The Solar System

• A planet is a large heavenly body which orbits our sun, and is held in orbit by the force of gravity.
• The name "planet" comes from the Latin word meaning "wanderer", which refers to the fact that the planets that we see do not follow the same path as the stars across the night sky.
• The planets are named after Roman gods, such as Mars, the Roman god of war.
• The Time of Revolution is the time it takes for a planet to orbit once around the sun. For example, Earth takes one year to revolve around the sun.

• The Time of Rotation is the time it takes for a planet to rotate or spin once on its own axis. For example, Earth takes one day or 24 hours to rotate once on its axis. All planets except Venus and Uranus rotate counter-clockwise. The side of the planet which is lit by the sun can be much hotter than the side which is dark.
• Some planets have their own moons or satellites. For example, Earth has 1 moon and Jupiter has 64 moons.
• All eight planets have an atmosphere composed of gases.
• The eight planets in order from the sun are:

Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.

• There are also five dwarf planets. A dwarf planet has a smaller mass than a planet and does not have enough gravity to clear the area around its orbit.
• The five dwarf planets in order from the sun are:

Ceres, Pluto, Haumea, Makemake and Eris.

• Distances in our solar system can be measured in kilometres or astronomical units. An astronomical unit is the distance between the sun and Earth (150 million kilometres).

Mercury

• Closest planet to the sun.
• Much smaller than Earth.
• Force of gravity is only one-third of Earth's.
• Very thin atmosphere containing hydrogen, helium, oxygen and sodium.
• Temperature varies from +427 °C on the lit side to –193 °C on the dark side.
• No moons.
• Revolution time is 88 days.
• Rotation time is 59 days.

Venus

• 2^nd planet from the sun.
• Incorrectly called the 'morning star' or 'evening star' because it is very bright in the sky at sunrise and sunset.
• Slightly smaller than Earth.
• Thick atmosphere of mostly carbon dioxide with clouds of sulphuric acid.
• Winds up to 360 km/h.
• Temperature is about +460 °C.
• No moons.
• Its Revolution time is 226 days, which is shorter than its Rotation time of 243 days.

Earth

• 3^rd planet from the sun.
• Fifth largest planet.
• Blue colour because of the oceans that cover most of the surface.
• Only planet known to support life.
• Thick atmosphere of nitrogen, oxygen and carbon dioxide.
• Temperatures range from +58 °C to –89 °C.
• Revolution time is 365 days.
• Rotation time is 1 day.

Mars

• 4^th planet from the sun.
• Reddish colour because of its rusted surface.
• Polar ice caps.
• Appears quite bright in the night sky.
• Atmosphere of mostly carbon dioxide, with some nitrogen, oxygen and water vapour.
• Temperatures range from +20 °C to –87 °C.
• Two moons.
• Revolution time is 1 year 321 days.
• Rotation time is 25 hours.

Asteroid Belt and Ceres

• The Asteroid Belt is between the planets Mars and Jupiter.
• It is occupied by thousands of asteroids that revolve our sun.
• The dwarf planet Ceres is in the Asteroid Belt.
• The Belt marks the end of the inner solar system and the start of the outer solar system.

Jupiter

• 5^th planet from the sun.
• Jupiter is the largest of the planets, and is over 1000 times larger than Earth.
• Clouds give it a banded appearance because it spins so fast.
• Great red spot.
• Atmosphere is mostly hydrogen and helium, with some methane and ammonia.
• Temperature is about –108 °C.
• 64 known moons including the largest moon in the solar system, Ganymede.
• Revolution time is 11 years 9 months.
• Rotation time is 10 hours.

Saturn

• 6^th planet from the sun.
• Yellow appearance, with a ring system made of rock, frozen gases including some water.
• At least 62 moons.
• Atmosphere is mostly hydrogen and helium, with some methane and ammonia.
• Winds up to 1700 km/h.
• Temperature is about –139 °C.
• Revolution time is 29 years 6 months.
• Rotation time is 10 hours.

Uranus

• 7^th planet from the sun.
• Blue colour.
• Atmosphere of hydrogen, helium and methane.
• Temperature is about –200 °C.
• 27 known moons.
• Revolution time is 84 years.
• Rotation time is 17 hours.

Neptune

• 8^th planet from the sun. Last of the large planets.
• Blue colour, with five thin rings.
• Atmosphere of hydrogen, helium and methane.
• Temperature about –201 °C.
• 13 known moons.
• Revolution time is 248 years.
• Rotation time is 6 days.

Dwarf Planets

• Eris is the largest dwarf planet and the farthest from the Sun.
• Eris's revolution time is 558 years.
• Pluto, Haumea, Makemake and Eris all orbit further from the sun than any of the eight large planets.
• NASA's New Horizons spacecraft photographed Pluto on 14 July 2015.