The inner solar system

Let's get our bearings...

The Sun

At the heart of our planetary system, is the Sun - our hydrogen burning star. Orbiting around it, all in the same counter-clockwise direction, are a number of rocks and planets. These are all orbiting practically in the same reference plane, and rotating about their axises in the same counter-clockwise direction (except Venus and variably Uranus).


The closet planet orbiting the Sun is Mercury. It is the smallest planet in the Solar System, which only takes 88 Earth days to complete its’ orbit. It has virtually no atmosphere, and surface temperatures can vary daily between -170 °C at night to over 400 °C during the day at the equator. Like the Earth, Mercury is classed as a Terrestrial Planet, as it has a rocky and metallic solid body.


The second planet from the Sun is Venus. It orbits the Sun every 224.7 Earth days, and rotates in the opposite direction to other planets (clock-wise); and it takes the longest time to rotate about it’s axis (243 Earth days). Like Mercury and Earth, Venus is also a Terrestrial Planet, with a similar size and mass to Earth. Venus has a dense atmosphere consisting of 96% carbon dioxide, and is the hottest planet in the Solar System, with an average temperature of over 460°C.

The Earth and the Moon

The third planet from the Sun is Earth. It orbits the Sun every 365.36 Earth days, and is the only ‘object’ in the known universe to harbour life (so far). The Earth and the Moon, are described in more detail in the section below, titled a 'A Double Planet.' And in relation to the other planets (including the moon, discussed more below), Earth is fortunate that their masses and configurations are as they are, because the: gravitational influences on each other and on the Earth produced the enduring emergent property of stability in the Earth's orbit. [1]


The fourth planet from the Sun is Mars. It orbits the Sun every 687 Earth days, and has a thin atmosphere, mostly of carbon dioxide - again 96%. And is the fourth (of four) Terrestrial Planets in the Solar System. Mars has two moons, 'Phobos' and 'Deimos,' and has water - although almost never in a liquid state due to the low atmospheric pressure; instead most of the water is held in polar ice caps and in underground ice.

The Asteroid Belt

Roughly positioned between Mars and Jupiter is the Asteroid Belt, which is a mass of rocky elements from 950km across to particles of dust. It is thought that the high gravitational energy of Jupiter has prevented these elements from forming together to make another planet. Of the 50,000 meteorites found on Earth thus far, 99.8% are thought to have come from the Asteroid Belt.


The fifth planet from the Sun, and the largest in the Solar System is Jupiter. With an atmosphere mainly composed of hydrogen and helium, it rotates around the Sun in 11.86 Earth years, and has the fastest axial rotation of all the planets - a dizzying ten hours. It is classed as a giant planet - the three others, the remaining Solar System Planets, Saturn, Uranus and Neptune are not shown. Jupiter is one-thousandth of the mass of the Sun, but is two and a half times the mass of all the other planets combined. And so, with this huge mass, and therefore huge gravitational field, Jupiter also influences the shape of the Solar System, and along with its 67 moons, it is believed by some astronomers to be the ‘vacuum cleaner’ of the solar system; partially shielding the inner planets from cometary bombardments (whilst sometimes deflecting some towards the inner planets).


[1] Harding, Stephan (2009 Second Edition) ‘Animate Earth: Science, Intuition, Gaia.’ Green Books, Cambridge, U.K.



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