(a) Describe the solar system. (b) Outline four characteristics each of the following planets: (i) Pluto; (ii) Mercury

(a) The solar system

The solar system is the family of the Sun and all the heavenly bodies that revolve around it under the pull of its gravity. It consists of:

• The Sun at the centre, a star that gives light and heat and holds the system together by gravity.
• Eight planets which revolve around the Sun in elliptical orbits: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune (Pluto is now classed a dwarf planet).
• Natural satellites (moons) that revolve around the planets, for example the Earth's Moon.
• Asteroids (minor planets) mostly found in a belt between Mars and Jupiter.
• Comets and meteors/meteoroids which also move round the Sun.

The inner planets (Mercury to Mars) are small and rocky; the outer planets (Jupiter to Neptune) are large gas giants.

(b) Four characteristics each

(i) Pluto

• It is the smallest body formerly counted as a planet and is now classified a dwarf planet.
• It is the farthest and coldest, lying at the outer edge of the solar system.
• It has a very long orbital period (about 248 Earth years to go round the Sun once).
• Its orbit is highly elliptical and tilted, sometimes bringing it inside Neptune's orbit; it has a moon (Charon).

(ii) Mercury

• It is the nearest planet to the Sun.
• It is the smallest of the eight main planets.
• It has the shortest orbital period (about 88 Earth days).
• It has no atmosphere and no moon, with extreme temperatures (very hot by day, very cold by night).