Jupiter with one of its moons in the foreground.
Unless you go outside at night and look up, it can be easy to forget we are living on the surface of a planet floating in space. We're not exactly hanging around all on our own though. There are 8 other planets nearby, and like Earth, they all orbit around a star that we call the Sun.
To help appreciate the scale of things, if you shrunk the universe until our system of planets, our Solar System, was the size of a small lunch box (15 cm), then the closest star to us, which may or may not have its own system of planets, would be 1 km away. That's a lot of empty space between our solar system and the next one!
In addition to the Sun and nine planets, our solar system contains thousands of asteroids in a belt-like distribution between the orbital paths of Mars and Jupiter. It also contains numerous comets, many of which have come from the "Kuiper belt" - a clumpy ring of material at the distance of the outermost planet Pluto. Interplanetary gas and dust also exists. In fact, on average, an impressive 3 tonnes of space dust falls to Earth every day.
Many of the planets have their own moons, eight of which are about the size of our Moon. Furthermore, Saturn, Jupiter and Uranus have rings of debris around them, thought to be the remains from the natural destruction of what were once moons.
In order of increasing distance from the Sun, the planets are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. They all orbit around the Sun in the same direction, tracing out a disk.
Apart from the outermost planet Pluto, the four closest planets to the Sun are the smallest and all have rocky surfaces, with the Earth additionally having oceans of liquid water.
During a typical day on Mercury, the planet closest to the Sun, the temperature reaches a sizzling 400 degrees, while at night it drops to a freezing minus 200 degrees.
Unlike Mercury, the next planet Venus has an atmosphere that helps to regulate its day and night temperature. But unlike Earth's atmosphere of oxygen and nitrogen, it is composed of carbon dioxide which traps the heat of the Sun, resulting in average temperatures of 400 degrees.
The third planet from the Sun is Earth. If it was only as big as a grape, then the Sun would be the size of a small car and located 100 car-lengths away. Pluto, the outermost planet, is 40 times more distant from the Sun than the Earth. Now scale all of this down to fit in your lunch box, and you soon realise there's a lot of empty space out there.
At present there are at least seven missions to place spacecraft in orbit around Earth's moon. They intend to study such things as its minerals and magnetism, and to also search for frozen water. During the 1990s two NASA spacecraft obtained data suggesting that ice may exist at the Moon's poles. Moreover, both China and the USA have plans to send humans to the Moon, considered to be a stepping stone to Mars and beyond.
Between Mars and Jupiter is the "Asteroid Belt", a region of space containing thousands of asteroids, thought by some to be the building blocks of a planet that never was. Moving further out, we encounter the "gas giants": Jupiter, Saturn, Uranus and Neptune.
Jupiter is so large that 1000 Earths would fit inside of it. It also rotates on its axis so quickly that the length of a day on Jupiter, the time for the Sun to rise, set, and rise again, is less than 10 hours. A year on Jupiter, the time for it to complete one revolution around the Sun, is 12 years.
Composed mostly of hydrogen and helium, although we can only see the upper atmosphere of these gas planets, they don't have surfaces that one could land a spacecraft on. Although, from the upper atmosphere of Saturn one would have a spectacular view of its 33 known moons and over a dozen system of rings.
Furthest from the Sun is the frozen world of Pluto. Some five and a half times smaller in size than the Earth, and smaller than many moons, it is often considered to simply be the largest object in the Kuiper belt.