The Earth in the Universe

The Universe and Human Beings

Human beings have been watching the Universe since ancient times. At first, people looked at the stars and constellations to guide themselves or to predict the seasons. 

First theories about the origin and structure of the Universe were simply speculations and myths, without any scientific support. The Ptolemaic Model, also called Geocentric Model, was the most accepted theory for centuries. According to this theory, the Earth was in the centre of the Universe and the Sun, the Moon and the rest of celestial bodies orbited around it. It is logical because, after all, we currently know that the Earth is moving, but we don't perceive it. And this model was useful for more than 1500 years.

But the model is not correct and new observations revealed some mistakes, many astronomical phenomena could not be explained by this theory, so it was replaced by the Heliocentric Model. In this model the Sun is in the centre of the Universe and the Earth and planets revolve around it.

Geocentric Model.

Nowadays we know that the Earth and the planets of the Solar System revolve around the Sun, although neither the Sun nor the Earth are in the centre of the Universe. The Sun is our closest star, and we live in a planet located in a Galaxy called the Milky Way.

Heliocentric Model.

Currently, the most accepted theory that explains the origin of the Universe is called “The Big Bang Theory". According to this theory, the whole Universe was concentred at a single point (singularity). 13.8 billion years ago, this point started standing, leading to the formation of the stars, planets and other celestial bodies. Galaxies are still moving, the Universe is expanding and the distance between galaxies is getting bigger and bigger throughout the time.

Measuring the Universe

The Universe is the space and time where we live. And all the things that we see, touch, feel or perceive are, in fact, a component of the Universe. This means that it groups all the things that exist, have existed or will exist in the future. It is a vast place, probably infinite. Only the part that we can see or study using telescopes, called the observable universe, is so large that light would take 91 billion years to cross it.

In such a  huge place, the units used to measure distances on Earth are not enough. The unit for distance in the International System is the metre, that is a good unit if we want to measure how tall we are or how far is the school from our house. For bigger distances on Earth, we usually use another unit, the kilometre. We can measure in kilometres the distance between two towns, or even the radius (6371km) or the circumference (around 40000km) of our planets.

But when we try to study the Universe, kilometres lose their meaning. The main issue is that the distances in the Universe are enormous and many people tend to think that the diagrams of the solar system that they can see in our books have something to do with reality. But they are not real, the schemes and drawings are a simply approach. If the Sun was the size of a basketball, the Earth would be a small 2mm diameter ball (more or less, the size of one of these letters). And if we put the basketball in the middle of a football pitch, the 2mm sized Earth would be orbiting beyond the goal. Neptune, the furthest planet in the Solar System would be orbiting more than 2.5km from the Sun.

The Moon is the Earth satellite and its closest celestial body It is 384400km from the Earth. It is a big number. The Sun, our closest star, is 270000000000000000km from us. That is, in fact, a huge number. But, as we have seen, it is really small if we compare it with the distance between the Sun and other celestial bodies, such as far extrasolar planets or other stars. We need other units to measure these kind of distances. 

Solar System

Astronomical Units (AU).

The Solar System is made up of the Sun and the celestial bodies orbiting around it. To measure distances between the Solar System elements the most common unit used by astronomers is the Astronomical Unit (AU).

We define Astronomical Unit as the average distance between the Earth and the Sun. So the Earth is 1AU from the Sun. Mars is 1.5AU from the Sun. Jupiter is 5.2AU from the Sun.

Galaxy

Light-Years.

Astronomical Units are still too small to measure distances between stars. So astronomers use another more appropriate unit, called the light-year (ly).

We define a light-year as the distance that light can travel in a year. Light travels at 300000km per second in a vacuum, so the distance it travels in a year is really enormous, around 9460700000000km.

Proxima Centauri, the closest star to the Sun, is about 4.25 light-years from us.

Parsec.

Currently astronomers tend to use another unit, called parsec (pc), to measure stellar distances. Parsecs are even bigger than light-years. 1 parsec is equal to 3.26 light years.

Components of the Universe

What is the Universe made up of? Basically, there are two main components, matter and energy.

We can perceive energy and its effects: light, radiations, etc.

