Embedded Files
Planets are very large objects, held in orbits around the Sun by gravity.
Gravity
Gravity
We know that all objects have mass, and that masses are pulled together by a force known as gravity.
The force of gravity is calculated as F = G.m1m2/r2 where m1 and m2 are the two masses and r is the gap between them. G is the gravitational constant (6.67x10-11 m3.kg−1.s−2).
You can see that this looks similar to the equation for Electric force between charges, which follows the same 'inverse square law'.
The Earth has a very large mass, which is why on Earth we feel the force of gravity pulling objects downwards towards the ground. We call this force the 'weight' of the object.
But the 'weight' of that object would be very different on another planet, because other planets have different masses.
It's also why the Moon is held in orbit around the Earth. The Moon and the Earth are pulled together by gravity. The only reason it doesn't fall to Earth is that it is moving at a speed of 1000metres/sec in orbit around the Earth - the outward pull of the fast orbit balancing the inward pull of gravity.
The force of gravity depends on the masses of each object, and the distance between them. There is equal force on both objects.
Image credit: Dennis Nilsson, CC BY 3.0 <https://creativecommons.org/licenses/by/3.0>, via Wikimedia Commons
The Solar system
The Solar system
We live on Earth which is just one planet in the solar system.
It's called the solar system because at the centre is the Sun (Sol) which is a medium sized star.
The Sun is a huge hot ball of gas, which glows brightly with a yellowish colour and has a huge mass. So it's pull of gravity on the planets is also huge.
The planets move around the Sun at very high speed, the outward pull of their orbits balancing the inward pull of gravity. (The Earth's orbit speed is 30,000metres/sec).
The planets do not slow down because there is no friction to slow them down.
If you look at the night sky you will see billions of stars. They will all have a system of planets orbiting them. Some will have more, some less. Some stars will be huge, some smaller. You might notice that some are blue, some white/yellow, some red, depending on their size and age.
Artistic rendition of our solar system. (Image credit: NASA/JPL).
The Earth and Planets
The Earth and Planets
The Earth is the planet which is best suited for Human life, because it has a breathable atmosphere, the temperature is generally ideal (for liquid water), and the force of gravity is not too high.
The inner planets (Mercury and Venus) are close to the Sun and so are far too hot. Venus also has a toxic atmosphere.
Mars is colder and has a very thin, non-breathable atmosphere.
The other outer planets (Jupiter, Saturn, Uranus, Neptune) are all huge gas giants, made up of toxic gases, and with no surface to walk on.
The Earth is constantly spinning. One whole spin equals 1 day.
Each planet spins at different rates, meaning their 'days' are different lengths.
The Earth does 1 orbit of the Sun in a year.
Each planet orbits the Sun at different rates. So their 'years' are different lengths.
The Earth is constantly spinning, with one whole spin taking 1 day. One year is the time the Earth takes to complete one orbit of the sun. The moon orbits the earth in ~27days, i.e. ~ 1 month.
Size of solar system
Size of solar system
The distances in the solar system are huge.
Even though the Earth is travelling at 30,000metres/sec it still takes 1 year to orbit the Sun.
The outer planets have much longer orbits.
It takes the light of the Sun over 8 minutes to reach the Earth. So if the Sun went dark we wouldn't know about it for 8 minutes.
It takes over 4 hours for sunlight to reach Neptune.
Distance to other stars
Distance to other stars
Our Sun is just an average star, one of hundreds of billions of stars in our galaxy (the Milky Way).
Our galaxy is just one of hundreds of billions of galaxies in the Universe.
Our nearest star (apart from the Sun) is Alpha Centauri. It is 26 trillion km from Earth. This means that its light takes 4 years to reach us.
It also means that travel to this star is very unlikely. The fastest a spacecraft (unmanned) is planned to travel is the Parker Solar Probe at around 200km/sec. Even if we could travel to Alpha Centauri at this speed it would take over 6,000 years.
The nearest large galaxy to our Milky Way is the Andromeda galaxy. It is 2.5 million light years away. We can easily see this galaxy in the night sky.
So the light from the Andromeda galaxy takes 2.5 million years to reach us.
You'll notice that we measure these huge distances in light years - 1 light year is the distance travelled by light in 1 year. This is because if measured in km, the numbers would be too big.
Page updated
Google Sites
Report abuse