This article featured in the February 2002 Beginners Magazine


The Sun actually has a quite extended family and its members can be categorised into a number of logical groups.  These groups starting nearest to the Sun are :






The Terrestrial Planets are MERCURY, VENUS, EARTH and MARS.  These planets are solid and made of metal and rock.

ASTEROIDS are lumps of rock and metal that have failed to form into a planet, between the planets Mars and Jupiter.

The Gas Giants are JUPITER, SATURN, URANUS and NEPTUNE.   These planets are comprised mainly of gas, Hydrogen and Helium with only small rocky cores.


JUPITER  is the largest of all the planets and is over ten times the diameter of Earth.  Unlike the inner planets Jupiter has no solid surface because it is comprised of gas although it may have a small rocky or metallic core.  Because Jupiter is so large the enormous pressure in the centre produces a lot of heat.  This heat is conducted outwards through the gas and drives huge storms in the outer atmosphere.  The surface of the planet has many coloured bands or ‘belts’ of different colours.  These belts are mainly different shades of browns with variations from white through orange to chocolate brown.  There are even vivid reds especially in the famous giant Red Spot.  The Red Spot is a massive storm larger than the size of the Earth which has been raging since before the invention of telescopes.


Jupiter has many moons and more are being discovered all the time.  Many are small and may be captured asteroids but four are large enough to have been planets, two, Ganymede and Callisto, are bigger than Mercury.


Jupiter has an orbit approximately 778 million km from the Sun and its diameter at the equator is 142,984 km (Earth 12,756 km).  Its volume is large enough to swallow all the other planets.  Despite being so large Jupiter rotates very fast, in fact, a day on Jupiter is equivalent to only 9.9 Earth hours.  This is so fast the centrifugal force of the spin causes the planet to bulge noticeably at the equator. 

Unlike the inner planets, Jupiter and the other giants are comprised mainly of the gases Hydrogen and Helium.  Being gaseous, Jupiter has a mean gravity (compared to water) of only 1.3 compared to the rock and metal composition of Earth which has a mean gravity of 5.5.  Jupiter actually weighs only 318 times as much as Earth despite having a volume 1400 times greater. Because the planet is made of gas it actually rotates as a fluid and the equatorial regions rotate significantly faster than the poles.

On Earth the main source of heat is the Sun but Jupiter actually produces more heat than it receives from the Sun.  The internal heating is what is left over from when the planet formed and has not yet had time to cool down.  The internal thermal energy heats the atmosphere and causes powerful currents to rise to the surface.  These currents carry chemical compounds from the interior to the surface.  The rapid rotation causes the surface of Jupiter to be marked by bands of coloured clouds known as Belts and Zones.  The darker areas are the Belts and the lighter, Zones.  The colours in the bands are caused by the presence of trace gases such as Methane and Ammonia.  The bands are very turbulent with wind speeds of over 200 km/h.  One famous feature is the Great Red Spot.  This is a gigantic storm system which has been raging for at least 400 years because it has been visible since Jupiter was first observed using telescopes in the early 1600’s.  The Great Red Spot is about twice the diameter of Earth and has wind speeds in excess of 500 km/s.  There are many other storm systems continuously raging in the atmosphere which produce gigantic lightning flashes over 1000 km long.

Jupiter’s Belts and Zones

With space probes much more is now known about Jupiter’s moons, especially the four Galilean moons (named after Galileo who discovered them).  Io the inner, is the most active volcanic place in the Solar System.  It is so close to Jupiter that is nearly torn apart by the gravitational forces of Jupiter.  The gravitational forces stretch and squash Io causing enormous pressure and friction which produces heat inside the moon.  This internal heat causes continuous volcanic action on the surface. Europa is also affected by the tremendous forces of Jupiter’s gravity.  The surface of Europa is covered by a 30km crust of ice. Radar scans have indicated that there may be a liquid salty ocean beneath the ice.  Ganymede is the largest moon in the solar system (3270 miles in diameter).  There are large areas on the surface cover with thick dark dust.  The strangest features are the massive fault lines cause by movements on the surface.  Callisto has one of the most heavily cratered surfaces in the Solar System.


SATURN  is the second largest planet but its most famous feature is the magnificent ring system.  The ring or to be more precise ‘rings’ are 275000km (170000 miles) across but only a few hundred meters thick.  They are made up of millions of small pieces of mainly ice and some rock varying in size from a few millimetres to a few meters across.  The rings are thought to be the remains of a comet or a small moon which was pulled apart by Saturn’s gravity.  In theory the rings should not last for very long but they appear to have been around for a very long time.  Small moonlets have been found orbiting within the rings which appear to stop the rings falling apart.  As these small moons orbit their gravitational influence pushes and pulls the ring particles into place, these moons are therefore called ‘Shepherd Moons’.  There is a gap in the rings called the Cassini Division, this can be seen quite clearly in medium sized telescopes.  When space probes photographed the ring close up it was found that there are in fact hundreds of rings and divisions.  The rings are as wide open as the can be just at the moment and will begin to close as we view the planet from a different position in its orbit.



Saturn, like Jupiter has dozens of moons with some large and some small.  Titan is the largest moon at 3500 miles in diameter and is one of the most intriguing of all the moons in the solar system.  It has a thick atmosphere and may have oceans, land, clouds and rain not of water but all made of Methane.  A space probe called Huygens  which is part of the Cassini - Huygens space probe is due to land on Titan in 2004.  The Huygens lander has a camera on board and will film the decent of the craft through the atmosphere by parachute.  Saturn has eighteen moons of various sizes.  Titan can be seen in most telescopes and perhaps three more may be seen in a 6 inch telescope.  Larger amateur telescopes may pick out up to seven.


URANUS  is quite hard to find in smaller telescopes but when found displays a small greenish blue disc in a 6 inch telescope.  No detail can be seen in small or even medium sized telescopes as the disc is too small.  It was the first planet to be discovered using a telescope because it is not visible to the naked eye.



Uranus is a gas giant but smaller than Jupiter and Saturn, it is also much further away from the Sun.  Uranus has a very odd characteristic, that is the poles of Uranus are tilted at 90º to its orbit plane.  It is thought that in the early period after the Solar System formed Uranus was involved in a catastrophic collision with large object, possibly another planet.  This collision may have destroyed the two planets which then reformed into the Uranus we see today. The odd tilt means that for half its orbit one pole points towards the Sun and for the other half the other pole point to the Sun.  In other words the summer and winters last for 42 Earth years each but each day is only about 11 Earth days long.  There are not many features visible in the atmosphere but Uranus like all the gas giants does have a ring system, it is however very faint.  Uranus has five fairly small moons at least one of which appears to have been smashed to pieces at some time and reformed.


NEPTUNE is the last of the Gas Giants. It is approximately the same size as Uranus and also appears blue / green.  Because it is further away than Uranus it is even harder to find but can be seen using a medium sized telescope. 



Strangely the atmosphere on Neptune is much more turbulent than on Uranus.  This is because Neptune produces much more heat in its core although the process is not yet fully understood.  Neptune boasts the second largest moon is the Solar System in Triton which has a diameter of 3300 miles and is just visible in medium sized telescopes.



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