Why are planets round, when other objects in space have funny shapes?

Discovery’s Director of the Henry B. duPont III Planetarium, David Mestre, explains.

David Mestre
David Mestre, Director of Discovery’s Henry B. duPont III Planetarium

There are a couple of reasons. One reason is part of the working definition scientists use for what makes an object a planet is that it IS round. Some objects in space are peanut or potato-shaped, for example.  These are not planets.

The key to what MAKES planets round is gravity. If an object has enough mass (meaning if an object is large enough), gravity will naturally act upon it. As this happens, the object is evenly attracted toward itself, which gives it its round shape, though many planets are not perfectly round or perfect circles.

Being big and round are not the only traits required for an object to be called a planet. For that to happen, the following 3 conditions must be satisfied:

  1. It has to orbit a star (the sun, for example)
  2. It has to be big enough for gravity to act upon it more than any other force, which makes it round (this is why part of the working definition for a planet is having a round shape)
  3. It has to be the only thing in its orbit (no two planets share an orbit)

If there are other things in a large, round object’s orbit, then it can be classified as a DWARF planet. That’s the deal with Pluto, whose orbit crosses the orbit with Neptune, a much bigger planet. Plus, there are other dwarf planets near Pluto that are like Pluto, so if we call Pluto a planet, then everyone will want to be called a planet, and that is annoying. A number of nearby dwarf planets are located in a region that is known as the Kuiper Belt.

Artist depiction of the Kuiper Belt
Artist’s depiction of the Kuiper Belt

Gravity is also working on the sun and other stars, giving them their round shape.  Stars, however, are much bigger than planets and since they are larger, their gravitational pull is tremendous.  At the core of a star, the gravity creates such monstrous heat and pressure that reactions like thermonuclear fusion (changing hydrogen into helium) take place rapidly and easily.  These reactions enormous amounts of energy (light and heat).

So, when we think about why planets are round, it has everything to do with size and gravity.

Want to read more about planets?

  • Scientists have published data about a possible Giant planet lurking in the outer reaches of our solar system.  Click to read more!
  • Right now, and again later this year, if you go outside at the right time you can see five planets aligned in the night sky.  Click to read more!
thumb_Ask a Scientist #1: Why Are Planets Round?