Unit 1:  Our place in the Universe                                          1   2   3   4  

4.2. The Solar System

4.2.1. Components of the Solar System

The Solar System is made up of the Sun, eight planets and smaller bodies

(dwarf planets, satellites, asteroids and comets)

Animation: The Solar System (National Geographic)

a) The Sun

It is a medium-size yellow star of G2 type.

It is composed by around 71% of Hydrogen, 27% of Helium and 2% of other elements (Carbon, Oxygen, Silica, Iron, Nitrogen, etc.)

Its nucleus reaches 15.10⁶ ºC of temperature, and its surface 6,000ºC.

It is located at 150 .106 km from the Earth. This distance is called astronomic unit.

b) The Planets

They are celestial bodies which revolve around the Sun and which mass is big enough to be spherical-shaped and have cleared the neighbourhood of their orbits.

According to their characteristics and location we can distinguish between:

- Inner or terrestrial (rocky) planets

They are Mercury, Venus, Earth and Mars.

They are the closest ones to the Sun.

They are small. Their surface is rocky and their atmosphere thin and slight.

They have few satellites or none.

- Outer planets or gas giants

They are Jupiter, Saturn, Uranus andNeptune.

They are the furthest from the Sun.

They are big. They have a rocky nucleus and a thick and dense atmosphere (liquid or gaseous)

All of them have a very large number of satellites.

Animation: Characteristics of the planets

c) Dwarf planets

They are celestial bodies that revolve around the Sun and have enough size to adopt a quasi-spherical shape, but that have not cleared their orbit of other celestial bodies.

In this kind are included a large number of astros located beyond the Neptune’s orbit, such as Pluto and some bodies located in the asteroid belt, such as Eris and Ceres.

d) Satellites

They are celestial bodies revolving around planets. Except Mercury and Venus,

all the planets have satellites.

They can be:

-  Regular-shaped. They are spherical and big. They formed at the same

  time that the planet. (E.g. The Moon)

-  Irregular-shaped. They are not spherical and are small. They seem to be

  asteroids trapped by the gravitational field of the planet. (E.g. Phobos)

e) Minor bodies of the Solar System

They are all the bodies that revolve around the Sun and are not planets, dwarf planets or satellites.

- Asteroids

They are small rocky bodies mainly irregular-shaped.

Most are in the Asteroid Belt, between the orbits of Mars and Jupiter, or in the Kuiper Belt, located beyond the orbit of Neptune.

Other important groups of asteroids are the Trojans, located in the orbit of Jupiter and Centaurs, in the orbit of Saturn.

When they fall towards the Earth they become incandescent due to the friction with the atmosphere. If they are small, they disintegrate before reach the ground (shooting stars).  But sometimes they are big enough and crash with it (meteorites).

- Comets

They are made up of ice and dust.

They describe very eccentric and irregular orbits around the Sun.

They come from the Oort cloud, an exterior zone of the Solar System where the remains of the nebula that originates it, are accumulated.

4.2.2. The origin of the Solar System

The Solar System has a series of features which any theory that try to explain its origin has to consider: 

  - The Sun, the planets and the satellites (except few of them) rotate in anticlockwise.

  - Planetary orbits are elliptical-shaped (with very low eccentricity)

  - Planetary distances from the Sun follow a fixed increase proportion (Titius-Bode Law)

  - All orbits are in the same plane (ecliptic plane) that coincides with the Sun equator.

  - The inner planets are small and dense and the outer ones big and slight.

  - All rocky bodies (planets, satellites and asteroids) have impact craters.

The current theory about the origin of our planetary system is the Nebular theory.

It can be summarise in this way:

1^st) Initial nebula collapse

Around 4,600 million years ago, a nebula compressed and collapsed. The reason seems to be the expansive explosion wave of a nearby supernova.

2^nd) Proto-sun formation

As dust and gas displaced, gravity started to act over them. As a result they began to revolve around the nebula centre. Particles were concentring in the central part and as they became more and more close together the number of collisions increased more and more.

These collisions provoked the progressive increment of the temperature in the centre of the cloud forming a proto-star (proto-sun). When the values were high enough the thermonuclear reactions started and the Sun began to glow.

3^rd) Proto-planets formation

The remains of the matter disposed in bands that turned around the Proto-sun.

 - The closest ones to the star contained heavier elements that came from the cosmic dust and they became the rocky planets.

 - The furthest bands to the Sun contained slighter elements that came from the interstellar gas and they became the gaseous planets.

Within each band, particles collided and joined together due to the gravity. As they grew in size, the number of collisions increased and the attraction force among them also.

The heat provoked by crashes melt particles and fused them. In this way, progressively bigger and bigger bodies were formed, the planetesimals. This process is known as accretion. Protoplanets were formed by accretion of planetesimals.

Eventually, these protoplanets cleared totally their respective orbits attracting all the small bodies that existed on them. So they became planets.

4^th) Formation of other celestial bodies

After sun and planets formation, there was still free matter. Part of it formed the satellites (from planetesimals that remained orbiting around the planets), asteroids (from planetesimals that did not became a planet), dwarf planets (from protoplanets that could not clear their orbits) and comets (from nebula remains that kept too far away of the gravitational attraction of the Sun)

4.2.3. Formation of the Earth

According to the Nebular theory, the Earth’s formation would follow this process:

1^st) Formation of the protoplanet Earth.

Within the nebular disc that surrounded the protosun, the accretion of planetesimals gave birth to the protoplanet Earth.

As it increased its size, also increased its gravitational field, what made easier the accretion of new planetesimals.

Due to its position in the Solar System, the most abundant planetesimals in the zone occupied by the young Earth would be those rich in iron and silica, although there would have others with lighter elements.

The constant collisions provoked the increment of the temperature. This kept the protoplanet at less partially melted.

2^nd) Differentiation by density

The melted state of the protoplanet allowed a distribution of its components, according to its density.

Iron sank until the deepest zones, forming the Core. This process is known like the “Iron catastrophe”.

At the same time, the gases of the interior of the planet escaped (degasification) and some of them formed the atmosphere. Among these gases, water vapour was very abundant.

3^rd) Surface cooling and formation of hydrosphere

Once the planetesimal shelling ceased, the Earth began to cool down. The decrement of the temperature of the surface rocks allowed that the water vapour condensed. Liquid water was progressively accumulating in the most depressed zones until 4,200 million years ago, the oceans were formed.

4.2.4. The origin of the Moon

Classical theories state that The Moon could be formed at the same time that the Earth and following a parallel process.

If they were correct, the Moon would have the same age than the Earth and because of its position in the Solar System the same density. However, the Moon is 100 million years younger than the Earth and its density is lesser (3.3 g/cm³)

Today, the most accepted theory proposes that during the first moments of the existence of the Earth, a rocky planet similar in size to Mars, collided with the Earth.

Part of the matter of this planet joined with the wretched materials of the Earth formed a remainders cloud that started to revolve around the Earth. The accretion of these materials would be the origin to the Moon.

The terrestrial materials which participated in the Moon’s formation were those of the Crust and the Mantle. These layers are made up of lighter materials than the Core. This explain that the density of the Moon were so low.

4.3. Answer these questions:

a. What are the differences between an asteroid and a comet?

b. Explain what the accretion of planetesimals is.

c. What is the “iron catastrophe”?

d. How was the Moon formed?