Anthony Garcia wrote in to ask, “Why are planets perfect spheres, or at least appear to be perfect?”
Nature loves spheres. It can’t get enough of them.
Soap bubbles are spherical because that shape most efficiently balances the outward pressure of the air within the bubble against the surface tension of the soap film.
When water splashes and for a brief instant a droplet of water is neither rising nor falling and is momentarily weightless, what shape does the droplet’s surface tension force the water to take? A sphere.
Nature loves spheres. In the case of bubbles and droplets of liquid, surface tension creates a sphere to minimize surface area.
Stars are perfect examples of natural spheres. The mass of a star is mind-bogglingly large and creates an equally mind-bogglingly large amount of gravity. What shape does Mother Nature give to so much mass to minimize its enormous volume? A sphere.
Stars like our Sun are huge, dynamic, energy-producing concentrations of Hydrogen and Helium, compacted by their enormous gravity into spheres.
The reason planets appear spherical is because gravity compresses the planet into a shape that most evenly distributes the gravitational force among the planet’s mass.
Whether it is shaping water droplets, stars, soap bubbles or planets, nature seeks to minimize the surface area needed to contain a given volume, and the shape that keeps volume at the absolute minimum a sphere.
Any object in weightless space larger than a couple of hundred miles in diameter has enough mass for its gravity to overcome large-scale irregularities and force it into a spherical shape. This gravitational compression also generates significant amounts of heat at the center of the planet. This heat melts, or at least softens, any solid materials within the planet, facilitating the planet’s collapse into a spherical shape.
Objects in space smaller than about 100 miles in diameter, such as most asteroids, comet nuclei and small moons, lack the mass to create a gravitational field strong enough to compress themselves into spheres. These little worlds often take on what I call the “sick potato” look.
The 12.5 mile-long, 7.5 mile wide asteroid Gaspra, imaged in October 1991 from a distance of 1,600 miles by the Galileo spacecraft en route to Jupiter.
A really large asteroid, such as Ceres (diameter = 600 miles), has enough mass for its gravity to compress it into a sphere.
The 600 mile-wide asteroid Ceres as seen by the Hubble Space Telescope.
However, “perfect” spheres are hard to find in space.
Pretty much everything is space rotates, and rotating a non-rigid sphere causes it to “bulge” at its equator from the centrifugal forces acting on it.
This spinning distorts large planets into a slightly squashed shape known as an “oblate spheroid.” This means that a planet’s diameter measured through its poles is smaller than the diameter measured through its equator.
Whereas the difference between the polar diameter and the equatorial diameter of Earth is a barely noticeable 0.3%, the oblateness of Saturn, a large, gaseous and rapidly spinning planet, is greater than 10%. You can easily see Saturn’s polar flattening through a telescope.
Saturn's polar diameter is 33,700 miles, but its equatorial diameter is 37,360 miles.
There may not be such a thing as a “perfect” sphere in nature, but there is no doubt that spheres, nature’s favorite shape, are perfectly lovely.
thnx i need more info on dis for my science project. by any chance you could put more details so i could pass my science class.
-Ms.Harwood’s student
Corey,
You might try looking at a couple of other places that talk about why planets are spheres. For example:
http://curious.astro.cornell.edu/question.php?number=19
and
http://www.pa.msu.edu/sciencet/ask_st/031198.html
Good luck with your homework!
Seth
Thanks for the information. It seems odd that we have so many spherical things in our universe yet very few of them (or none of them) are actually spheres?
My brother would like to know how the gaseous planets stay spherical… Does this apply to them too? And if not, could you explain why the gaseous planets stay spherical??
Amber – love the question!
Yes, gravity will cause any object in space with significant mass to collapse itself into a spherical shape – including gasses. That’s why stars, including our Sun, are spherical, too. Even if all you’re talking about is a lot of hydrogen and helium, if there’s enough of it to exert a significant gravitational pull on itself then you’ll end up with a spherical collection of matter as the mass redistributes itself to respond to gravitational forces equally from the gravitational center of its mass.
Notice that I’m saying “spherical” rather than “perfect spheres.” If the only force acting on a collection of matter was gravity, then you would end up with a perfect sphere. However, there are generally a lot of other forces at work on stars and planets, rotation being chief among those forces.
Centrifugal forces cause a rotating sphere to bulge at the equator, causing the equatorial diameter of the sphere to increase and the polar diameter (the diameter measured between the sphere’s north and south pole) to decrease. The faster the rate of rotation, and the lower the density of the sphere (star or planet) the greater the equatorial bulge.
And since pretty much everthing in the Universe is spinning, pretty much all spheres are thought to actually be “oblate spheroids.”
One type of massive collection of matter that is almost certainly spherical _and_ almost certainly rotating very rapidly is a Neutron Star. A Neutron Star’s surface gravity is fantastically powerfull, and they often spin at a rate of several rotations PER SECOND. Are Neutron Stars oblate, too? Or is their surface gravity so powerful that in spite of their rotation they are somewhat oblate?
Maybe someday someone will get close enough to one to make such a measurement. That’s the stuff of sci-fi stories.
I also need more info on why are they actually spherical, but anyhow I cant complain as I enjoyed your piece of knowledge.
Bence, spheres are nature’s way of evenly distributing gravitational attraction. Every part of a sphere at any given distance from the sphere’s center experiences exactly the same gravitational attraction as any other part of the sphere at that same distance. Think of any shape other than a sphere and you’ll quickly see lots of places on the objects surface are signifcantly different in their distance from the object’s center of mass. That’s fine for small objects like asteroids, comets and spaceships, but for anything that’s more than a couple of hundred miles in diameter the gravitational forces will just naturally pull the mass into a spherical shape in order to evenly distribute the mass around the center of gravity. I hope that helps – Seth
love the web site. going to tell my astronomy teacher about this website. AWESOME LAYOUT
Thanks! We love hearing that kind of feedback.