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Our planet, has been formed by the accumulation of very small dust particles that were part of a large disc (planetary disc) orbitating around our star in the first times of its life.
When in a certain zone of this disc the dust accumulated more by chance, the weak gravitational force of this amount of dust began to attract other dust around and the more the dust was concentrated in that point, the more strong its gravitational attraction became.

More of these first nuclei could join together to form bigger ones, so that a planet started to form.
After many millions of years, the planet has already formed so that the dust, compressed by the gravity force, now enormous, has become rock, often melt at very high temperatures in the core of the planet, like inside our Earth, where the magnetic fields created by the great amount of iron and the energy released by radioactive elements, keep melt, hot and perhaps liquid that rocks.
The planet continues to orbit around its sun swiping away all the dust it finds along its path until the planetary disc of dust doesn't exist any more, except for some residual dust that continues to be attracted by the planet, in more and more reduced amounts, but NEVER reaching zero.
So, still today, also our Earth continues , after 4,5 billions of years since its formation, to attract some dust from the surrounding space.
The mass of this dust is very little if matched with Earth dimensions, but still about 500-1000 tons/years.
If our Earth were much more massive, like Jupiter and Saturn, it would attract also the gases in the space (H2, He, and CH4) lighter and more volatile than dust, formed by heavier elements, like metals and, infact, those planets are mostly gaseous, while the Earth, like Venus, Mercury and Mars is rocky, only made from dust.
But why, in a solar system like our is, the rocky planets are the closest to the star and the gaseous are the farthest, with the exception of Pluto?
Because the solar wind of ionizated particles emitted by our star, invested the planetary ring and pushed the gases farther than dust particles, made of more heavy elements that, instead, tended to remain near the Sun to create rocky and dense planets.

Learn more about this author, Aldo Bonincontro.
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