Young Astronomers

12 to 16 Years Old


There are 8 planets in our solar system, four inner planets and four outer planets.  The inner planets are rocky and comprise of Mercury, Venus, Earth and Mars.  The four outer planets are made of gas and comprise of Jupiter, Saturn, Uranus and Neptune.

All the planets are round in shape, and all orbit around the Sun in a roughly circular orbit, with Neptune being the exception.

Venus and Uranus both rotate clockwise, all the other planets rotate counter-clockwise.



Average distance from the Sun: 58 million km
(36 million miles)
Surface temperature:  -180°C to 430°C
(-290°F to 800°F)
Diameter: 4,875 km
(3,030 miles)
Length of day: 59 days
Length of year: 88 Earth days
Number of moons: 0

Mercury is the smallest planet in our solar system, and its size means it cannot hold on to its atmosphere.

Although it is 18 times smaller than Earth, it is much denser due to its larger core of iron and nickel, and because Mercury rotates slower on its axis, its core produces a magnetic field that is 100 times weaker than that of Earth.

The temperatures on Mercury are high enough to melt lead, but at the poles it is so cold that there may be water ice at the bottom of the deep craters.

The first spacecraft to visit Mercury was Mariner 10 in 1974.  It sent back over 12,000 pictures showing the planet having a heavily cratered surface.  The best known crater is the Caloris Basin which is 1,300 km (800 miles) across.  This crater may have been formed by an impact that sent shock waves around the planet as on the opposite side to the crater is a jumble of mountains.

When Mercury passes in front of the Sun, this is known as a transit.  The next transit is on 9 May 2016.

Space missions to Mercury:

Mariner 10 – Launched 3 November 1973 and carried out three flybys in 1974 and 1975.  During these flybys Mariner 10 took photos of half of the planet’s surface, and sent back data on the planet’s magnetic field, and temperatures.

Messenger – Launched 3 August 2004.  It carried out three flybys in 2008 and 2009; then in March 2011 a year-long science orbit of Mercury began.  Messenger was intentionally crashed into Mercury on April 30th 2015 after four years orbiting Mercury.



Average distance from the Sun: 108 million km
(67 million miles)
Cloud-top temperature: 460°C
Diameter: 12,100 km
(7,520 miles)
Length of day: 243 days
Length of year: 224.7 Earth days
Number of moons: 0

Venus is the second planet from the Sun and is very similar to Earth in terms of size, mass and composition, but its thick layer of clouds makes it unsuitable for water and life.

It is so hostile that even the few spacecraft that have landed on the surface only lasted for a few hours.

The planet is covered in thick clouds that are made of sulphuric-acid and a blanket of carbon dioxide gas which traps the heat.  Any person landing on Venus would die from a combination of acid burns, roasting, crushing and suffocation.

There are also more than 1,600 volcanoes on Venus.

The highest mountains on Venus are the Maxwell Montes which rise 12 km (7.5 miles) above the surface.

The wind speed on Venus is about 350 km/hr (220 mph); the clouds can circulate around the planet in about 4 days.

About 80% of the Sun’s light is reflected off the atmosphere, making Venus the second brightest object in the night sky, and 20% of the light reaches the surface.  Its brightness makes Venus the Evening Star and the Morning Star.

Venus spins clockwise so the Sun rises in the West and sets in the East, and its day is longer than its year.

Space missions to Venus:

Pioneer Venus 1 – Launched 20 May 1978 and entered the orbit of Venus on 4 December 1978.  The mission lasted 14 years.
Pioneer Venus 2 – Launched 8 August 1978 – launched 3 probes on 20 November 1978 which reached the Venusian atmosphere on 9 December 1978 which ended the mission.
Venera 13 – Launched 30 October 1981.  Venus flyby and landing on 1 March 1982.
Venera 14 – Launched 4 November 1981.  Landed on Venus on 5 March 1982.
Magellan – Launched 4 May 1989, entered the orbit of Venus on 10 August 1990, mission ended 12 October 1994.
Venus Express – Launched 9 November 2005.  Mission ended 16 December 2014.

When the Venera 13 and 14 landers landed on Venus in March 1982, they sent back pictures showing an orange sky and a desert covered in different size rocks which appear to be flat, suggesting they are made of thin layers of lava.  It is estimated that at least 85% of Venus is covered in volcanic rock.

There were a number of missions to Venus, some were flybys by spacecraft on their way to other planets/comets etc.



Average distance from the Sun: 149,598 km
(92,956 million miles)
Average temperature: 16°C
Diameter: 13,000 km
(8,000 miles)
Length of day: 23.934 hours
Length of year: 365.26 Earth days
Number of moons: 1

Earth is the fifth largest planet in the solar system and the third from the Sun.  It has an atmosphere of approximately 78% nitrogen, 21% oxygen, with trace amounts of water, argon, carbon dioxide and other gases.  71% of the surface is covered in water, and it is the only planet with plant and animal life on it (as far as we know).

The age of our planet is 4.6 billion years and we spin at a speed of 1,609 km/hr (1000 mph).

