Why are Planets Round?

Criteria for being a planet As telescopes became more powerful, scientists were able to see farther, and in more detail, than ever before. Very quickly, several planet-sized objects were discovered far beyond Pluto, orbiting our sun exactly as Earth and its neighbors. But are they actually planets?

This led to a debate among scientists around the world - what exactly IS the definition of a planet? Up until now, a planet didn't really have a definition other than "a big round thing that went around the sun". But the discovery of these objects underscored the need to come up with a better definition of the term "planet".

After much discussion, a new set of criteria was established; if an object met all three of these criteria, it would be considered a "planet":

Must orbit the sun
Must be massive enough for gravity to compress it into a round shape
Must have cleared the area surrounding its orbit

This third requirement resulted in the removal of Pluto from the list of known planets; Pluto's orbit is heavily influenced by Neptune's gravity, and Pluto passes very near other, smaller, objects that are also orbiting the sun.

But what makes a planet assume a round shape?

If you place a small amount of water on a surface on Earth, the water flattens and spreads out as Earth's gravity pulls on all parts of it. However, there are other attractive forces working at the same time - cohesive forces between hydrogen bonds in neighboring water molecules act to hold water together (like a crowd of people linking arms with each other), and gravity between all of these water atoms are pulling them all together. Again, with the massive force created by the Earth's mass, these other forces are difficult to observe. As a drop of rain falls from the sky, it forms a roughly spherical shape (not a teardrop, as you might expect), although wind resistance flattens it out a bit.

It is only when water is poured out in the microgravity of space that you can see the effect that an object's gravity has on itself; water pulls itself into a sphere.

If the early planets were hot enough to melt, then their gravitational attractions from within would also pull their matter into a spherical shape. As they cooled, the planets would then be essentially "frozen" into this shape. Smaller objects either did not experience enough heating to allow them to form these liquid spheres, or are remnants of larger objects that broke apart after cooling. There is additional evidence to support this part of the theory, but it will be explored in a later module.

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