It is likely that there are many planetary systems scattered throughout our vast universe. However, the universe is vast - so vast that we have difficulty observing them. As such, we can only really know about our own Solar System, that is the system of objects under the primary influence of the Sun. Even here we have lots of gaps in our knowledge. How did the planets form? Why is the Sun a single star in a sky full of binary systems? Why does Jupiter have its Great Red Spot? Is there life on Europa? We have theories that try to answer some of these questions, but there is still a lot we need to learn about our own solar system, before we can start looking elsewhere.
Here's a brief overview overview of this fascinating system.
The Solar System
How did it get here?
The solar system probably formed from a cloud of mainly hydrogen gas, with various other heavier elements mixed in. Gravity may made part of this cloud condense enough to ignite nuclear fusion in part of it. Thus the Sun might have been born. The other matter in the nebula of gas surrounding the young star probably condensed to form protoplanets, some of which collided with each other, and some of which survived, eventually making up the planets and other bodies in the system which we see today. The composition of the planets at various distances from the Sun can be explained by the heat generated by the Sun - the planet Mercury only has a thin sillicate mantle because sillicates have difficulty existing that close to the Sun.
We can be pretty sure that the objects in the Solar System formed at around the same time - the Sun didn't capture planets which had already formed somewhere else.
The nearest1 star to the Sun is Proxima Centauri, part of the Alpha Centauri system. The Solar System is located about two thirds of the way out from the centre of the Milky Way Galaxy, on a spiral arm. Our Galaxy has two companions, the Greater and Lesser Magellanic Clouds, and, in the wider Universe, the nearest large galaxy is Andromeda, M31.
A great star. Not too many X-rays, not too little infrared, just the right size for carbon-based life to evolve in the system. Sol is stable over long periods of time, though it's expected to start running down in about 200 million years, unless some preventative maintenance is carried out.
The Terrestrial Planets
Hot enough to boil tin on the sunward summer side, cold enough to freeze ice in the dark winter. No atmosphere. Mercury is a quite inhospitable place.
Due to an out-of-control greenhouse effect, not only is the sky constantly covered in clouds, but it rains all the time (not unlike England). Any visitors would have to wear comfortable rain gear which isn't dissolved by concentrated sulphuric acid. You'd think that humans would take a hint from this example just next door, since Venus is in many ways Earth's sister planet, except for the atmosphere. They don't, of course, and continue to burn fossil fuels, polluting the air and contributing to the same factors that makes Venus such a hot-house.
The Earth has got this huge satellite called the Moon, an amazing satellite that, despite not having an atmosphere, has been visited safely by humans. Both bodies dominate each other's skies, and have complicated gravitational interactions. As a result, the Earth's seas vary in height as much as two metres every rotation. The Moon's rotation has perfectly matched its orbital period (also due to these interactions) and Earth's rotation is slowing down for the same reasons.
This planet has fascinated humans for centuries because it is so near. Giovanni Schiapirelli thought that he saw straight canals on Mars, which led to suspicions of intelligent life. No one really knows why Martians should be green, but that's how they're always portrayed...
The asteroid belt constitutes a vast ring of lumps of rocks kept in order by the gravitational influences of Jupiter and Mars. Whether they are the remains of an unformed planet, an exploded planet, or just debris is not known. The belt is mostly confined to the ecliptic plane2, so man-made satellites can hop over the belt on their way to the outer planets.
The Jovian Planets
Being the biggest planet in the system, the metallic hydrogen core of Jupiter generates some pretty intense magnetic fields. It also has a system of 16 satellites, the largest of which is Ganymede. Jupiter's anomaly is its obvious storm systems, including the 'Great Red Spot'. The whorls and patterns on the upper atmosphere are well worth looking at.
Saturn has a ring system to die for, made from high-albedo3 ice, kept in place by members of its 18-satellite contingent.
Big. Blue. Weird. And the butt of some terrible jokes...
First, someone's knocked it over. Its axis of rotation is almost 90° to the plane of the ecliptic, so its polar regions get more sun than the equator. The equator is still somehow warmer than the poles, though, and no one knows why. Alone of the four largest planets, it doesn't generate more heat than it receives, leaving it cold. To cap it all, Uranus' magnetic field is at 60° to titshe axis of rotation.
Uranus has the most moons, too.
Neptune seems like an imitation of the weirdest features of the other three giants. It's got some twisty rings, like Saturn; it's got a storm spot like Jupiter; and it's got a tilted magnetic field like Uranus. The only new weird thing it adds to the broth is that the winds are unaccountably stronger than they should be.
The Outer Reaches
Pluto & Charon
These two make up the the second double planet in the system, along with the Earth and the Moon, but these two are even closer in size. This has lead to both planets matching rotation with orbit, so the same sides face each other all the time.
Pluto is quite eccentric, and swaps order with Neptune every hundred years or so. And did we mention it's cold? When it wanders back out to perihelion - its furthest distance from the Sun - the atmosphere freezes and falls as snow. Take a coat.
The Kuiper belt starts just outside the orbit of Pluto, and is in the plane of the ecliptic like the asteroid belt.
We don't know that the Oort Cloud exists. Its existance has been suggested as the source of some types of comet. It is thought to be a spherical shell completely surrounding the Solar System at a distance of about a light year, but we don't know if it is, or how it got there.