Planet Earth: Characteristics

Planet Earth

Planet Earth: Components and Characteristics.

The Earth is one of the inner planets in the Solar System. It is the biggest rocky planet, although it is much smaller than the gaseous planets. Its equatorial radius is 63718.1km long and its estimated mass is 5.792x1024kg.

It is located between Venus (its closest planet) and Mars, and it is, on average, 150000000km from the Sun. It has an atmosphere rich in nitrogen, oxygen, carbon dioxide and water vapour. 

The distance from the Sun and the atmospheric density of the planet lead to an average temperature of 15°C, from -90°C in the coldest regions to 60°C in the warmest ones, providing the optimal conditions to support life.

Planet Earth has three components:

• Atmosphere.
• Hydrosphere.
• Geosphere.
• Biosphere.

As we have just studied, the Earth’s atmosphere is the layer of gases that surrounds the planet and it is rich in nitrogen, oxygen, carbon dioxide and water vapour. Oxygen is very important for living beings, because they use it to respirate. Carbon dioxide is vital for plants and other autotrophic organisms, because it is necessary to carry out photosynthesis.

Atmosphere is, besides, essential to regulate the Earth’s temperature.

The hydrosphere is the layer of water that covers the Earth surface. The average temperature and its variations allow the presence of water in the three different states. 70% of Earth's surface is covered by liquid water, but we can also find significant amounts of solid water (ice) in the poles and some gaseous water in the atmosphere (water vapour).

The water of the planet can change from one state to another: liquid water can solidify when it is very cold forming solid water, or evaporate in hot places forming water vapour, for instance. This process is called water cycle.

The geosphere is the solid part of the planet. Its surface is mainly made of silicates. The most abundant component of the core is, however, iron. The movement of the iron in the core generates the Earth's magnetic field. This magnetic field is the main component of the Van Halen Belt, that protects the planet against dangerous radiation from the outer space.

Another important characteristic of the geosphere is the volcanic and seismic activity. It is related to the tectonic plates, that are dynamic components of the Earth's crust. The dynamism of the tectonic plates is responsible for changes in the Earth's surface, such as crust creation and destruction, mountain building, movement of continents and geological phenomena like volcanoes and eruptions, earthquakes or rifts.

Finally, the biosphere is made up of the living things of the planet.

Earth Movements

Planet Earth is not an static structure, it is moving in space. It has two basic movements: rotation and revolution.

Rotation is the spin movement of the Earth on its own axis.

Revolution also called orbit, is the movement of the planet around the Sun.

Rotation

The Earth spins on its axis. It rotates from the west to the East. The planet takes 24 hours to complete one rotation. When it is day on the half of the Earth facing the Sun, it is night on the opposite side.

The rotation axis is not perpendicular to the geographic equator, but is 23.5° tilted. Due to this, the incidence of the Solar rays that reach the planet is different in different regions of the planet and different moments of the year, depending on the part that is directly facing the Sun. This phenomena is related to the seasons and the duration of nights and days. 

On the one hand, when the inclination angle makes the northern hemisphere directly face the Sun, the temperatures in this hemisphere are higher and the days are longer. We are talking about the summer in the norther hemisphere. At the same time, the temperatures in the Southern Hemisphere are lower and the days are shorter. We are talking about the winter in the Southern Hemisphere. As we can see, when it is summer in the northern hemisphere it is winter in the Southern Hemisphere. 

When the inclination is maximum and the day is longest and the night shortest, we talk about summer solstice in the northern hemisphere and winter solstice in the Southern Hemisphere. It happens the 21st of June. 

On the other hand, when the inclination angle makes the Southern Hemisphere directly face the Sun, the temperature in this hemisphere is higher and the days are longer. We are talking about the summer in the southern hemisphere. At the same time, the temperatures in the northern Hemisphere are lower and the days are shorter. We are talking about the summer in the Southern Hemisphere and the winter in the northern Hemisphere.

