The Solar System for Beginners

For thousands of years astronomers have been studying the Sun and the objects orbiting it. Their knowledge has changed humankind’s perceptions over the centuries – few people now believe in a flat Earth or that the Earth is the centre of the Universe. Today it is known that the Solar System has many more objects than early astronomers could have believed, and new additions are recorded nearly every day.

The planets, of which only five were known, were called ‘wandering stars'. Mercury, Venus, Mars, Jupiter and Saturn were plotted along with the Sun and the Moon to move around the Earth at the centre of Universe. Nicolaus Copernicus corrected these beliefs in the 16^th century with his discovery that the planets and the Earth orbited the Sun. The size of the Solar System was established soon after. 

The telescope began to make a massive difference to our understanding of our local skies. In 1609, Galileo discovered four satellites orbiting Jupiter - meaning the Earth was not the only planet with a moon. In 1781, William Herschel discovered the planet Uranus. At nineteen times further from the Sun than the Earth this meant the Solar System was now double the size previously believed. 

Uranus' orbit indicated another body was out there and in 1846, Johan Galle found Neptune. It was less than one degree from its predicted position indicated by Uranus' orbit. 

Astronomers around the turn of the 18^th century discovered the asteroid belt and this accounted for a gap in the regular sequence of planetary orbits. The Solar System was now known to be 30 astronomical units across (30 times the Earth to Sun distance). In 1930, Pluto (now no longer considered a planet) was discovered making the Solar System even larger. More moons were discovered orbiting Saturn, Uranus and Neptune. Then with the advent of space telescopes more and more moons were added to the list. 

As well as planets and moons the Solar System contains trillions of small and tiny objects. Many of these are asteroids within the asteroid belt. Around a billion of these are bigger than one kilometre across. A group of asteroid-like objects have been found between Jupiter and Saturn. Also the Edgeworth-Kuiper belt orbits just beyond Neptune, but astronomers are debating whether it contains rocky asteroids or spheres of dust and snow. 

Only as recently as 2003, Sedna was discovered way beyond the orbit of Neptune. Sedna is 1800km across and orbits the Sun every 10,500 years. Astronomers expect to find many more strange bodies like Sedna. 

The Oort cloud was suggested about 50 years ago - a vast spherical cloud comprising about a trillion orbiting comets. These are so distant that they are beyond our vision. But we do see comets when some of their orbits bring them closer to the Sun, like Halley's comet. 

Last but not least we should mention the meteoroids which were added to the Solar family in the 1860s. Regular annual meteor showers such as the perseids and orionids are caused by dust particles hitting the Earth's atmosphere. The orbits of the dust clouds move close to the Earth's orbit at the same time every year.The dust clouds are believed to be from decaying comets.