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There are eight planets in our solar system. Each of the planets circles (orbits) the Sun.  Most of the planetary bodies of the Solar System orbit the Sun in nearly the same plane, known as the plane of the solar system. This is likely due to the way in which the Solar System formed from a protoplanetary disk.  

In order of increasing distance from the Sun, the planets are Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU), Mars (1.5 AU), Jupiter (5.2 AU), Saturn (9.5 AU), Uranus (19.2 AU), and Neptune (30 AU).


You can see the correct order of the planets from the Sun (we've ignored relative distance) in Picture 1.

How was it all formed?

The amazing diversity expressed by the various planetary bodies and the structure of the Solar System itself, can be explained by the widely accepted origin theory: the nebula hypothesis. 

The theory suggests that the Solar System was formed from a huge rotating disc of hydrogen and helium gas, along with a smattering of lesser materials.  As the majority of the material was drawn to the centre of the disc it triggered nuclear reactions which resulted in the birth of a relatively bright, G-type star - The Sun.

The remaining 10% of material in the protoplanetary disc started to collide and stick together by a process called accretion. The rotation of the disc prevented all the material forming a single object.  The rocky material formed the inner terrestrial planets, where it was too hot for ices to accrete.  Further out from the Sun, where it wasn't so hot, ices formed, giving rise to the outer gas giant planets.

Solar system poster with planets and their names Elements of this image furnished by NASA.
Image by Andrew Russian



The four proximal planets are Mercury, Venus, Earth and Mars.

They are known as the Terrestrial (Earth-like) Planets, as they are formed of materials similar to those of Earth.

They have densities ranging from

3.9 - 5.5 g per cm cubed.

Image by NASA (Cassini)



The distal planets, Jupiter, Saturn, Uranus and Neptune are collectively described as the gas giants.

They have radii greater than 24 000 km.

Densities close to:

1 g per cm cubed.

They are predominantly or wholly fluid.

Astronomical Units (AU)

The Earth is one astronomical unit from the Sun. Therefore by definition 149.6 x 10^6 km



Absolute Temperature 

The SI Unit for absolute temperature is Kelvin (K).


0 K = -273°C

273 K = 0°C




Is the measure of mass per unit of a substance:

density =

mass (kg) / volume (m^3)

The SI Unit is therefore kilograms per cubic metre:

kg m^-3

Image by Neven Krcmarek



The rocks brought back by Apollo missions have been dated at 4400 Ma.

The Moon has a solid crust, mantle and core.  It lacks the volatile 

materials found on Earth.

The surface is made of the dark-coloured Maria: basalt lava flows and the lighter highlands formed of anorthosite.

Image by Federico Beccari

The Solar System describes the relationships between the Sun and the planets, satellites/moons, dwarf planets, comets and asteroids. 

The Sun is not a planet, it is a star. It's a huge ball of gas (mainly Hydrogen and Helium). At its centre nuclear reactions release huge amounts of energy.  The Sun is therefore very hot.

The temperature of the sun at its core is 15 000 000 K and at its surface is 5770 K

The Sun is the largest body in the Solar System and accounts for 99.8% of the total mass.

A planet is a sizeable celestial body that has sufficient gravity to clear its own orbit. It's all down to size! All dwarf planets in the Solar System are smaller than our Moon.

Asteroids are rocky objects which failed to form a planet.

A comet is composed of ice and silicate material (rock/dust).  The outer crust evaporates as it approaches the Sun.  The orbit of a comet tends to be elongated.  Most originate from the Kuiper Belt or Oort Cloud in the outer reaches of the Solar System, beyond Neptune (30 AU).

Mercury - NASA.jpg


Heavily cratered surface with signs of volcanism.

A weak magnetic

field suggests an iron-rich core.

Venus - NASA_edited.png


Desert surface has craters, shield volcanoes and structures resembling basaltic lava flows.

Earth - NASA.jpg


The surface is 67% liquid water.  Landmasses with volcanoes, high mountains, extensive rivers and lakes; desert areas including icecaps and arid zones; few impact craters.

NASA - Mars.jpg


Large shield volcanoes; and features that may have been formed by running water.  No liquid water at the surface but water ice at the poles.  



 Swales, boulders and hummocky ridges - flood debris. 

Jupiter - Hubble - NASA.jpg


A relatively small rocky or metallic core which is under huge pressure.  Internal pressure generates huge amounts of heat, radiation and a powerful magnetic field.

SATURN - Cassini - NASA_JPL-CalTech.jpg


A rocky core enveloped by liquid hydrogen.  The iconic rings consist of ice debris.

Uranus - Hubble - NASA.jpg


Uranus is spins on its side. Most of its mass is a dense fluid of "icy" materials – water, methane and ammonia – above a small rocky core.

Neptue NASA.jpg


An ice giant.

Predominantly composed of a super dense fluid of ices: water, methane and ammonia. Enveloping a small rocky core.



 Captured by the Soviet U 'Venera 13 Probe' (1981) 

What ever happened to Pluto?
The rise of the Dwarf Planets.

Following the discovery of Eris, in the distant Kuiper Belt, it became apparent that there were likely numerous planetary bodies like Pluto still to be discovered.  Debate raged in the scientific community regarding the classification of planets. eventually in 2006 The International Astronomical Union agreed on a new classification: The Dwarf Planet.

The key difference between a planet and a dwarf planet is that the latter does not dominate its region in space around the Sun. Pluto, for example, has not cleared its orbit of similar objects while Earth or Jupiter have no similarly-sized planetary bodies on the same path around the Sun. A dwarf planet has to be massive enough to be plastic and have a gravity that can maintain a hydrostatically equilibrious (roughly spheroid) shape - Haumea is shaped like a rugby ball. A dwarf planet may not be a satellite of another planetary body, it has to directly orbit the Sun in its own right. There are likely thousands of dwarf planets waiting to be discovered beyond Neptune.


The International Astronomical Union have recognised these planetary bodies as dwarf planets because they are massive, round, and orbit the Sun, but haven't cleared their orbital path: Ceres, Pluto, Haumea, Makemake & Eris.


Except for Ceres, which lies in the main asteroid belt (between Mars and Jupiter), these dwarf planets are located in the Kuiper Belt (in the outer solar system, beyond Neptune).



 Ceres is the largest object in the asteroid belt and the only dwarf planet located in the inner solar system.



Pluto is smaller than Earth’s Moon. It has blue skies, spinning moons, mountains as high as the Alps. 



Haumea is about 1/14 the radius of Earth.

It's the first known Kuiper Belt Object to have rings.



Slightly smaller than Pluto, Makemake is the second-brightest object in the Kuiper Belt



Eris is one of the largest known dwarf planets in our solar system -  about the same size as Pluto.

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