Science Questions

Why are planets round?

Mon, 14th Apr 2014

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Paul Anthony Weston asked:

Why are planets round? Why not other shapes?


Tamela - That's a great question. It comes back to this idea that Ė well, this Figure 1: An artistknowledge that gravity is a central force, it's acting radially inwards and pulling mass, anything with mass towards its centre of mass. So, to go back to the formation of planets, we expect that stars and planets wouldíve condensed and collapsed down from these massive clouds of gas and dust, these nebulas. As they were collapsing, they were attracting other bits of matter and the natural equilibrium state is for everything to be as close to that centre as possible and to be in a sphere. It wants to be equidistant in a sense. Bigger objects have more mass and that strength of gravity is greater. So, it really starts to smooth without the surface. A smaller object might be a cube and material properties of that cube can keep it up without gravity pulling it into a sphere.

Chris - Itís a bit like raindrops I suppose isnít it because you've got a droplet of rain coming down. If it can forms a sphere, but then it sort of smears out a bit with the air current pushing it. Itís the same sort of phenomenon Ė the water sticking, pulling itself together.


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A spherically shaped planet represents the minimum amount of energy that the body can have. If the shape is otherwise then any action taken by gravity will act to reduce the amount of energy and that means driving toward the shape of a sphere. Pmb, Sat, 5th Apr 2014

Radioactive decay will tend to heat the larger solar-system objects, and allow them to "differentiate", separating out the dense metals near the center, and the less-dense silicate minerals near the surface. Collisions with other solar system objects will provide additional heating (provided the collision is not big enough to fragment the object). During this molten state, an object's own gravity will pull it into a near-spherical shape (although it may have a slight equatorial bulge if it was rotating quickly; the Earth has a slight equatorial bulge).

Looking at a list of spherical objects in the solar system, it appears that this differentiation will happen to objects larger than 400-500km radius - which includes all the planets, many moons plus the larger asteroids.

It is likely that this happened to many more objects in the solar system, only they have been subsequently shattered by collisions. We know this because many meteorites show the differentiated nature of "stony" meteorites vs "nickel-iron" meteorites. evan_au, Sun, 6th Apr 2014

It is also because earth's gravity for instance pulls equally in all directions from its center, almost as much as it pulls to its surface from all directions. The center of the earth is virtually weightless. If there were more gravity production in one portion of the earth, say a very large continent with seas on the other side of the globe, as in early earth, that large land mass would pull more strongly from the center, tending to bulge itself upward and split. It seems the western hemisphere was forced away from the larger area of present day Europe, Africa and Asia in this manner The earth's crust is only floating on the magma after all. It is moving around with the upward surges that its own gravity produces and is trying to equalize itself. There are more details at Floyd Baker, Thu, 9th Apr 2015

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