The planets of the Solar System

The planets of the Solar System: structure, characteristics, size and names of the nine planets, terrestrial or Jupiter, that revolve around the Sun

What you will learn

  • The structure of the Solar System and the mathematical laws that govern it
  • The characteristics of the planets of the Solar System


  1. Introduction to the planets of the Solar System
  2. The planets of the Solar System
  3. The laws of Kepler and Newton
  4. The classification of the planets of the Solar System
  5. Mercury
  6. Venus
  7. Mars
  8. Jupiter
  9. Saturn
  10. Uranus
  11. Neptune
  12. Pluto
  13. Watch the video: How long do planets take to circle the sun?
  14. Key concepts


1Introduction to the planets of the Solar System

The planets of the Solar System have until very recently been considered an unexplored and unknown world, unattainable . In reality today, thanks to the images provided by numerous Russian and American space probes, we have been able to understand what is hidden in the “sky” in addition to the Sun and the Moon .

2The planets of the Solar System

Illustration of the planets of the Solar System – Source: Shutterstock

The celestial bodies that form the Solar SystemThe Solar System is made up of the Sun and various celestial bodies : the planets , their satellites, a thousand comets , an indefinite quantity of meteorites and 1,800 small planets called asteroids.
The planets in the solar system are nine , are all characterized by an almost spherical shape and by a light reflected from the sun . According to the increasing distance from the latter, they are: Mercury , Venus , Earth , Mars , Jupiter , Saturn ,Uranus , Neptune and Pluto .

The movements of the planetsThe planets are therefore of celestial bodies characterized by a cold outer surface , by significantly smaller than those of the Stars and by a motion of revolution around the sun . It is thanks to this movement that we can see them change position in the sky with respect to the fixed stars.
With the exception of Pluto and Mercury , when the planets move around the Sun they make elliptical orbits in which one of the two foci is occupied by the Sun. For greater precision it is important to remember that, in addition to the motion of revolution, there is the motion of rotation , a movement in which theplanet rotates on itself around an axis of rotation. Both motions of the planets proceed counterclockwise from West to East.
The time taken by the planet to rotate on itself is defined as day and has a variable duration depending on the celestial body concerned: if the movement affects the Earth, the day lasts 24 hours. , if it affects Jupiter it lasts less than 10 hours while on Venus it lasts 243 Earth days.

3The laws of Kepler and Newton

Kepler’s laws on the motion of revolutionThe planets are characterized by two types of movement : a motion around the planet itself said rotational motion and a motion around the Sun said revolution motion .
The motion of revolution of the planets is described by Kepler’s three laws :

  • 1st law of Keplerthe planets move around the sun in elliptical orbits. The Sun occupies one of the two foci of the ellipse. When the Earth is closest to the Sun it is in perihelion, when it is further away it is in aphelion.
  • 2nd law of Keplereach planet moves on its orbit in such a way that the vector ray that connects it to the Sun sweeps equal areas in equal times.
  • Kepler’s 3rd lawthe ratio between the square of the revolution times of the planets and the cube of their average distance from the Sun is constant.



Kepler’s Laws


The law of universal gravitationHowever, Kepler’s three laws failed to explain the existence of an elliptical orbit in which the planets moved, but a further law was needed, known as the law of universal gravitation , formulated by Newton .
This law explains that between two bodies there is a force of attraction ( F ) directly proportional to the product of the masses ( m, m ‘ ) and inversely proportional to the square of their distance ( r ). The relation that expresses Newton’s law is:

where G is the universal gravitational constant equal to 6.67 ∙ 10-11 Nm2 kg-2.

The force of gravityThe force of gravity is in general a weaker force than the electromagnetic force but it is quite important if the mass of one or more bodies in question is large enough, as in the case of the planets . The planets and the Sun are characterized by a reciprocal force and they move, attracted by the planets present, around a common center of gravity which is very close to the center of the Sun. The elliptical orbits described are therefore perturbed and cannot be described by the three laws of Kepler.


On January 3, the Earth reaches its perihelion, while on July 4, the aphelion.

4The classification of the planets of the Solar System

Terrestrial-type planets and Jupiter-type planetsThe solar system planets are divided into two categories : the planets Type Earth and planets type Jovian .

