The solar system consists of everything that is controlled and powered by our closest star, the Sun. Humankind, the Earth, and all objects that orbit the Sun have all been created from the same gas and dust that existed about 4.6 billion years ago.
The force of gravity brought the swirling hydrogen gas and dust together, causing it to become hotter and hotter and beginning the process of nuclear fusion to ignite the Sun. And so from the molecular gas and dust the Sun was born, and the solar system began its life. With the ignition of the Sun came the control and power of the remaining dust and gas orbiting the central powerhouse. The spinning clouds of hydrogen and gas, slowing and clumping together over a few hundred million years created the planets, moons, asteroids and comets about which we continue to learn through our exploration of the solar system. And so from the same fundamental particles the laws of the universe have created an incredibly diverse range of worlds, from the Earth to the giant gas planets. The device on which you are reading this article originated from the same source as that which created the Sun and the planets.
There are eight planets. In order of distance from the Sun these are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The inner planets are rocky planets (Mercury, Venus, Earth and Mars) whilst the outer planets (Jupiter, Saturn, Uranus and Neptune) are gas giants. The two most distant planets, Uranus and Neptune, are also referred to as 'ice giants' with a different make up to Jupiter and Saturn. It is thought that the constitution of the planets was determined by the nature of the gas and dust particles orbiting the newly created Sun with heavier elements such as iron collecting nearer the Sun and the gas and ices surviving in the more distant regions.
The solar system also includes the moons (six of the planets have them), dwarf planets, asteroids and comets, all rotating around the Sun under its gravitational influence.
Asteroids are rocky, airless worlds that orbit our sun, but are too small to be called planets. Tens of thousands of these "minor planets" are gathered in the main asteroid belt, a vast doughnut-shaped ring between the orbits of Mars and Jupiter. It was once thought that they were the remains of a planet that broke up, but today we believe most are debris left over from the formation of the Solar System. Not all asteroids are contained between Mars and Jupiter. Many have orbits that bring them through the inner Solar System. There are even several which can come close to the Earth, crossing our own orbit like celestial jaywalkers. They are called Near-Earth Objects (NEOs) and can be considered Potentially Hazardous Objects, though the threat of impacts is considerably less than it was billions of years ago when the Moon was given its spectacular crater scars.
In 1986, astronomy's regulators the International Astronomical Union made the controversial decision to strip Pluto of its planet status and classify it as a member of a new category, the dwarf planet. At the same time, the largest known asteroid, Ceres, was promoted to the same status. Three recently discovered bodies in the same outer zone of the Solar System as Pluto became dwarf planets too - they are Haumea, Makemake and Eris.
The region beyond Neptune where we find Pluto, Haumea, Makemake and Eris is known as the Kuiper Belt after Gerard Kuiper who suggested it was there. Sensitive electronic cameras are discovering more and more icy bodies and numbers probably run into many hundreds of thousands.
The Solar System does not end with the Kuiper Belt. More than a thousand times further still, and stretching for a vast distance, is thought to lie the Oort Cloud, an enormous reservoir of icy fragments that is the original source of comets and named after Dutch astronomer Jan Oort who promoted the idea of its existence.
The solar system is a big place at least to a human's comprehension of size. Although the eight planets are all within 4.5 billion kilometres (2.8 billion miles) from the Sun, its influence stretches way beyond the eight planets, with the solar winds travelling 17 billion kilometres until they hit interstellar space and the Sun's gravitational influence stretching out to the Oort cloud of ice particles tens of billions of kilometres from its source. These distances make a journey to Mars or the moon seem like a trip next door.
Though the solar system is vast in human terms, when you put our solar system into universal context the size and numbers of space are absolutely stunning and difficult to comprehend. The Sun is one of about 200 billion stars in the Milky Way galaxy which itself is one of several hundred billion galaxies in the universe. So the Sun is an average size star and just one of billions of billions of stars in the universe.
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