Our tiny little world has gazed out upon the cosmic ocean for thousands and thousands of years. Ancient stargazers and astronomers have observed points of light that appeared to move among the stars. They named these objects planets, which to them meant wanderers, and they later named them after Roman deities, like; Jupiter the king of the gods; Mars, the god of war; Mercury, the messenger of the gods; Venus, the god of love and beauty, and Saturn, the father of Jupiter and the god of agriculture. These stargazers also witnessed comets with sparkling tails, and also meteors or shooting stars apparently falling from the sky.
When the telescope was invented and put to use, three more planets have been discovered in our solar system: Uranus in 1781), Neptune in 1846, and Pluto in 1930. There are also hundreds of thousands of small bodies including asteroids and comets. Most of these asteroids orbit in a region between the orbits of Mars and Jupiter, whilst the home of comets lies much farther beyond the orbit of Pluto, in the Oort Cloud.
The four planets that are closest to the Sun – Mercury, Venus, Earth, and Mars – are described as the terrestrial planets because they have solid rocky surfaces. The four large planets beyond the orbit of Mars – Jupiter, Saturn, Uranus, and Neptune – are routinely referred to as gas giants.
The quite small, distant planet of Pluto has a solid but icier surface than the terrestrial planets. Practically every planet – and some of their moons – have some type of atmosphere. Earth’s atmosphere is primarily made up of nitrogen and oxygen. Venus has a thick atmosphere of carbon dioxide, with traces of poisonous gases such as sulphur dioxide. Mars’ atmosphere, which is extremely thin, is carbon dioxide. Jupiter, Saturn, Uranus, and Neptune have mainly hydrogen and helium based atmospheres. When Pluto moves near to the Sun, it has quite a thin atmosphere, but as Pluto travels to the outer reaches of its orbit, its atmosphere freezes then collapses to the surface of the planet. Pluto then acts like a comet.
141 natural satellites have been discovered – which are also referred to as moons – in orbit around the many planets in our solar system. They range from bodies larger than our own Moon, to small pieces of floating debris.
Many of these were discovered by planetary spacecraft. Some of these have moons that have atmospheres, like Saturn’s Titan. Some even have magnetic fields, like Jupiter’s Ganymede. Jupiter’s moon Io is the most volcanically active body in our solar system. An ocean may lie beneath the frozen crust of Jupiter’s moon Europa, while images of Jupiter’s moon Ganymede show historical motion of icy crustal plates. Some planetary moons may actually be asteroids that were captured by a planet’s gravitational pull.
Captured asteroids presently counted as moons, may include Phobos and Deimos, several satellites of Jupiter, Saturn’s Phoebe, many of Uranus’ new satellites, and possibly Neptune’s Nereid.
From 1610 to 1977, Saturn was thought to be the only planet with rings. We now know that Jupiter, Uranus, and Neptune also have ring systems, although Saturn’s is by far the largest. Particles in these ring systems range in size from dust to boulders to house sized, and may be rocky and/or icy.
Most of the planets also have magnetic fields which extend into space and form a magnetosphere around each planet. These magnetospheres rotate with the planet, sweeping charged particles with them. The Sun has a magnetic field, the heliosphere, which envelops our entire solar system.
Ancient astronomers once upon a time believed that the Earth was the centre of the Universe, and that the Sun and all the other stars revolved around the Earth. Copernicus proved that Earth and the other planets in our solar system orbit our Sun. Little by little, we are charting the Universe, discovering new planets all the time, and searching for the existence of other life forms. Are there other planets where life might exist?
The space-race is now undoubtedly under-way, with numerous public and private corporations vying to be the one to discover a habitable planets.
The 21st century will see space tourism expand exponentially to new heights.