Why are all the planets round, yet so many asteroids are all different shapes?

Question

I just visited the Griffith Observatory in Los Angeles. They had models of all the planets in our solar system, and they were all nearly perfectly round. The tour guides said that they really are shaped that way, but could not adequately explain why. When I saw pictures of the asteroid belt, it showed many different shapes, so it just doesn't make sense that the planets are round balls, regardless of their size, and asteroids are not.

Answer 1

the planets have a spherical shape because of their volume, mass and rotation.

Answer 2

Planets are round because their gravitational field acts as though it originates from the center of the body and pulls everything toward it. With its large body and internal heating from radioactive elements, a planet behaves like a fluid, and over long periods of time succumbs to the gravitational pull from its center of gravity. The only way to get all the mass as close to planet's center of gravity as possible is to form a sphere. The technical name for this process is "isostatic adjustment."

With much smaller bodies, such as the 20-kilometer asteroids we have seen in recent spacecraft images, the gravitational pull is too weak to overcome the asteroid's mechanical strength. As a result, these bodies do not form spheres. Rather they maintain irregular, fragmentary shapes.

Answer posted July 30, 2001


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UNDERSTANDING PHYSICS, VOLUME I, MOTION, SOUND and HEAT
by Isaac Asimov

An object at sea level, for instance, may be 6370 km from the center of the earth, but at the top of a nearby mountain it may be 6373 km from the center, and a stratoliner may take it to a height of 6385 km from the center.

Even if we confine ourselves to sea level, the distance to the center of the earth is not always the same. Under the action of gravity alone, the earth would be a perfect sphere (barring minor surface irregularities)-a fact pointed out by Aristotle-and then the distance from sea level to the earth's center would be the same everywhere. A second factor is introduced, however, by the fact that the earth rotates about its axis. This rotation means, as Newton was the first to recognize, that the earth cannot be a perfect sphere.

As the earth rotates about its axis, the surface of the earth is continually undergoing an acceleration inward toward the center of the earth (just as the moon does in revolving about the earth). If this is so, then Newton's third law comes into play. The earth's center exerts a constant force on the earth's outer layers to maintain that constant inward acceleration as the planet rotates; the outer layers must, therefore, by action and reaction, exert a force outward on the earth's center. The force directed inward is usually called a centripetal force, and the one directed outward is called a centrifugal force (the words coming from Latin phrases meaning "move toward the center" and "flee from the center," respectively).

The two forces are oppositely directed and the result is a stretching of the earth's substance. If you were to imagine a rope stretching from the earth's surface to the earth's center, with the earth's surfaces pulling outward at one end of the rope and the earth's center pulling inward at the other end, you would expect the rope to stretch by a certain amount; the earth's substance does exactly that.

If every point on the earth's surface were rotating at the same speed, the stretch would be the same everywhere and the earth would be perfectly spherical still. However, the earth rotates about an axis, and the nearer a particular portion of the earth's surface is to the axis, the more slowly it rotates. At the poles, the earth's surface touches the axis and the speed of rotation is zero. At the equator, the earth's surface is at a maximum distance from the axis and the speed of rotation is highest (just over 1600 kilometers an hour).

The interacting forces are zero at the poles, therefore, and increase smoothly as the equator is approached. The "stretch" increases, too, and a bulge appears in the earth, which reaches maximum size at the equator. Because of this equatorial bulge, the distance from the center of the earth at sea level at the equator is 21 km (13 miles) greater than the distance from the center of the earth to sea level at either pole.

The earth, therefore, is not a sphere, but an oblate spheroid.

To be sure, 21 km in a total distance of 6370 km is not much, but it is enough to introduce measurable differences in the value of g. What with the equatorial bulge and local differences in altitude, there are points in Alaska where the value of g is over 9.82 m/sec2, whereas at the equator it is barely higher than 9.78 m/sec2. That represents a difference of nearly one-half of one percent and is reflected in weight.

In other words, the weight of an object changes measurably from place to place on the earth's surface, as a spring balance would show. A man who weighs 200 pounds at the poles would weigh 199 pounds at the equator. To a chemist or physicist interested in the mass of an object (many properties depend on the mass), the measurement of weight as a substitute for mass would introduce serious inaccuracies.

Answer 3

Gravity and mass. Planets have such mass and material to create gravity. Asteroids do not have enough gravity to pull them into a sphere. Also, asteroids are broken off chunks of other larger objects such as planets. When the planet was created, it was molten, the gravity pulled it into a sphere and then it cooled and stayed that shape. The asteroid was already cooled when it was broken off.

Answer 4

An object has to be about 500 miles in diameter for gravity to have a decent shot at pulling it into a roughly spherical shape. The larger asteroids (Ceres, Pallas, Juno, Vesta, and some others) are spherical.

Answer 5

Because space is a vacuum, and the planets are spinning. When something spins in a vacuum it becomes an orb. Truth be told though, the Earth is shaped more like an EGG than a ball. Also, the astroids don't have enough mass to spin and become circular. As they travel through space they lose material from their tails (that's why they look spikey),

Answer 6

The planets are round because their own gravitational force. A planet has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (near spherical) shape. Asteroids are not large enough to have significant gravitational force to make them round.

Answer 7

Planets are more massive than moons in general. A small moon or asteroid doesn't have enough gravity to pull its shape into a sphere, but a large moon or a planet does.

Answer 8

Planets have just as many imperfections on the surface as asteroids (just look at how many hills, mountains, and cliffs there are on earth), but planets are so big that the surface imperfections are much less noticeable.

Answer 9

To be honest the Earth not not round at all. If you remove all the water from a model, you will find it is sorta shaped like an asteriod, with mountians and valleys .

Answer 10

They have a much bigger mass and therefore have a much greater gravitional effect. They are able to pull themselves into a round shape while asteriods with a smaller mass arn't able to replicate the same effect