When the inclination is maximum and the day is longest and the night shortest, we talk about summer solstice in the southern hemisphere and winter solstice in the northern Hemisphere. It happens on the 22nd of December. 

Summing up, during the summer in the northern hemisphere, this part of the Earth is exposed to more direct sunlight because this hemisphere faces the Sun. That is why the days are longer than nights. During the winter in the northern hemisphere, this part of the Earth is exposed to less direct sunlight because the rays reach the earth more transversely.

When the inclination angle is perpendicular to the incident rays day and night last just the same, twelve hours. This occurs in autumn and spring. We are talking about autumn or spring equinox. It happens on the 21st of March and the 22nd of September. 

Orbit (Revolution)

Orbit is the movement of the planet around the Sun. The Earth moves around the Sun in a plane called ecliptic plane. It takes 365.25 days to complete a revolution around the Sun.

This orbit is not circular, but elliptical, so the distance between the Sun and the Earth changes. The point where the Earth is closest to the Sun is called perihelion. It happens during the northern winter, the 3rd of January. The point where the Earth is furthest to the Sun is called aphelion. It happens during the northern summer, the 4th of July. 

Moon

The Moon is the Earth's natural satellite. It is much smaller than the Earth, although it is the fifth larger satellite in the Solar System, and the largest one in relation to the planet it orbits. Its equatorial radius is 1737.1km long (0.273 Earths) and it weighs 7.324x1022kg (0.012  Earths). Its atmosphere is extremely tenuous (we could nearly say that it doesn't has an atmosphere).

It is, by far, the closest relevant celestial body, and the brightest one at night.

The Moon has two movements, rotation on its axis and revolution around the Earth.

Rotation

The Moon rotates on its axis. It takes 28 eight days to complete a rotation, just the same it takes to complete a revolution around the Earth. As a result, the Moon always presents the same side to the Earth. This phenomena, called tidally locked rotation, is a consequence of the gravitational force of the Earth.

Revolution

The Moon revolves around the Earth, in an anticlockwise direction. As we have studied, it takes 28 days to complete a revolution.

Although the Moon always presents the same face to the Earth, we only can see the part of the Moon that is reflecting the sunlight. The shape of the illumination portion of the Moon is called Lunar Phase.

The lunar phase depends on the position of the Moon in relation to the Earth and the Sun. When the sunlight illuminates the whole surface that is facing the Earth it is called a full moon. When the sunlight illuminates the opposite side of the Moon and the portion of the Moon that faces the Earth is dark, it is called a new moon.  

Effects of the Moon on the Earth.

Tides

The gravitational attraction of the Moon has some relevant consequences on Earth. The most important one is the movement of sea water when attracted by the Moon, that causes tides. In the part of the Earth closest to the Moon, the sea level is slightly higher. In the opposite side of the Earth, the sea level is a also higher, because of the water movement from the lateral parts to the front. Tides are the manifestation of the sea water movement, following the Moon’s revolution. 

Tides mainly lead from the gravitational attraction of the water by the Moon, but also by the Sun. Due to this, tides are higher when the Moon and the Sun are aligned with the Earth. This fact occurs with the new Moon and the full moon.

 Eclipses

Eclipses occur when one celestial body projects its shadow against another celestial body, fading or obscuring it.

We can see two different types of eclipses from the Earth: Solar eclipses and lunar eclipses.

Solar eclipses occur when the moon passes in front of the Sun during the day, partially hiding it and projecting its shadow to the Earth. This happens when the moon crosses the ecliptic plane of the Earth during the day, being the three celestial bodies perfectly aligned. The moon's shadow crosses the Earth, obscuring a portion of the planet. 

Lunar eclipses occur when the moon crosses the ecliptic plane during the night, and it is, at the same time, perfectly aligned with the Sun. According to this, it only happens during a full moon. The Earth blocks the Sun, so it cannot illuminate the moon. Summing up, the earth's shadow obscures the moon.

