Why are planets spherical but meteorites irregularly shaped?

31 October 2010

Question

Hi Chris,

Thank you SO very much for all your truly informative topic's of discussion. You just reaffirm how small we really are in this spectacular universe of ours!

Chris, I hope this is not a stupid question, but something has puzzled me a little bit in the observation of our planet earth, the planets, the moon, the sun,

the stars. They all seem to be in a very distinguished shape and that being of a circle. Such a perfect shape?

How come we see meteorites hurtling through space, being odd shaped, distinguishably irregular? Their composition being made up of metals/rock etc?

Thank you Chris, your valued explanation would be truly appreciated for this puzzled mind of mine. J

Regards

Karin

Answer

If you look at things from a long way away like the Sun, like the Earth, like Mars - yes, they do look like big circles in the sky, that's true. If you zoomed in a bit closely, you'd see that actually, the surfaces are quite lumpy and bumpy. The Earth has mountains and volcanoes, and so does Mars.

So the surface isn't completely smooth, but yes, they have been pulled into a circular shape. Whereas, smaller objects like things in the asteroid belt can be an irregular shape.

What's different between the things in the asteroid belt and big planets like the Earth and even big blobs of gas like the Sun is that the Earth, Mars and the Sun are very big, therefore they have enormous amounts of mass, and therefore, they have an enormous amount of gravity. And what that gravity is doing is pulling all of the particles together, and the way in which the particles can arrange themselves so they are as close as they can be to each other is into a spherical shape. It gives you the best surface area to volume ratio. So in other words, everything is surrounded by everything else and pulling towards everything else as tightly as it can.

A smaller object like an asteroid doesn't have the same mass. It's much smaller and they're much therefore more loosely bound together and there isn't the gravity to pull the material together into a spherical shape. If you kept adding material then it would accrete slowly to make a planet.

In fact, the asteroid belt is a failed planet. It's rubble left over from planet building, probably because there wasn't sufficient gravity there to pull a planet together and hold it together, up against the gravitational tugs of all of the other planets forming in the solar system. So that's really the reason.

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