Pluto the dwarf

Since its discovery in 1930, Pluto had been considered the ninth planet in our solar system. However, more recent discoveries of dwarf planets larger in size and mass than Pluto have made some astronomers question its status. 

In 2006, the International Astronomical Union decided upon a conclusive definition of what constituted a planet. Pluto's low mass - not even a fifth of the mass of the moon - excluded it from that definition. Now Pluto is considered a dwarf planet.

Bound together by gravity

When the International Astronomical Union (IAU) defined planets in 2006, part of that definition included the requirement that a planet has enough mass that its self-gravity causes it to reach hydrostatic equilibrium. The planet is able to resist compressive forces in space to hold together and stay rounded in shape.

Planets also "clear the neighbourhood" around their orbits. This means that there are no other bodies of the same size in its orbit. The Sun has a strong enough pull to keep the planets and their bodies around it.

Measuring our solar system

Understanding the size of planets and where they are.

Before the development of radar, astronomers measured the distance between planets through trigonometry, a process where distance to an object is derived from measurements of angles and distances taken between two known positions. Today, radar is the predominant method of measuring distance and is allows for more accurate measurements to be attained. This process works by astronomers timing how long it takes the radar beam, which is travelling at the speed of light, to travel to an object and back. By multiplying the speed of light by time taken, then dividing that in two, scientists can derive the distance of the object.
Once distance has been derived, the mass of the object can be ascertained by monitoring the orbital periods of circling satellites. To do this astronomers measure the angular separation between the satellite and the object and then use trigonometry to convert that angular separation into distance. Astronomers can then use Kepler's third law to determine total mass.

What and where are the asteroid belts?

There are a few asteroid belts in our solar system, but none can compare to the main belt, a massive ring between the orbits of Mars and Jupiter. Here the dwarf planet Ceres, the large asteroids 2 Pallas, 10 Hygiea and 4 Vesta, and millions of small asteroids and dust particles orbit the Sun. Most of the larger asteroids have elliptical orbits and an orbital period of a few years. Some astronomers believe that the main belt's contents are left over from a planetary collision of from a planet that never formed due to the strong gravitational pull of Jupiter.

Head to Head - Largest Planets

Uranus~The Big One
Diameter at equator: 25,559km
Average distance from the Sun: 2.88 billion km
Orbital period: 84.02 years
Mass (Earth=1): 14.37 Earth masses

Saturn~The Bigger One
Diameter at equator: 60,260km 
Average distance from the Sun: 1.4 billion km 
Orbital period: 29.5 years
Mass (Earth=1): 95 Earth masses

Jupiter~The Biggest One
Diameter at equator: 142,985km
Average distance from the Sun: 778 million km
Orbital period: 11.86 years
Mass (Earth=1): 318 Earth masses

Earth to Saturn in a Mini Metro

How long would it take to reach the planets in a moderately priced car?
Can't afford that ticket on the next spaceship out of town? Well fear not, for if you are the patient type and hold an interplanetary driving licence then you can drive to that Earth colony orbiting Saturn in next to no time...well, relatively speaking. In our souped-up Mini Metro, travelling at an average speed of 120mph, any traveller can reach Saturn in only 842 years. Better stock up on travel sweets then...

• Mars-134 years At 120mph you could drive to the planet named after the Roman god of war in only 134 years.
• Jupiter-459 years Mars a little too dusty? Then why not visit Jupiter, only 459 years of 120mph driving away.
• Neptune-2,497 years One for colder climates? Then Neptune should be top of your list. At 2,497 years distance, though, it is a long drive, so make sure you take regular breaks and keep at 120mph! 

Journey through the Solar System

Bound to the immense mass of the Sun by gravity, the contents of our solar system are numerous and spectacular.
The solar system formed about 4.6 billion years ago, when part of a giant molecular cloud had a gravitational collapse. The centre became the Sun, which comprises more than 99 per cent of the solar system's total mass. The rest became a dense, flat rotating disk of gas from which planets formed, called a protoplanetary disk. In our solar system, most of that disk became the eight planets, each of which orbits the Sun.

There are two different categories of planets: gas giants and terrestrials. The gas giants are the four outer planets: Jupiter, Saturn, Uranus and Neptune. They are much bigger than the terrestrial planets and are mostly made of helium and hydrogen, although Uranus and Neptune also contain ice. All of the outer planets have ring systems made of cosmic dust. These planets comprise more than 90 per cent of the rest of the solar systems mass.
The four inner planets are very close to the Sun. To grant perspective, for example, the distance between Jupiter and Saturn is larger than the radius of all the inner planets together. These terrestrials are made up from rocks and metals, have no ring systems and have a low number of moons. they include Mercury, Venus, Earth and Mars. Except for Mercury, the inner planets also have recognisable weather systems operating in their atmospheres. 
In addition to the eight main planets, there are also dwarf planets such as Pluto. The five dwarf planets are Ceres, Pluto, Haumea, Makemake and Eris. In addition, the solar system is home to numerous small solar system bodies, which include all minor planets, asteroids and comets.