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The sun is a star, which is primarily hydrogen gas which has gathered in significant volumes to heat the core and start a nuclear fusion of hydrogen into helium.

In our solar system, we have generally two types of planets:
The inner planets, Mercury, Venus, Earth and Mars which are solid, such as the earth with a crust.

The outer planets in our system, JUPITER, SATURN are so called gaseous planets, which means they are made primarily of collections gas, similar to the sun. These planets consist of hydrogen and helium.

Uranus and Neptune are also gaseous planets, but they also have methane gas in addition to hydrogen and helium, and both have rocky inner cores consisting of frozen water. The inner cores are as big as earth itself.

If Jupiter or Saturn during their creation had collected more gas, they could have become the sun. As it is today, they do not have enough mass to start nuclear fusion even as their cores are heating up due to the gravitational pressures.

In other words: some planets consist mainly of iron, carbon, methane and other heavier elements. In our solar system, it is especially the inner planets which have these heavier elements which form the "solid" surfaces we associate with our own earth.

Other planets are large collections of various gasses, primarily helium and hydrogen which account for 99% of the primordial elements in the universe. Collections of hydrogen gas which become so big (and create so high pressures) that nuclear fusion starts to take place turned into stars. Other collections of hydrogen gas may exists as hydrogen planets.

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15y ago

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