The planets

The rocky composition and the relatively high density of 3.9–5.6 grams per cubic centimetre of the inner planets Mercury, Venus, Earth, and Mars is earth-like. Apart from a small core of rock, the gas giants Jupiter, Saturn, Uranus, and Neptune are composed largely of hydrogen and helium and reach a density of 0.7–1.6 gram per cubic centimetre. Due to the absence of an atmosphere, extreme differences between day and night temperatures prevail on Mercury. Whereas the temperature on Venus, due to its proximity to the Sun and an extreme greenhouse effect, is about 500 °C, the temperature on Mars is usually below freezing and on Jupiter it is below –150 °C. The temperatures of the gas giants are measured in the lower cloud layer because the gas giants have no solid surface and gas changes into a liquid state in the depths. In 2006, Pluto was demoted from its status as a planet: it is located – along with a number of recently discovered similar objects – close to the Kuiper Belt of comets. Furthermore, Pluto's composition is similar to that of a comet – a conglomeration of different kinds of ice (water, methane, ammonia, carbon monoxide etc.) and rock.