ACCRETION THEORY: BACKGROUND AND EXPLANATION - PHYSICAL - 2024

The t eoría accretion (or accretion) in astrophysics, explains that the planets and other celestial bodies are formed by the condensation of small dust particles are attracted by the force of gravity.

The idea that planets form in this way was put forward by the Russian geophysicist Otto Schmidt (1891-1956) in 1944; He proposed that a huge cloud of gas and dust, in the shape of a flattened disk, surrounded the Sun in the early solar system.

Figure 1. Artist's concept of the protoplanetary disk, from which planets are formed by accretion. Source: Wikimedia Commons.

Schmidt claimed that the Sun had acquired this cloud in conjunction with another star, which, carried by its movement through the galaxy, passed at the same time through a nebula rich in dust and gas. The closeness of the other star helped ours to capture matter that later condensed.

Hypotheses about the formation of the solar system fall into two categories: evolutionary and catastrophic. The former affirm that both the Sun and the planets evolve from a single process and go back to the ideas proposed by Inmanuel Kant (1724-1804) and Pierre Simon de Laplace (1749-1827).

The second point to a catastrophic event, such as a collision or proximity with another star, as triggers for planetary formation. Initially, the Schmidt hypothesis fell into this category.

Explanation

Today there are observations of young star systems and sufficient computational power to perform numerical simulations. This is why catastrophic theories have been abandoned in favor of evolutionary ones.

The nebular hypothesis of the formation of the solar system is currently the most accepted by the scientific community, maintaining accretion as the planet-forming process.

In the case of our own solar system, 4.5 billion years ago, gravitational pull gathered tiny particles of cosmic dust - ranging in size from a few angstroms to 1 centimeter - around a central point, forming a cloud.

This cloud was the birthplace of the Sun and its planets. It is speculated that the origin of the cosmic dust could be the previous explosion of a supernova: a star that collapsed violently and scattered its remnants through space.

In the densest areas of the cloud, the particles collided more frequently because of their proximity and began to lose kinetic energy.

Then the gravitational energy caused the cloud to collapse under its own gravity. Thus was born a protostar. Gravity continued to act until it formed a disk, from which first rings were formed and later planets.

Meanwhile, the Sun in the center compacted, and when it reached a certain critical mass, nuclear fusion reactions began to occur within it. These reactions are what maintain the Sun and any stars.

The highly energetic particles were propelled from the Sun, which is known as the solar wind. This helped clean up the debris, throwing it out.

Formation of the planets

Astronomers suppose that after the birth of our star king, the disk of dust and gas that surrounded it remained there for at least 100 million years, allowing enough time for planetary formation.

Figure 2. Diagram of the Solar System today. Source: Wikimedia Commons.

On our timescale, this period looks like an eternity, but in reality it is only a brief instant in universe time.

At this time larger objects, about 100 km in diameter, called planetesimals, were formed. They are the embryos of a future planet.

The energy of the newborn Sun helped evaporate gases and dust from the disk, and this shortened the birth time of the new planets considerably. Meanwhile, the collisions continued to add matter, since this is precisely accretion.

Models of planetary formation

By looking at young stars in formation, scientists are gaining insight into how our own solar system formed. At the beginning there was a difficulty: these stars are hidden in the visible frequency range, because of the clouds of cosmic dust that surround them.

But thanks to telescopes with infrared sensors, the cosmic dust cloud can be penetrated. It has been shown that in most of the nebulae in the Milky Way there are stars in formation, and surely planets that accompany them.

Three models

With all the information gathered so far, three models have been proposed about planetary formation. The most widely accepted is the accretion theory, which works well for rocky planets like Earth, but not as well for gas giants like Jupiter and the other outer planets.

The second model is a variant of the previous one. This states that rocks form first, which are gravitationally attracted to each other, accelerating planetary formation.

Finally, the third model is based on the instability of the disk, and it is the one that best explains the formation of the gas giants.

The nuclear accretion model and rocky planets

With the birth of the Sun, the remaining material began to clump together. Larger clusters formed and light elements such as helium and hydrogen were swept away by the solar wind to regions further away from the center.

In this way, the heavier elements and compounds, such as metals and silicates, could give rise to the rocky planets close to the Sun. Subsequently, a geochemical differentiation process was launched and the various layers of the Earth were formed.

On the other hand, it is known that the influence of the solar wind decays with distance. Away from the Sun the gases formed by light elements can gather. At these distances, freezing temperatures promote the condensation of water and methane molecules, giving rise to gaseous planets.

Astronomers claim that there is a border, called the "ice line" between Mars and Jupiter, along the asteroid belt. There the frequency of collisions was lower, but the high rate of condensation gave rise to planetesimals of much larger size.

In this way the giant planets were created, in a process that curiously took less time than that of the formation of rocky planets.

The accretion theory and exoplanets

With the discovery of exoplanets and the information gathered about them, scientists are fairly certain that the accretion model is the main process of planetary formation.

It is because the model very adequately explains the formation of rocky planets like Earth. In spite of everything, a good part of the exoplanets discovered so far are of the gaseous type, of a size comparable to that of Jupiter or much larger.

The observations also indicate that gaseous planets predominate around stars with more heavy elements in their cores. On the other hand, rocky ones are formed around stars with light nuclei, and the Sun is one of these.

Figure 3. Artist's representation of the exoplanet Kepler 62f around its star, in the constellation Lyra. Source: Wikimedia Commons.

But in 2005, a rocky exoplanet was finally discovered orbiting a solar-type star. In a way this discovery, and others that followed, indicate that rocky planets are also relatively abundant.

For the study of exoplanets and their formation, in 2017 the European Space Agency launched the CHEOPS satellite (Characterizing ExOPlanets Satellite). The satellite uses a highly sensitive photometer to measure light from other star systems.

When a planet passes in front of its star, it experiences a reduction in brightness. By analyzing this light, the size can be known and whether it is gaseous or rocky giant planets such as Earth and Mars.

From observations in young systems, it will be possible to understand how accretion occurs in planetary formation.

References

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5. Taylor, N. How Did the Solar System Form? Recovered from: space.com.
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