  • The terrestrial planetsare characterized by a density 5 times higher than that of water , by an extremely rarefied atmosphere and are mainly made up of rocky materials, metals and minimal quantities of gas. Earth- like planets such as Mercury , Venus , Earth and Mars belong to this category .
  • The Jupiter-type planets, on the other hand, are characterized by a density 1.2 times higher than that of water and by high percentages of Hydrogen, Helium and ” ice “. The planets of the Jupiter type are distinguished from those of the terrestrial type by the presence of a very dense atmosphere, consisting mainly of Hydrogen, Helium, Methane and Ammonia.
    The parameters that contribute to the presence of such a dense atmosphere are two: the mass of the Jupiter planets and their distance from the Sun. The first affects the atmosphere by retaining atmospheric gas molecules while the second allows young planets to maintain a low temperatureand avoiding the gas particles a thermal agitation such as to allow an escape velocity . Jupiter- like planets such as Jupiter, Saturn , Uranus and Neptune belong to the Jupiter type category .

A further classificationA further classification of the planets based on their position relative to the asteroid zone classifies them into inner planets ( Mercury, Venus, Earth and Mars ) and outer planets ( Jupiter, Saturn, Uranus and Neptune ).


Photo of the planet Mercury – Source: Getty-Images

The smallest terrestrial planetMercury , the smallest terrestrial planet and the closest to the Sun, has a higher density than the other planets and is characterized by a rotation period of 59 terrestrial days and a revolution of 88.97 terrestrial days . The temperature of Mercury in the areas hit by the sun’s rays reaches about 700 K due to the enormous proximity to the Sun and the slowness of the rotational motion.

A high thermal excursionThe smallest planet in the Solar System is devoid of water , with a metallic core similar to that of the Earth and with an almost non-existent atmosphere: this last feature allows for greater heat absorption and a marked thermal excursion . A peculiar element of Mercury is the presence of an unaltered original morphology and the total absence of volcanic activity.
Studies carried out to date on the planet Mercury hypothesize its similarity to the Moon .


The transit of Venus on the solar disk – Source: Loop

An Earth-like planetVenus is the only planet with Earth-like characteristics and the only planet in the Solar System to be easily identifiable , just before sunrise and immediately after sunset, due to its brightness and its position next to the Sun.
Venus is considered a very hot planet so that its average daytime temperature is around 720 K due to the presence of carbon dioxide in the atmosphere which generates a real greenhouse effect .


In the past, moreover, astronomers, observing Venus both in the morning and in the evening, came to the conclusion that there were two different planets identified with the names of Lucifer and Stella del Vespero.

The components of the planet VenusThe very dense atmosphere that characterizes Venus is constituted, besides high percentage of carbon dioxide (97%), also from hydrochloric acid , hydrofluoric acid , sulfuric acid in small percentages and water . The high temperatures, the high atmospheric pressure and the type of composition of the air present have led to the conclusion that Venus is a planet unsuitable for any living form . Some recent studies have also made known the presence on the surface of plateaus, of several craters resulting from probable impacts with meteorites and the presence of structures similar to volcanoes to indicate a remote volcanic activity.


In France Venus was called L’Etoile du Berger or The Shepherd’s Star because it indicated to the shepherds the time to return with the flock.


A suggestive image of the planet Mars taken by the Hubble Space Telescope – Source: Loop

The red planetMars is a planet belonging to the Solar System , slightly smaller than Earth and visible to our eyes as a reddish-colored body .
The atmosphere that characterizes Mars is very thin , consists of high percentages of carbon dioxide and small quantities of water vapor and is often involved in large dust storms in which strong winds, with speeds up to 100 km / h, erode the surface of the planet by changing it. The dust that is raised on these occasions and that remains suspended in the air is responsible for the typical reddish color.

The climatic characteristicsThe sandstorms mainly characterize the autumn and winter months. Mars too , like the Earth, is characterized by an alternation of (Martian) seasons with lower durations and temperatures than terrestrial ones: spring and summer are very short and hot in the southern hemisphere while autumn and winter are longer and definitely more rigid.


Learn more

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Is there life on Mars?The “red” planet and consequently its surface have been the subject of study for some time . From numerous analyzes it had emerged that on Mars in the past there were a river network, a strong volcanic and tectonic activity and deep craters caused by impacts with meteorites. Precisely on the basis of these data, Mars was initially considered a ” hospitable planet ” but this hypothesis was definitively denied by the data provided by particular probes: no activity connected to any living form could be present on Mars .