Ríos: Curso Alto, Medio y Bajo.

En un río se puede distinguir un tramo alto, un tramo medio y un tramo bajo.

En el curso alto predomina la erosión y el transporte sobre la sedimentación. Es la zona de los torrentes, las pendientes fuertes, las velocidades de flujo altos y el río tiene una gran capacidad de transporte. 

La carga es menor que la capacidad, el río erosiona, se encaja, el río discurre por valles my estrechos en forma de V muy cerrada, muy estrecha.

Garganta del cares (por Javier Mendia García)

En el tramo medio y en el tramo bajo los valles son más anchos, sobre todo el el curso más bajo y el fondo está cubierto por sedimentos acumulados por el propio río, formando una llanura aluvial. Alternan la erosión, el transporte y la sedimentación, según el curso. Pero la erosión ya no es tan fuerte como en el curso alto y la sedimentación es mucho más importante.

La forma del perfil de los valles formados por ríos es típicamente en forma de V, más estrecha en tramos altos y más abierta en los tramos medio y bajo. Y son importantes los tipos de materiales que deba escavar. Si son muy competentes, el río formará gargantas o desfiladeros.

Existen algunos rasgos geomorfológicos asociados a los cursos fluviales. Así, en las zonas de montaña en las que hay barrancos, cuando alcanzan los puntos de menos inclinación, el río tiende a descargar toda su carga sedimentaria y se forman abanicos fluviales. Si en una franja de un borde montañoso se unen varios abanicos que se unen lateralmente, se forman los denominados piedemotes. Son típicos de zonas áridas. Están asociados a los ríos en sus cursos altos.

 

Abanicos fluviales (image de la NASA)

En los cursos medios y bajos el río tiende a circular por canales que va encajando en las llanuras aluviales. En las zonas de curso medio, predominan los canales anastomosados, que dejan en el medio islotes con arena, cantos, etc. Estos islotes van cambiado a cada crecida del río.

En las zonas bajas o zonas de poca pendiente se forman canales meandriformes. Como indicamos, son típicos de zonas con bajas pendientes y de cursos bajos de los ríos y en ellos el canal describe curvas muy marcadas. En su zona cóncava, el río tiende a escavar, a erosionar y en la zona convexa en cambio tiende a depositar sedimentos. Por este motivo los meandros tienden a ir haciéndose cada vez más acusados. Cuando el meandro es muy antiguo, es frecuente que el río acabe atajando y dejando una zona de cauce sin agua denominada meandro abandonado.

Estructuras de los ríos en el curso medio.

Cuando en una cascada la roca de arriba es más dura que la de abajo, la cascada tiende a retroceder, pero no a desaparecer.

Meandros (en Alaska)

Otra forma característica son las marmitas de gigante o pilancones. Se trata de pozos en los que entran cantos que giran por la acción de la corriente, provocando que el hueco se agrande. Es decir, el canto hace de taladradora. Se dan con más facilidad cuando el canto gira en un hueco que ha sido excavado en una zona especialmente blanda. 

Cascadas (por Klaus Eltrop)

De vez en cuando podemos encontrar un tipo especial de meandros denominados meandros encajados. Se dan cuando un río atraviesa una llanura aluvial formada por sedimentos del propio río. El río va profundizando hasta encontrar la roca dura que se sitúa debajo. Los meandros encajados se producen cuando el río alcanza esta roca dura y ésta le impide cambiar el cauce. Es decir, una vez llega a la roca dura, el meandro ya no puede cambiar o dibujarse y la roca dura de la parte inferior refleja la forma de los meandros. Por este motivo, los meandros que se forman son estables, fijos.

En muchas ocasiones las llanuras de inundación tienen un perfil convexo en lugar de un perfil cóncavo. Se debe a que su cauce natural es la llanura entera y el lugar por donde está discurriendo en este momento es el lecho menor. Es decir, el lugar por donde está circulando en este momento está rodeado por dos diques, los diques del lecho menor denominados malecones. Los malecones están formados por los materiales más gruesos que lleva el río.

