My granddaughter ask me why are planets round,and other space objects are not (asteroids etc)?

Question

My granddaughter ask me why are planets round,and other space objects are not (asteroids etc)?

Answer 1

According to the universal law of gravitation, there is a force of attraction between every two particles of matter that is directly proportional to the product of their masses, and indirectly proportional to the square of the distance between them.  That means, the more mass there is, the stronger the force of gravity.

Roundness is the most natural shape for a liquid to take in space because when all the particles are attracted to each other, a round object has the lease amount of potential energy.

Smaller objects, like astroids, do not become round because their hardness is not overcome by the force of gravity between the parts.  Larger objects, like planets, become round because there is so much mass, that the force of gravity is able to overcome the hardness of the planet itself, and it acts similar to a liquid.

But even planets are not PERFECTLY round and smooth.  Earth, for example, has mountains.  But if you'll notice, the ocean is a lot more smooth than the land.  That's because water flows more easily than rock, and is able to reach a lower state of potential energy.

Answer 2

Thats actually a very interesting question, and one that is hard to understand at first.

It all has to do with mass, pressure, and gravity.

Before we can discuss what goes on with these three, we need to know what they are.

Mass is the amount of matter in an object. Not its weight... that is the amount of gravitational force affecting the object. Mass is different. Its just the amount of matter in an object. Matter being anything consisting of atoms. That's actually a lesson your granddaughter is going to need in physics class, so its good to teach them early.

Pressure is an pushing force. A force can be one of two things: a push, or a pull. Pressure is a force pushing outwards. If you pick up a water bottle full of water, and squeeze on it, its hard to keep it that way... its the pressure in the bottle that increases. So basically, pressure is a force pushing outward when matter is compressed. The more compressed it is, the more pressure.

Gravity is a pulling force. Objects with a large mass have a large amount of gravity. The larger the mass, the more gravitational pull an object creates. Every object that has matter, and mass, has a certain amount of gravitational pull. Even a paper clip has its own gravitational pull. Because it is so small, it isn't noticeable at all, and can only be found through calculation. If you want to do something fun, weigh yourself in pounds, multiply it by 2.2 to get it in kilograms, and then multiple that by .000000000066726. That is your gravitational force in Newtons, the measurement of force named after the physicist, Newton.

Now that we know the basic concepts of pressure, gravity, and mass, we can discuss why certain objects are spherical, and why others are oblong, such as asteroids.

Ok, so lets start with planets. Planets are these huge spherical objects, with extremely dense cores. So all the other layers are being pulled into the dense core. This is a result of a large amount of gravity. Now, because this matter is being compacted, there is a pressure pushing outward. If the gravitational pull is large enough, and there is enough pressure, the gravitational force of the planet and the pressure become balanced, making a spherical object. Because these forces aren't perfectly balanced, the planets aren't always perfectly spherical. In Earth's case, we are a bit oblong. The equatorial diameter is lager than the polar diameter, basically meaning that the Earth being pulled... this is why we call Earth an "oblate spheroid". When the object becomes a spherical shape as a result of pressure and gravity becoming balanced, it is known as "hydrostatic equilibrium".

But when objects have lower masses, such as that of an asteroid, there is not enough self created gravity for the pressure and the gravity to become balanced, so it never forms into a sphere, and instead they are very oblong, and oddly shaped. Because gravity is a weak force, objects have to be very massive in order to obtain hydrostatic equilibrium. Asteroids simply don't have enough matter to have a large enough gravitational pull.  

Answer 3

Earth is spinning think of it like dough or clay if you spin it long enough it will become round. That's why they spin pizza in the air. Earth isn't clay or flour tho but after age and time it will shape into place.

Answer 4

Why is a bubble round? Spheres are the most efficient objects in the universe. In space, if one releases water, it's gravity will cause a spherical shape. Asteroids are the results of planets that have come apart due to different reasons, such as a core destabilization for instance. Chance however is not how the planets came to be, as all the objects in the universe show a sign of something other than chance. Earth at one time was a near perfect sphere, then something happened which caused the Earth to burst out from one area and turn the moon's face into the sea of tranquility you see today. Due to the type of the shot that occurred, the force that hit the moon had to be tens of billions of nuclear warheads of pressure to get that far and do that much damage. Since the moon's face always faces Earth an asteroid colliding with it is highly impossible because it would have had to swerve straight from earth to hit it where the damage is. The moon's surface does not turn, but it does move away from Earth, and that would be a reason why.

Answer 5

Because the core of most planets is hot and so layers of varying temperature of rock and magma will naturally form a spherical shape around the core to cover the most area possible. Asteroids however are just debris from other objects in space, and have no core which could sustain life or heat.

Answer 6

The short answer? Time & gravity & chance.

Answer 7

The planets, just like earth, are rotating around an axis.   Asteroids are just flying through space.