Matter can form different structures called Astronomical Objects. They can be classified according to their size and physical properties. The following are the most important astronomical objects:

• Stars.
• Planets.
• Dwarf Planets.
• Satellites.
• Asteroids.
• Comets.
• Others.

Stars

Stars are massive luminous spherical bodies. In general, they are big and bright celestial corpus. Their main chemical components are hydrogen and helium. They are so enormous, and  have such an enormous mass, that the hydrogen and helium in their core r

The Sun.

eact (in nuclear fusion reactions) releasing huge amounts of energy.

The emitted energy is merely electromagnetic radiation. Visible light is a kind of electromagnetic radiation that we can perceive with our eyes. Other important radiations are the infrared rays, that transmit heat. Some radiations emitted by stars are extremely deleterious for living beings, but luckily they do not reach the Earth in significant quantities; X-rays, Gamma Rays and Ultraviolet rays are three typical examples.

Stars are not solid structures, they are frequently a plasma of gases condensed by gravitational forces. These gases are usually at extremely high temperatures, due to the nuclear reactions that take place in them. And, as we have said, the main chemical components are hydrogen and helium, although other heavier elements can also be found in them, above all in the inner core.

Stars are formed when clouds of gases and dust are pulled together by gravitational attraction. As a result, all the stars have a big cloud of non-collapsed materials surrounding them called Nebula. 

Nebula

During the first phases of formation, nebula are bigger, and some outer components of this surrounding materials join, forming other celestial bodies that will orbit the stars, such as planets, satellites or asteroids.

The group of a stars with all the celestial bodies orbiting it is called Solar System. The Sun is the star of our Solar System.

Stars are classified according to their size and temperature. These two characteristics are related to their colour. The Sun is a yellow star.

Stars are not static structures, they evolve and change and some types of stars change into other when the nuclear reactions consume some components. At the end of their life, some stars can transform into other celestial bodies, such as Supernovas, Neutron Stars or Black Holes.

Stars sometimes group to form star clusters, that are groups of hundreds or even thousands of stars. Galaxy clusters sometimes group forming galaxies. Galaxies can have millions of stars.

Our Solar System is in one of the arms of a spiral galaxy called the Milky Way.

The Sun is the nearest star to Planet Earth. It provides us with light and heat, and it is absolutely essential to support life.

The Sun is in the centre of the Solar System. It is much bigger than the rest of celestial bodies that orbit it. In fact, Sun contains 99% of the total mass of the Solar System.

Planets

Planets are spherical bodies orbiting a star across a clean orbit, what it means is that no other celestial body share its orbit. Planets are held to their star by gravitational forces.

Planets are always smaller than stars and never emit electromagnetic radiation (nor light or heat), because they are not large enough to support nuclear reactions. They are, however, bigger than other celestial bodies like comets or asteroids.

Planet Movements

Planets have two movements: rotation and revolution, also called orbit.

Rotation is the spinning movement of the planet around its axis. The period a planet takes to complete a rotation is known as a day.

Revolution is the movement of a planet around a star. The time a planet takes to complete a revolution around its star is known as a year.

All the planets in the Solar System revolve around the Sun in the same direction: anti-clockwise as seen from the Sun northern pole. All the planets in the Solar System but Venus and Uranus rotate in an anti-clockwise direction.

There are two kind of planets, rocky and gaseous planets. Rocky planets are smaller and denser, whereas gaseous planets are bigger and less dense. Inner planets in the Solar System (closes to the Sun) are rocky planets, outer planets are gaseous planets.

Planets in the Solar System

There are eight planets in the Solar System. The four closest planets, Mercury, Venus, Earth and Mars are rocky planets. The next four planets, Jupiter, Saturn, Uranus and Neptune are gaseous planets.

Mercury

Mercury is the closest planet to the Sun. It is also the smallest planet in the Solar System.

It doesn't have atmosphere and its rotation period is only a bit longer than its revolution, in other words a day is nearly as long as a year. Due to this, one of the sides faces the Sun for a long time. The side of the planet that faces the Sun is very hot and the other side, however, is very cold because the lacking of atmosphere provokes that it loses a lot of heat.

Mercury.

Venus

Venus is the second planet of the Solar System. It is bigger than Mercury, but a bit smaller than the Earth. It rotates in clockwise direction. Its rotation period is longer than its revolution, in other words its day is longer than its year.

