Why are planets such as Earth colder than the Sun?

When the Solar System was formed, almost all the dust and gas in the original cloud was drawn to the centre by gravity. The pressure and temperature caused by the compression of that gas caused the Sun to initiate nuclear fusion in the core and the Sun was born. The rest of the gas and dust gradually coalesced into the planets and other bodies, and at first, those bodies were pretty hot, again because of the pressure produced by gravity. However, being a great deal smaller than the Sun, nuclear fusion never got started, and everything apart from the Sun cooled quite quickly in astronomical terms. Of the whole Solar System, about 99.8% of the matter is in the Sun, so you can see the planets are very, very much smaller.

Based on your question I am assuming you don't really understand what the difference is between a star and a planet.

As a result of the physical properties of the Sun, it has a luminosity of about 2x10^30 watts which is basically 2 with six millions after it watts. The Sun produces that energy or rather emits it into space every second.

The physical properties of the Earth meanwhile are completely different and do not produce any energy in the same context as the Sun. The very very small amount of solar radiation which reaches Earth is simply reflected back into space by the atmosphere.

Because the heat source is in the centre of earth, although the rocks that are on top of them are very thin - thinner than a apple skin if everything was to scale they are unbelievable insulation. The other fact to bear in mind, is if the planet were as hot as the sun we wouldn't be here discussing it - so in theory a planet could be as hot as star but there wouldn't be any kind of life on it.

The sun is a star. It is heated by the nuclear fusion inside it.

Earth is a planet. Just a peace of rock with only a little heat inside from radio activity and left over from its formation.

Maybe read a little in Wikipedia. It helps.

The Sun is a slow thermonuclear explosion - hot.

The Earth is a lump of rock (with a thin film of gas and water) - cold.

Because the Sun is a flaming ball of gas & the planets aren't

Warrior Knights assemble! This face-palm is far too great for me to do alone. My brothers prepare for this may be our last battle against the dark barbarians of retards.