The Earth is tilted at 23½° which gives us our seasons.  When the North Pole is pointed at the Sun, it will be summer in the northern hemisphere, but when the North Pole is pointed away from the Sun then it will be summer in the southern hemisphere.

Over a 26,000 year cycle the direction of the Earth’s axis also changes.  This means that after 13,000 years the seasons for winter and summer reverse before returning to their original positions after another 13,000 years.



Average distance from the Sun: 228 million km
(142 million miles)
Surface temperature: -125 to 25°C
(-195 to 77°F)
Diameter: 6,800 km
(4,200 miles)
Length of day: 24.5 hours
Length of year: 687 Earth days
Number of moons: 2

Mars is the fourth planet from the Sun and had rocks which contain iron which has rusted.  The fine red dust makes Mars appear orange-red.

The two poles have permanent ice caps.  The north pole has an ice cap made of water ice and is 3 km (1.8 miles) thick, and the south pole’s ice cap is made of carbon dioxide and is thicker and colder at -110°C (-166°F).

The atmosphere on Mars is very thin and contains mainly carbon dioxide.

The winds on Mars can reach speeds of up to 400 km/h (250 mph), this creates dust storms which can reach 1,000 m (3,000 ft.) in height, covers vast areas of the planet and last for months.

Mars has the largest volcano in the solar system called Olympus Mons.  It is 600 km (373 miles) across and would cover most of England.  Its height is 26 km (16 miles), that is 3 times higher than Mount Everest, and at the top is a crater 90 km (56 miles) across.

The Valles Marineris is a system of canyons 4,000 km (2,485 miles) long.

There have been more missions to Mars than any other planet:

Mariner 4 – 1964 (flyby)
Mariner 7 – 1969 (flyby)
Mariner 9 – 1971 (orbiter)
Mars 5 – 1973-1974 (orbiter)
Viking 1 – 1975-1976 – first successful lander
Viking 2 – 1975-1978
Mars Pathfinder – 1997 – delivered first successful rover
Mars Global Surveyor – 1997-2006
Mars Express – 2003
Beagle 2 – 2003 – UK lander believed to have crashed on Mars but in 2015 it was found intact, but the solar panels had not deployed properly to uncover the radio antenna, it was therefore unable to contact Earth
Phoenix – 2008
Spirit – 2004
Opportunity – 2004 – still operational
Curiosity – 2012 – still operational



Average distance from the Sun: 780 million km
(484 million miles)
Cloud top temperature: -143°C
Diameter: 143,000 km
(89,000 miles)
Length of day: 9.93 hours
Length of year: 11.86 Earth years
Number of moons: 63

Jupiter is the fifth planet from the Sun.  It is gas giant and the largest planet in the solar system; the other 7 planets could fit inside Jupiter.

Jupiter has a core of rock, metal and hydrogen compounds.

The rest of the planet comprises of an inner layer of metallic hydrogen, an outer layer of liquid hydrogen and helium, and the part that we see is hydrogen and helium gas.

The gases near the core are liquid due to the high temperatures, but it is cold nearer the surface of the planet.

Jupiter comprises of 90% hydrogen (H) gas.  The remainder is helium (He), methane (CH4), ammonia (NH3), water (H2O) and ethane (C2H6).  It is the liquefaction of these gases that make clouds.

The most famous feature on Jupiter is the Great Red Spot.  This was first recorded in 1664, but has halved in size since then.

The white bands of clouds circling Jupiter are known as zones, and the brown bands are called belts.

The clouds at the equator move at more than 45,000 km/h (28,000 mph).

There are some thin, dark rings of dust encircling the planet.  These were discovered by Voyager 1 in 1979.

The four main moons of Jupiter are called Io, Europa, Ganymede and Callisto, and are known as the Galilean Moons.

Io is the most volcanically active body in the solar system with temperatures reaching up to 1,500°C (2,730°F) in some hot spots.  The volcanoes are caused by Jupiter’s gravity pulling on Io and heating it up.

Europa is similar in size to our Moon and is covered in ice which is continually being renewed.  There may be liquid water about 10-20 km (6-12 miles) below the surface.

Ganymede is the largest moon in the solar system with a diameter of 5,260 km (3,270 miles), and is larger than Mercury.  It may have a 3-layer interior:

-  small, iron-rich core
-  rocky mantle
-  icy shell on top

There may be a salty ocean 200 km (125 miles) beneath the surface.

Callisto is the moon that is furthest away from Jupiter, and its surface is heavily cratered and dates back billions of years.  It is a bit smaller than Mercury, is a mixture of ice and rock, and like Ganymede may have a salty ocean beneath its surface.

