Solar System Composition and Planet Formation

Composition of the Solar System

The Sun is the star of our planetary system. It has a medium size between a sphere of incandescent gases and other stars. It owes its energy to the thermonuclear reactions that occur in its nucleus, where it reaches a temperature of 15 million degrees. On its surface, it is 6,000 degrees Celsius. The sun revolves around its axis.

Planets are celestial bodies orbiting the Sun, whose mass is sufficiently large for them to be almost spherical and have cleared their orbits. They differ among their "inner" or terrestrial planets. These include Mercury, Venus, Earth, and Mars. They are near the Sun and have a small, rocky surface. The outer planets are Jupiter, Saturn, Uranus, and Neptune. They are further away from the Sun and have a large size, and are not rocky.

Dwarf Planets are celestial bodies orbiting the Sun and have a mass sufficiently large for them to adopt a nearly spherical shape, but have not swept their orbits. These are Ceres, Pluto, and Eris.

Satellites are celestial bodies that revolve around planets. The Moon is the satellite of Earth. Except for Mercury and Venus, all planets have satellites.

Minor Solar System Bodies are neither planets nor satellites. This includes:

• Asteroids: Irregularly shaped rocky bodies. The majority are generally found in the asteroid belt. Other important groups are Trojans and Centaurs.
• Comets: Small celestial bodies orbiting beyond Neptune, in the Kuiper Belt. They are constituted by ice and dust particles.

Planetesimal Theory

It is divided into 6 parts:

1. Initial Nebula: About 4,600 years ago, a rotating nebula of gas and dust, whose dimensions were superior to the solar system, began to contract.
2. Gravitational Collapse: The contraction or collapse formed a large central mass and a rotating disk around it.
3. Formation of the Protosun: The collision of particles in the central mass released a lot of heat. The nuclear fusion of hydrogen began. This marked the birth of a star, the protosun, inside the nebula.
4. Formation of Planetesimals: The dust and gas particles in the rotating disc formed around the protosun followed a process of grouping. This formed granules of a few millimeters, and larger bodies whose collisions and fusions originated between sizes with some hundreds of meters.
5. Formation of Protoplanets: The collisions of the planetesimals and their union originated primitive planets or protoplanets.
6. Orbit Sweeping: In the accretion process, each protoplanet was clearing its orbital zone of planetesimals.

Origin of the Moon

There are 3 hypotheses:

• Sister: The Moon was formed at the same time as the Earth, in its orbital zone and following a process parallel to our planet. The Moon is 100 million years younger than our planet, and being in the same area of the solar system, it would have formed from the same type of planetesimals, so their densities should be similar. However, this is not the case, as the density of the Earth is 5.5 g/cm³, and the Moon's is 3.3 g/cm³.