fill more than 10% of the composition of Uranus and Neptune. so the attention, hydrogen, and helium can not condense in the protoplanet disk. In other words, both are captured by a giant planet in the form of gas. And that means, in order for Jupiter and Saturn to form as they are today, they must be able to capture a large amount of hydrogen and helium gas before the protoplanet disk disappears.

The life or the presence of a protoplanetary disk on a young star is only a few million years old. The observation results show that the life of the gas disk is only 1 - 10 million years. Thus, giant planets must be formed within that span of time. Long? Not really. For the astronomical scale, this time is very short. By comparison, Earth is estimated to take 30 million to 100 million years to form until its present state. Another problem, in a very short span of time, is the giant planet's core whose size can reach a dozen times the mass of the Earth must be formed and very quickly capture the surrounding gas.

The protoplanet disk will disappear or the material runs out because of the stellar wind in this solar wind, photoevaporation and planet formation.

The question is, how is a giant planet formed until the Solar System can have four giant planets? To be able to capture large amounts of gas, it takes a massive core. The more massive, the greater the attraction force. For the case in the Solar System, it takes at least a planet's core with a mass of at least 10 Earth masses to accumulate a large amount of atmosphere. And the core of this planet must be formed very quickly in just a few million years and immediately catch the gas.

As described earlier, the first step in the formation of a giant planet is the formation of the planet's core. for the case of giant gas planets such as Jupiter and Saturn, the formed nucleus must have a minimum mass of 10 Earth masses.

To be able to form the planet's core as needed, astronomers perform various simulations to trace the birth of the giant planet. In order to qualify, the core formed in a short time should start with a planetesimal formation which is also large enough so that planetesimal will be more efficient to pull the other planetesimal around it and form the core. One of the models proposed is a gravel accretion model (an object of a few centimeters - meters). And it turns out this model is effective enough to form a giant planet in a short time.

Building a Giant Planet from Gravel

The gravel accretion model begins with the assumption that the early planetary population formed very quickly after the birth of the star. The rest of this planetesimal formation is a pebble that is thought to play an important role in shaping the core of the giant gas planet.

The accretion that begins with a gravel-sized object is effective enough to attract and accumulate the surrounding gravel to join the core. The accretion process progresses rapidly and the planetary core embryo grows from the mass of Pluto to the mass of the Earth in just over a thousand years! And within a few thousand years, the core mass reaches the mass of the Earth. Very fast!

Apparently, there is another problem. Accretion processes that take place quickly and the nucleus can reach several times the mass of the Earth in a short time does not create the Solar System that we know today. What is formed from the accumulation of gravel on the protoplanet disk is not some of the planet's core but 100 new planets with Earth-like mass. These newly formed planets are spreading in very tall orbits and very large slopes. That is, the modeling results do not match the observations.

This problem is solved when the pebbles are formed slowly. During the formation of gravel, planetesimal also has enough time to interact with the surrounding matter. In these conditions, planetesimal can propagate smaller planetesimal out of the gravel gravitational scope. The result, once the pebbles formed and interacted with the surrounding matter, formed several giant planets.

The result of pebble accretion modeling yields 5 objects with Earth mass or slightly larger and 2 giant gas planets formed in the span of 10 million years! But the formation of objects whose mass is like Earth does not occur in just 1000 years. It takes more than 400 thousand years for the core to form before it captures the gas around it. The giant planet in the gravel accretion model is formed in the range of 5 - 15 AU. That is, in order for us to have Uranus and Neptune at its present location, both must migrate. And it is important to be able to explain the distribution of small objects in the outermost areas of the Solar System. The migration of the two planets can result from gravitational disturbances in the system.

One of the causes proposed is the presence of the fifth giant planet in the early formation of the Solar System. The planet is then ejected out of the Solar System and causes the migration process on the giant planet.

Although in general, the gravel accretion model can answer the question of the formation of giant planets, in fact, we are still looking for answers.