[Karin Page]

Karin Page  asked the Naked Scientists:
   
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

What do you think?

[chris]

Hi Karin

you are right, at low resolution (ie looking from a long way off), planets and stars appear to be round balls, while coments, asteroids and some natural satellites are often irregularly shaped.

Of course, the surface of a planet or a star isn't really smoothly spherical - Earth and Mars both have tall mountains (Olympus Mons is over 20km tall!) and the Sun has tendrils of plasma curling from its surface; but in the grand scheme of things these bodies are grossly "round" and the reason for this is gravity.

Gravity is a function of mass, so the more "massive" an object (like a star or a planet), the greater its gravitational field. Gravity acts through the centre of a mass, meaning that everything around the centre is pulled inwards.

Consequently, the arrangement or shape that enables the maximum amount of matter to get as close as possible to the centre is a sphere and hence large bodies naturally adopt a spherical shape under the influence of their own gravity.

So what about the asteroids, comets and some mini-moons (natural satellites) I mentioned earlier?

The answer is that these objects are generally much smaller than planets and stars and therefore have a lower mass and therefore a smaller gravitational field. Consequently, although their gravity is sufficiently strong to hold material together it isn't sufficient to pull that material into a spherical shape and so the object remains irregularly proportioned. If futher material was accreted, however, it would slowly assemble to form a sphere.

Chris

[kenhikage]

Karin, I think if you remove gaseous atmospheres from planets -- or artificially bind them to asteroids -- then the issue may be clearer.  []

[tommya300]

Karin, I think if you remove gaseous atmospheres from planets -- or artificially bind them to asteroids -- then the issue may be more clear.  []

Can you apply that to Jupiter

[kenhikage]

Who knows, but I would bet on it.

[kenhikage]

After hearing the podcast I see that I was wrong. [:0] Interesting that bit about, to paraphrase, if the Earth were shrunk to the size of a billiard ball it would be smoother than a billiard ball, despite mountains. I wonder if it's true for Mars. And, I wonder, if asteroids were shrunk to that size -- obviously they wouldn't be spheroids -- but, how smooth would they seem?  []