Our Solar System consists of the Sun with planets in orbit around it. Most planets have at least one satellite (moon) in orbit around them. Gravity provides the centripetal force needed to keep objects in orbit.
We can explore space using manned or unmanned spacecraft.
Our solar system consists of:
- a star (the Sun)
- planets and dwarf planets in orbit around the Sun
- satellites (moons) in orbit around most of the planets
- comets and asteroids in orbit around the Sun
The Earth is just one of the eight major planets orbiting the Sun. There are also dwarf planets, such as Eris, Pluto, and Ceres. (Pluto is no longer considered to be a major planet).
- The Sun's gravity keeps the planets, dwarf planets, comets and asteroids in orbit.
- A planet's gravity keeps its satellites in orbit.
The Sun
Our Sun is just one of at least 200 billion stars in our galaxy, the Milky Way. Stars are very hot and give off their own light, which is why we can see the Sun during the day, and the stars in the night sky.
A galaxy is large group of many millions of stars. The observable universe contains around 80 billion galaxies. The universe also contains other objects, such as black holes.
Planets
The time for a planet to do a single orbit (i.e. to go round the Sun once) is called the planet's year. The further out a planet is, the longer is its year.
The orbits of the planets in the solar system are almost circular,with the Sun near the centre. The orbits all lie in the same plane, and the planets all go round in the same direction.
A mnemonic for remembering the order of the planets (up until the time that Pluto was demoted from being a major planet to being merely a dwarf planet) was:
My Very Elegant Mother Just Served Us Nine Peas
where the first letters of each word indicates a planet e.g. M=Mercury.
(You may prefer this mnemonic: My Very Easy Method Just Speeds Up Naming)
Dwarf planets
Ceres, as well as being a dwarf planet, is the largest asteroid. It lies within the asteroid belt between Mars and Jupiter. Pluto orbits further away from the Sun than Neptune, while Eris orbits further out still.
Moons, asteroids & comets
Moons orbit planets. Most are tiny. Only a few are as large as our Moon, which is nearly a sixth of the diameter of the Earth.
Asteroids are much smaller than planets. Most of the them are situated between the planets Mars and Jupiter, but some come close to the Earth.
Comets have different orbits to those of planets, spending much of their orbital time far from the Sun. They are similar in size to asteroids, but are made of dust and ice. The ice melts when the comet approaches the Sun, and forms the comet’s tail.
Gravity and orbits
The orbits of the planets in the Solar System are almost circular - with the Sun near the centre. Many diagrams show the orbits very squashed from top to bottom. This is to give a sense of perspective, or to fit the diagram into a page in a book.
Circular motion requires a centripetal force. Without it, the object will fly off in a straight line. The Sun's gravity keeps the planets, dwarf planets, comets and asteroids in orbit. The gravity of a planet keeps its satellites in orbit.
Other solar systems: exoplanets
 |
An exoplanet is a planet outside our own Solar System, orbiting another star. It is now known that a big fraction of all stars have planets, suggesting that tens of billions of exoplanets must exist in the Milky Way Galaxy alone. The discovery of exoplanets has increased interest in the possibility of extraterrestrial life. Another name for exoplanet is extrasolar planet. |
|
| The HR 8799 planetary system, imaged in infrared. The blurred blob in the centre of this image represents the host star with most of its light blocked off, so that the three giant orbiting planets (the red dots) can be seen. (Credit: National Research Council Canada). |
[ This page has been adapted from www.bbc.co.uk/schools/gcsebitesize/science
]