Venus has a dense atmosphere, rich in carbon dioxide. Although it is further from the Sun than Mercury, it is the hottest Planet of the Solar System due to the greenhouse effect provoked by its atmosphere. This is the closest planet to the Earth and it is the brightest astronomic object in the night sky after the Moon.

Venus.

Earth

Planet Earth is the third planet in the Solar System. Its distance from the Sun and the density of its atmosphere, rich in nitrogen and oxygen, provide an average temperature which ranges from -90°C to 60°C (depending on the place). This temperature allows that about 71% of its surface is covered by liquid water. Probably due to these two factors it is the only known planet that support life. Planet Earth has one satellite, the Moon.

Earth

Mars

It is also called the red planet, due to the colour of the iron oxide, that is very abundant in its surface. This planet has a thin atmosphere and an average temperature slightly lower than in the Earth, from -135°C to 35°C. Mars has two small satellites.

Mars

Jupiter

Jupiter is the largest planet in the Solar System, it has 2.5 times the mass of all the planets combined.

It is a giant gaseous planet with at least 67 satellites and a faint ring.

It has an extremely fast rotation, but it takes more than 11 earth years to complete a revolution. Its average temperature is -110°C in spite of its dense atmosphere.

Jupiter

Saturn

It is the second largest planet in the Solar System. It most well known characteristic is its prominent ring system, made of dust, rocks and small asteroids.

It also has 62 satellites with formal designation, although some asteroids of the ring system could be considered as small satellites.

Just like Jupiter, it rotates quite fast (its day is less than ten hours long). It has a long orbit, so it takes nearly 30 Earth year to complete one revolution around the Sun. It is a bit colder than Jupiter, its average temperature is -139°C.

Saturn

Uranus

Uranus is the only planet which rotation axis is almost horizontal. Its rotation is also inverse, it has clockwise rotation. It has 27 satellites and a faint ring in vertical position, perpendicular to its rotation axis.

It is the second furthest planet. This long distance, added to its atmospheric characteristics make this planet the coldest in the Solar System, its average temperature is -224°C.

Uranus takes more than 80 Earth years to complete a revolution around the Sun. 

Uranus

Neptune

Neptune is the farthest planet in the Solar System. It is really far, more than 30AU from the Sun.

Due to this, its average temperature is really low, around -210°C, although it is a bit higher than in Uranus.

It has 14 known satellites and also has a faint ring.

Neptune.

Dwarfs Planets

Dwarfs Planets are spherical bodies, smaller than planets, orbiting a star. They usually have strange, eccentric orbits. All of the known dwarfs planets are orbiting the Sun beyond Neptune.

Currently there are five recognised dwarfs Planets: Pluto, Ceres, Haumea, Makemake and Eris, although its estimated that there may be hundreds. The most famous one is Pluto (that was relegated to this category in 2006).

Pluto

Satellites

Satellites are spherical celestial bodies orbiting a planet.

All the planets of the Solar System but Mercury and Venus have satellites. The large gaseous planets have extensive satellite systems. Jupiter and Saturn have more than 60 satellites, for instance.

The two satellites that orbit Mars are very small. The Moon, however, is a big satellite, the bigger one in relation to its planet. In fact, only half of the satellites in the Solar System are comparable in size to the Earth’s Moon.

Calisto (Jupiter's moon)

Asteroids

Asteroids are celestial rocky bodies with irregular morphology. They are smaller than planets and dwarfs planets, although vary greatly in size.

The majority of known asteroids orbit between Mars and Jupiter. This huge group of asteroids orbiting together form an asteroid belt.

Another asteroid belt is known as the Kuiper Belt, that is made of asteroids orbiting beyond Neptune.

Asteroid (recreation).

Comets

Comets are celestial bodies that orbit the Sun in extremely elliptical orbits. Their nucleus is made up of a mass of gas, dust and ice. When they move close to the Sun, some of the ice evaporates, creating the tail of the comet (so when they are far away from the Sun, they do not have a tail).

Due to their eccentric elliptical orbits, comets have wide range of orbital periods.

Most of them were originated in the Kuiper Belt or in the Oort Cloud.

Comet.