Gravity and the Universe

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In this section we will look at gravity much further away – how it works on other planets besides the Earth, how it holds our solar system together; and then further away again to ‘black holes’.

What are Planets?

We know that they are objects in space that rotate around a star. Stars are balls of gas which shine and twinkle as we see them through the atmosphere that surrounds the Earth. There are billions of stars in Space.

The Sun is a big star. From Earth it looks like a ball of yellow but it is really made up of lots of hot gases and flames. The Sun and this system of planets that orbit (circle) around it make up our Solar System. Asteroids (minor planets between Mars and Jupiter), comets, moons (bodies that orbit around planets) also form part of the Solar System.

Our Solar System was once regarded as the central part of the Universe. Today we know that it is just one speck compared with the rest of the Universe.

Planets and Gravity

Gravity keeps the planets orbiting around the Sun, and the moons orbiting the planets.

Neptune's moon
Courtesy of NASA

The Earth has just one moon. Most of the other planets have many moons, for example Jupiter has 16 known moons, and Saturn has at least 18. Only Mercury and Venus have no moons.

The planets and other objects which orbit the Sun travel in flattened circles called ellipses. If the pull of gravity is strong enough it will pull gases towards a planet or moon to form an atmosphere.

Flash

Did you know?

Pluto, the smallest planet and one of the coldest places in our Solar System (minus 230 C!), is no longer classified as a planet. In 2006 the International Astronomical Union re-defined the word ‘planet’, and Pluto is now classified as a ‘dwarf planet’.

There are now officially 8 planets of the solar system rather than 9!