Small vehicles capable of exploring the surface have been brought to Mars.


Photo of the planet Jupiter – Source: Loop

The structure of Jupiter, the largest planet in the Solar SystemThe largest planet in the Solar System is Jupiter . Jupiter is characterized by a mass 2.5 times higher than that of all nine planets of the Solar System and has very different characteristics from the Earth but very similar to the Sun.
The “big planet” is formed by a gaseous outer layer , consisting mainly from Hydrogen and Helium, to a lesser extent from methane, ammonia and water vapor, and from a compact inner layer in which metallic hydrogen is present at a pressure of 3 million atmospheres.

The atmosphere of JupiterThe atmosphere, the outermost layer, has a particular structure of light and dark colored bands in which there are irregular spots and swirls. The best known atmospheric structure, present below the equator, is ” the great red spot “, a vast perturbation comparable to a cyclone that affects a larger area of ​​the earth’s surface.

The duration of the Jovian days and yearsA substantial difference with the Earth is the length of the clearly different “Jovian” days and years : a year on Jupiter , in fact, corresponds to 12 terrestrial years and a day lasts just over 10 hours.


Learn more

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The satellites of JupiterThanks to the use of special probes, the main characteristics of Jupiter have been brought to light , revealing the existence of a thin ring , enormous quantities of energy emitted in the form of infrared radiation and the presence of numerous satellites.
The main satellites , called Galilean satellites , are Io , Europa , Ganymede and Callisto .
Io is similar in size to the Moon, has a rocky composition and is characterized by a conspicuous presence of sulfur and intense volcanic activity. Europeit also has the dimensions similar to the Moon and presents a rocky composition, while Ganymede and Callisto are the planets with greater dimensions and with less density.


Photo of the planet Saturn – Source: Getty-Images

The characteristics of SaturnSaturn is a planet of the Solar System characterized by an important mass , about 95 times larger than that of the Earth, and a very large surface .
Saturn is able to radiate more energy than it receives from the Sun , is mainly made up of Hydrogen and is surrounded by a band of 7 rings of solid substances present around the equatorial plane.
A characteristic that distinguishes Saturn is that, despite having a rather high rotation speed, it takes almost 30 years to complete a full circle around the Sun.


The densest planet in the Solar System is Earth while the least dense planet is Saturn.


Photo of the planet Uranus – Source: Getty-Images

The characteristics of Uranus, the turquoise planetUranus is a Jupiter planet characterized by a high medium density and an atmosphere rich in Hydrogen , Helium and Methane . It is precisely the presence of methane that gives the planet a soft turquoise color .
The planet Uranus consists of three layers : a more superficial layer in which molecular hydrogen is present, an intermediate layer containing water and metals in the liquid phase and an inner layer characterized by a rocky core.
The turquoise planet also has an axis of rotation which lies on the plane of the orbit around the Sun and a magnetic axis inclined by 60 ° with respect to the axis of rotation.


The Voyager 2 probe reached the planet Uranus in 1986.


Photo of the planet Neptune – Source: Getty-Images

Neptune, a partly unknown planetNeptune is a planet of the Solar System in part still unknown due to the distance that does not allow a clear vision. It differs from other planets for an atmosphere in which hydrogen, helium and methane are present , a band structure and a system of rings around the planet itself.
The discovery of Neptune dates back to 1846 when, based on Newton’s Law, it was understood that the perturbations of the movement of Uranus were caused by the gravitational attraction of an external body.
The information available to us on Neptune is not yet sufficient to allow a more in-depth study of the planet .


Pluto in an image published on NASA’s Twitter profile – Source: Ansa

The smallest and coldest planet in the Solar SystemPluto was discovered in 1930 and is presented as the planet of the Solar System characterized by the slowest motion of revolution due to the enormous distance from the Earth.
It is the only planet in the Solar System not yet reached by a space probe and it is the only celestial body that, due to the eccentricity of its orbit , alternates periods in which it is closer than Neptune to the Sun with others in which is located at the far end of the Solar System .
Pluto , the smallest planet on Earth , is considered “ cold”: Its average temperature is close to 53 K due to the high distance from the Sun and consequently to the impossibility of the latter to heat it.
Often, due to low temperatures, Pluto is considered as a mass of ice and associated with a satellite, Charon , which has a rotation speed equal to that of Pluto itself.