Space missions to Jupiter include:

Pioneer 10 – launched 03/1972 – flyby
Pioneer 11 – launched 04/1973 – flyby
Voyager 2 – launched 08/1977 – flyby
Voyager 1 – launched 09/1977 – flyby
Galileo – launched 10/1989 – orbiter and probe
Ulysses – launched 10/1990 – solar orbiter – flew by Jupiter – took some measurements
Cassini – launched 10/1997 – flyby
New Horizons – flyby on its way to Pluto – launched 01/2006
Juno – launched 08/2011 – orbiter – to arrive July 2016
JUICE – due for launch 2022 – orbiter



Average distance from the Sun: 1,400 million km
(870 million miles)
Cloud top temperature: -180°C
Diameter: 120,540 km
(74,900 miles)
Length of day: 10.6 hours
Length of year: 29.4 Earth years
Number of moons: 62

Saturn is the sixth planet from the Sun and is a gas giant.  As well as being the second largest planet in the solar system, it is also surrounded by an impressive ring system made from ice and rock.

The ring system comprises of 3 large and bright rings, C, B and A.  Then there are the faint outer rings, F, G, and E.  The D ring is inside the C ring.

The B ring is the widest at 25,500 km (15,800 miles) across and 5-15 m (15-50 ft.) thick.

There are gaps between some of the rings, the largest being between the A and B rings, called the Cassini Division.  These gaps are caused by the gravity of Saturn’s moons sweeping the area clear.

Sometimes white areas appear on Saturn; these are giant storms with wind speeds up to 1,800 km/hr (1,100 mph).

Storms also occur at Saturn’s poles, and like the hurricanes on Earth, they have an “eye”.

Like Earth, Saturn can have auroras (rings of light).

Saturn has 62 moons, most of which are small.

The main moons which lie within Saturn’s rings are:


The largest moons that lie outside Saturn’s rings are:

Phoebe – has its own ring made of ice and dust
Hyperion – tumbles as it orbits Saturn and looks like a sponge
Iapetus – covered in dust on one side which comes from Phoebe
Titan – second largest moon in the Solar System, bigger than Mercury, has its own atmosphere.  The Cassini orbiter launched a probe, Huygens, to land on titan on 14 January 2005, the first and only spacecraft to land on a world in the outer solar system.

Space missions to Saturn:

Pioneer 11 – launched 5 April 1973 – flyby 1 September 1979 – final contact 24 November 1995
Voyager 2 – launched 20 August 1977 – flyby 26 August 1981
Voyager 1 – launched 5 September 1977 – fly by 12 August 1980
Cassini – launched 15 October 1997 – arrived 1 July 2004 – Orbiter - launched Huygens lander to Triton – 24 February 2004 – landed 14 January 2006 – mission still ongoing



Average distance from the Sun: 2,870 million km
(1,784 million miles)
Cloud top temperature: -216°C
Diameter: 51,120 km
(31,765 miles)
Length of day: 17.25 hours
Length of year: 84 Earth years
Number of moons: 27

Uranus is the seventh planet from the Sun.  It is a blue gas planet with storms that travel around the planet at twice the speed of Earth’s hurricane winds, and spins in the opposite direction to Earth.

It was discovered by William Herschel in 1781.

It is thought that Uranus may have collided with a planet-size body which caused it to tip on to its side, this means its poles face the sun which each pole having 21 years of sunlight then 21 years of darkness.

Uranus is surrounded by 13 dark, thin rings which are made up of dust and boulders.  These rings are less than 10 km (6 miles) across.

The shepherd moons, Cordelia and Ophelia, keep the particles of the rings in place.

The main moons of Uranus are:

Miranda – the smallest with canyons 12 times deeper than the Grand Canyon
Ariel – the brightest moon
Umbriel – heavily cratered
Titania – one of the largest moons
Oberon – the first moon to be discovered in 1787 by William Herschel

The only space mission to Uranus was a flyby by Voyager 2.



Average distance from the Sun: 4,500 million km
(2,800 million miles)
Cloud top temperature: -220°C
Diameter: 49,500 km
(30,760 miles)
Length of day: 16 hours
Length of year: 165 Earth years
Number of moons:  13

Neptune is the eight planet from the Sun, and like Uranus is a blue gas planet, the colour being caused by methane gas in its atmosphere.

Neptune was discovered by Johann Galle in 1846 after its position was calculated by John Couch Adams and Urbain Le Verrier.

The core of Neptune produces heat which rises causing large storms and which are the fastest in the solar system.  These winds can travel around Neptune at speeds of up to 2,000 km/hr (1,240 mph).

Neptune had a feature called the Great Dark Spot, but this only lasted a few years.

Another short-lived feature was a white cloud called Scooter which took only 16.8 days to go around the planet.

Neptune’s largest moon is called Triton, which is smaller than our Moon but larger than Pluto.  It has a surface temperature of -235°C (-391°F), making it one of the coldest places in the solar system, and is covered in frozen nitrogen gas.

Neptune’s gravity is pulling Triton closer to the planet.

Neptune has 6 inner moons, the largest of which is Proteus.  Proteus takes 27 hours to orbit Neptune.

Five of the moons orbit at a distance of more than 15 million km (9 million miles) and are probably captured comets.

Like Saturn and Uranus, Neptune has a ring system comprising of six very narrow, dark rings.  There are four moons which lie inside the ring system, with Galatea and Despine acting as shepherd moons, keeping two of the rings in shape.

The two brightest rings are called Adams ring and Le Verrier.

Like Uranus, there has only been one mission to Neptune and that was a flyby by Voyager 2.