Why all planetary objects are spheriacl in shape,but not asteroids?

Question

earth ,sun,moon...........all.Even when we look up & see sky we find it circular......WHY??????

Answer 1

For a celestial object to be considered a planet, it must satisfy two conditions:


The object must be in orbit around a star, but must not itself be a star
It must have enough mass for the body's own gravity to pull it into a nearly spherical shape

Answer 2

Gravity is a central force. This means that it tries to pull everything as close to the center of the mass generating the gravity as possible. This is why water spreads out when you spill it. If you have a pile of water, the stuff on top flows down the sides to get closer to the center. If you imagine a big blob of water in space, forgetting for the moment that it would tend to boil in vacuum, its own self gravity will try to pull it all to its own self center. It can't all get there because there is more water underneath it. A sphere is the shape at which all of it can be as close as possible to the center. A physicist would say the water is at its lowest potential energy.

Now, why don't smaller objects like asteroids go round under their own self gravity? This is for the same reason that a rock won't spread it when you put it on the ground. There are other forces opposing the gravity. In this case it is the forces of the chemical bonds within the rock that resist the atoms moving apart from one another. In a liquid, the molecules are not bonded to one another, so they are free to move individually (another weaker force, called surface tension, is what keeps a spill of water from spreading out until it is only 1 molecule thick, and gives it its characteristic curve at the edge of the puddle.)

In an asteroid, these same forces are strong enough to keep the thing from becoming a sphere. In larger objects, the pressure is large enough that even rock can flow, almost like a liquid, so it forms a ball. This kind of motion is called plastic flow, and is why mountains can only get so tall on earth. The biggest mountains are large enough that the rock at their base inside becomes plastic and tends to squish out from under. Mars, with its lower gravity, can have much larger mountains, such as Olympus Mons.

Answer 3

Many celestial bodies whether stars, planets or moons have properties that make them spherical.

Firstly consider stars. They are comprised mainly of incandescent gases with an extremely high gravitational pull. It would be very unusual for them to be any other shape.

Secondly Planets. They are mainly comprised of matter ejected from stars or explosions of other larger bodies and in their formation would have been molten and probably spinning. Again it would have been un-natural for them to be any other shape.

Moons. Many moons are offshoots from the forming planets and, again would have been molten or very hot so the spherical shape again would be the one adopted. Some moons are not spherical but mis-shapen depending on their size.

Asteroids are mainly caused by the collision of drifting objects in space thus causing them to break into irregular shapes. As this is not a heat generating explosion the pieces do not become spherical and will go into orbit around the sun at whatever point in the solar system they are.

Answer 4

Asteroids are simply chunks of rock and metal that orbit the sun in the same way that planets orbit. They come in all sizes and shapes. Some are as big as boulders, some are as big as mountains, and some are as big as the moons that orbit many of the planets. They can be globe-shaped or pear-shaped or have strange irregular shapes like any other big rock. Their shapes often change as they smash into each other as they orbit. The smaller bits and pieces that break off asteroids are called meteoroids.


I think that planetary objects are mostly spherical (or close to it) because of their large mass and therefore large force of gravity. Asteroids are generally not as big and their gravity therefore is not strong enough to make them spherical. They also smash into each other constantly, as said above, and therefore have irregular shapes.

Answer 5

The more massive an object is, the better chance it will be spherical. More mass = more gravity. Gravity pulls toward the center of mass (on earth it's called the center of gravity because we are already object to the Earth's gravity), thus the larger the object, the more proportionate an object's mass will be from its center. This is what causes the spherical shape. Asteroids are (for the most part, or they would be called planetoids or plutons) simply to "small" to be spherical.

Answer 6

Planets (...and also stars) form within immense drifting clouds of gas and dust. Every bit of gas and dust generates a tiny gravitational force. In relatively dense regions of the drifting cloud (...called a 'nebula') two or more bits of gas or dust may be close enough so that they're attracted to each other by their gravity. They clump together, which increases their overall gravity. This larger clump is surrounded by more bits and pieces which are drawn in from all directions. The original clump grows in size and produces more gravity, which in turn pulls in even more material from all directions. The result is a growing sphere.

Asteroids are simply planets that didn't quite grow up :)

Answer 7

Very few are circular, just in our little solar system, we've only found 8, not including the sun and moons. But that's not very many when there are billions of objects circulating within our own little solar system. There are billions of 'suns' with their own solar systems, and all of those are within our 'universe' and we have found that there are more universes beyond ours, so billions and billions of more suns and solar systems. I wonder if any of the entities out there speak english? lol

Answer 8

Because they all are circling on their axis but asteroids are not circulating on their axises.I need 10 points.

Answer 9

because of gravity
because your living on a circular planet

Answer 10

it's the gravity thing.