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Gravity really - if we didn't have gravity big chunks of the earth would break off and spin uncontrollably in to the great expanse of the universe. Also bits off the sun would find themselves hurtling in our general direction - not healthy lol :)

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Each of the small bits of gas & rock had their own (tiny) gravity. That's what caused them to attract each other in the first place, so they'd all stick together. After that you got a sort-of avalanche effect, where the more bits of "stuff" that came together all added their gravitational force, which made the combined gravity stronger, which attracted more bits.

The only thing that limited the size of the various planets was the amount of "stuff" close enough to the planets' orbits to be attracted to them.

The reason that planets are mostly spherical (unless something bad happened to them) is that a sphere contains the most amount of stuff, closest together. Although there is a secondary effect due to the spinning of the planets, which tends to flatten them out very slightly, so that top-to-bottom is slightly smaller than side-to-side.

Edited by pete_l
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Now you're getting technical lol - but it's all down to gravity pulling stuff together from large and dense clouds of molecular dust. Sometimes a sun (or star) is formed followed by planetary formation within the star's gravitational field. Give this a good read - seems to cover it in some detail:

Formation and evolution of the Solar System - Wikipedia, the free encyclopedia

Hope that helps :)

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As well as gravity, the other crucial ingredient is heat (which arises through gravity). If the object is large enough then pressure at the centre caused by the weight of material above produces melting, so that the object assumes a more spherical shape. The densest material tends to sink to the centre, so the object typically ends up with a metallic core.

Small asteroids are far from spherical: they have come together through gravity but their interiors have never experienced enough pressure to melt. If two small asteroids gravitate together, the result can be a dumbell-shaped object (e.g. asteroid Toutatis).

Edited by acey
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I can offer a quite different explanation! They are approximately round by definition. It was agreed that, in order to exclude poor Pluto and his fellow potato shapes, only bodies sufficiently massive to have gravitational fields capable of pulling themselves spherical would merit the title planet...

Only joking, it's the same explanation with a twist.

Olly

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Olly, Pluto is extremly spherical and nowhere near potato shaped. Pluto is kept out the planet club on grounds it has not swept its orbit clean of other bodies, what a lazy lazy dwarf planet...

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So did earth have gravity when it was first formed to pull it all together ?

Gravity is simply the term we use to describe how the mass of a large object (such as the Earth) attracts other objects to it. Indeed, anything with mass pulls on everything else around it and so can be said to have "gravity". The greater the mass the greater the effect and that's why a large object overcomes the attraction of a smaller one. Indeed, in still waters two objects of sufficient mass will be attracted to and drift towards each other (as I think was noted in the film "Master and Commander- The Far Side of the World").

Of more interest (and one physicists are still pondering) is what gives objects mass. This is why scientists are searching for the elusive Higgs Boson at CERN and Fermilab (and evidence for which may have recently been found).

Brinders

Edited by Brinders
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As well as gravity, the other crucial ingredient is heat (which arises through gravity). If the object is large enough then pressure at the centre caused by the weight of material above produces melting, so that the object assumes a more spherical shape. The densest material tends to sink to the centre, so the object typically ends up with a metallic core.

Small asteroids are far from spherical: they have come together through gravity but their interiors have never experienced enough pressure to melt. If two small asteroids gravitate together, the result can be a dumbell-shaped object (e.g. asteroid Toutatis).

Great answer , its all starting to make sense now , Thanks :)

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Olly, Pluto is extremly spherical and nowhere near potato shaped. Pluto is kept out the planet club on grounds it has not swept its orbit clean of other bodies, what a lazy lazy dwarf planet...

By Gad, you're right! My apologies. I'm not very 'planetary' and thought from (defective) memeory that Pluto was pretty irregular. I must have been thinking of Charon.

Thanks for the correction.

Olly

Edited by ollypenrice
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