According to the information provided by the space probes, Pluto cannot be considered either a terrestrial planet or a Jupiter type.

13Watch the video: How long do planets take to circle the sun?

Key concepts

·                                 The solar system

The Solar System consists of 9 planets , satellites, comets, meteorites and asteroids.
The planets of the Solar System have:

  • almost spherical shape
  • light reflected from the Sun
  • cold outer surface
  • dimensions smaller than the Stars

The planets are characterized by two motions:

  • Revolution motion around the Sun
  • Rotational motion in which the planetrotates on itself around the Sun

The planets move around the Sun in elliptical orbits.
The time it takes for the planet to rotate on itself is called a day.

·                                 The laws of Kepler and Newton

Revolution motion of the planets described by Kepler’s three laws:

  1. The planetsmove around the Sun in elliptical orbits. The Sun occupies one of the two foci of the ellipse.
  2. Each planetmoves on its orbit in such a way that the vector ray that connects it to the Sun sweeps equal areas in equal times.
  3. The ratio between the square of the revolution times of the planetsand the cube of their average distance from the Sun is constant.

Universal law of gravitation (Newton):
between two bodies there is a force of attraction (F) directly proportional to the product of the masses (m, m ‘) and inversely proportional to the square of their distance (r).

·                                 The classification of the planets of the Solar System

The planets of the Solar System are classified into :

  • terrestrial planets: rarefied atmosphere, density 5 times higher than water density, made up of rock materials, metals and gas. They belong to this category: Mercury, Venus, Earth and Mars
  • Jupiter-type planets: very dense atmosphere, density 1.2 times higher than that of water. They belong to this category: Jupiter, Saturn, Uranus, Neptune
  • inner planetsMercury, Venus, Earth, Mars
  • outer planetsJupiter, Saturn, Uranus, Neptune

·                                 The terrestrial planets


  • Smallest terrestrial planet
  • Planet closest to the Sun
  • Temperature around 700 K due to the enormous proximity to the Sun
  • Atmosphere non-existent
  • Free of water
  • Metallic core
  • Similarity to the Moon


  • Earth-like planet
  • Clearly visible before sunrise and after sunset
  • Dense atmosphere
  • Inhospitable planet dueto high temperatures, high pressures and air composition
  • Presence of craters, plateaus and similar volcanoes


  • Slightly smaller than Earth
  • Visible to our eyes as a reddish body
  • Rarefied atmosphere
  • There are big dust storms
  • Presence of Martian seasons
  • Considered an inhospitable planet

·                                 Jupiter-type planets


  • Largest planet in the Solar System
  • It consists of a gaseous outer layer and a compact inner layer
  • It has characteristics similar to the Sun and different from the Earth
  • The atmosphere features patches and irregular swirls
  • There are 4 main satellites: Io, Europa, Ganymede, Callisto


  • Important mass and extensive surface
  • Made up of Hydrogen
  • Present band with 7 rings around the equatorial plane
  • It takes 30 years for a full circle around the sun


  • Atmosphere rich in hydrogen
  • Three layers: one superficial (molecular hydrogen), one intermediate (water and metals in the liquid phase) and one internal (rocky core)
  • Axis of rotation lies on the plane of the orbit around the Sun.
  • Magnetic axis inclined by 60 ° with respect to the earth’s one


  • Partly unknown due to its remoteness
  • Band structure
  • Ring system around
  • Atmosphere consisting of Hydrogen, Helium and Methane


  • Slow revolution motion
  • Planet distant from Earth
  • The only planet not reached by a space probe
  • Eccentricity orbit
  • Cold Planet(- 53 K)

Questions answers

  • How many planets are there in the solar system? 

There are 8 planets in the solar system

  • What is the largest planet in the solar system? 

Jupiter is the largest planet in the Solar System

  • How many dwarf planets are there in the solar system?
  1. Ceres, Pluto, Haumea, Makemake and Eris.
  • What are the inner planets?

Mercury, Venus, Earth and Mars.

  • What are the coldest planets in the solar system?

Pluto, Neptune, Uranus, Jupiter and Saturn.


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