Our Solar System is 4.6 billion years old and was formed inside a diffuse cloud of interstellar gas and dust called a nebula. At its center is a giant ball of exploding hydrogen (75%) and helium (24.9%) called the Sun, which took less than 1 million years to form. Our solar system and its planets, moons, and smaller objects then took a further 100 million years or so to essentially complete its formation.
Size
In astronomy, the mean distance between the Earth and the Sun is called an astronomical unit (AU), and is roughly equal to 93 million miles (150 million km). To put this into perspective, our solar system has a diameter of around 100,000 AU, meaning that if our Sun was the size of a basketball, then Pluto (39 AU) would be a grain of sand located 1,000 meters away, while the edge of our solar system (50,000+ AU) would extend a further 1,000 times beyond Pluto.
Structure
The Sun
Sun: Our nearest star is a yellow dwarf with a diameter of 864,576 miles (1.3914m km), compared to 7,917.5 miles for the Earth. The Sun (sol) burns with a surface temperature of 5,505 °C (9,941 °F) and contains 99.86% of the entire mass of our solar system. In fact, the Sun’s gravity is so strong that it overpowers all other objects around it for a distance of two light-years. These objects rotate around the Sun in nearly circular orbits and include the 8 planets, as well as asteroids, comets, moons, dust, gas, and some dwarf planets.
Inner and Outer Solar System
Planets (0.39 To 30 AU): The 4 inner, terrestrial planets are Mercury, Venus, Earth and Mars, which are all basically balls of rock and metal. Beyond the asteroid belt are situated the 4 outer, ringed planets of Jupiter, Saturn, Uranus and Neptune, which are essentially gigantic balls of hydrogen and helium with gaseous outer layers and liquid interiors. In general, the nearer the Sun the hotter the planet will be; therefore Mercury, for instance, is hotter than the furthest planet Neptune.
Asteroid Belt (2 to 4 AU): Between inner planet Mars and outer planet Jupiter lies the asteroid belt, a region occupied by millions of rocky objects including over 90,000 asteroids. Around half of this region’s mass is contained within its four largest asteroids, namely Ceres, Vesta, Pallas, and Hygiea.
Kuiper Belt (30 To 50 AU): The first of our solar system’s two reservoirs of cometary material is a circumstellar disc consisting mainly of icy debris called the Kuiper Belt. Amongst the trillion or so icy bodies it contains are thousands that are bigger than 62 miles (100 km) wide, as well as the dwarf planets Eris, Pluto, Haumea and Makemake.
Oort Cloud (5,000 To 50,000 AU): There is a massive thick “bubble” of up to a trillion comets encompassing our solar system known as the Oort Cloud. If Neptune lies 30 AU away, then the Oort Cloud may extend beyond 50,000+ AU or nearly a light-year from our sun. While short period comets (less than 200 years to orbit the Sun) originate in the Kuiper Belt, long-period comets (over 200 years) originate from the Oort Cloud.
The nearest star to our own Sun is Proxima Centauri, which is located more than 4.3 light years away and is part of a triple star system called Alpha Centauri.