Llanuras de perfil convexo.

En una crecida grande, el río se extiende por toda la llanura, pero los cantos pesados los deja en el borde del lecho menor, pues al desbordarse pierde energía, suelta materiales de gran tamaño en las orillas y lo que llevará en suspensión serán materiales más ligeros, como arenas, arcillas o limus. Por eso el borde tiende a formarse con orillas más elevadas.

 

Terraza fluvial

Todo esto provoca que, en ocasiones, esté más elevado el lecho por donde circula el agua que el resto de la llanura de inundación. La formación de malecones puede provocar, en ocasiones, que los afluentes se encuentren con los diques y que estos impidan al afluente unirse directamente con el río principal, circulando ambos paralelos hasta que se unan en un punto donde lo existan malecones o estos sean de menor altura.

También se puede dar, en ocasiones, la formación de capturas fluviales. Imaginemos que hay dos vertientes, una de ellas con más pendiente y con el nivel de base de los ríos próximo a ella, frente a otra pendiente más suave y que tiene más alejados los niveles de base. Si hay peor clima en la pendiente más inclinada y llueve más, en esta pendiente habrá también más erosión. Los ríos de la pendiente más suave pueden llegar a capturar la cabecera de los ríos de la otra pendiente, atrapando así el agua y llevándosela la zona de pendiente más hundida.

Captura fluvial.

Las terrazas fluviales son otra estructura que puede aparecer en algunos ríos. Los ríos, a lo largo de su historia, experimentan variaciones en la capacidad de erosión y sedimentación debidos a cambios climáticos o de nivel de base, cuando por ejemplo el nivel de base profundiza.  El río pasa una temporada en la que escarba con fuerza, seguido de otra temporada en la que deja de escarbar, a la que sucede otra etapa de excavación y así sucesivamente. Esto originará la formación de terrazas fluviales.

Cabe destacar que estas también pueden ser formadas debido a levantamientos isostáticos de la corteza terrestre.

Otra posibilidad es la formación de terrazas encajadas. Se forman cuando los episodios de erosión son más cortos que los episodios de la sedimentación. Es decir, en los ciclos erosión-sedimentación, predomina la sedimentación.

En cualquiera de los casos, en las terrazas fluviales la llanura más baja es la más reciente.

Terrazas fluviales y terrazas fluviales encajadas.

Al estudiar los ríos, resulta muy importante el estudio del perfil vertical de sedimentos. Los sedimentos depositados en horizontal combinan lentejuelas o lentejones de gravas y arenas entre arcillas. Corresponden a posiciones del lecho menor. Los sedimentos de los lentejones son los depositados por el cauce cuando cambia de posición. Las arcillas son depositadas en las subidas, durante las crecidas.

En los cursos altos, los depósitos que dejan dan lugar a los abanicos fluviales. Hay una mezcla de materiales de distinto tamaño, mezcla de rocas distintas y poco redondeamiento en los cantos, ya que el transporte es muy pequeño, muy corto. Se dice que los sedimentos están poco calibrados, por el tamaño y que presentan poca madurez, por lo poco redondeados que se encuentran.

Estos depósitos fluviales cuando son compactados se convierten en rocas sedimentarias. Son diferentes en diferentes niveles. Las rocas sedimentarias que generan los depósitos fluviales son brechas y conglomerados en los abanicos fluviales; en los abanicos aluviales tienden a ser solo conglomerados; así como conglomerados y areniscas con pizarras en los depósitos de las llanuras de inundación.

 

Delta de ebro (por Gons)

Esto no quiere decir que todos los conglomerados, areniscas o pizarras sean de origen fluvial. Lo que, sin embargo, es cierto, es que las calizas no son nunca de origen fluvial, siendo de origen marino la mayoría de ellas, o de origen lacustre otras.

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.