Why can't a planet be other than spherical shape?

Question

I have always wonder why planets have spherical shapes. Does it have a scientific explanation for it?
or Is it one of the characteristic for a thing in space to be called 'planet'?

Answer 1

The reason planets have to be spherical is because of gravity. Smaller planets and moons are not always round-take Uranus' moon Miranda, for example. Gravity causes the edges of a planet to pull at one another, until the planet it rounded out. Also, the planets are spinning, which creates spherioid shapes. The reason it's not completely spherical is because if the rotation speed is too fast and the planet too large-take Jupiter, which is huge and spins once around every 10 hours. Jupiter bulges at the center, but it's still spheroid.
In another universe, maybe, where there isn't gravity, planets might be another shape. Personally, I would LOVE to see a doughnut-shaped planet (a torus). I saw a poster with this very question in my fifth grade classroom and it had a bunch of different shapes and the complications they would pose if they could exist, and I became obsessed with a torus-planet ever since.
And by the way, there are non-spherical objects in space, but they aren't planets. Asteroids aren't big enough (sometimes) to pull themselves into a sphere. Most are shaped sort of like pyramids. But I've yet to see a torus. :(

Answer 2

Gravity

The most efficient shape for mass is a sphere. Stars are spherical for the same reason. The gravity attraction and the mass has to be great enough to collapse the volume down to the most compact form and that is a sphere.

A spherical shape is a requirement for a planet because anything formed in the method that a planet forms will have to go into a sphere. Planets started off as clouds of gas and dust. They gathered together because of gravity and as the collection grew it got harder. Eventually the collection got hard and dense enough to be a planet. If the collection is too large then it becomes a star. Planets are just stars that never got big enough to self ignite.

Because of this system of planetary and stellar formation anything that doesn't have a spherical shape had to be formed in some other manner. It is either a collection of other things or a fragment of something larger.

The asteroids in the asteroid belt were assumed to once have been a planet or a series of proto-planets in the same orbit. Gravitational stress, and collisions form the fragments broke the planet(s) into pieces.

The Ort Cloud is a shell of dust around the solar system. The Kiper Belt is another asteroid belt. It is a collection of fragments from planets that might have been. These planets were either destroyed by the tug of gravity or collision with other fragments. A large fragment could be confused with a planet, but if it is not spherical shaped then it can't be a planet.

Pluto is still a planet, but it is called a dwarf planet. It was created by the normal method of planetary formation and it is not a fragment of something else. It is a planet, but it is a planet of many that have the same size. If Pluto remained a planet then there would be several other planets also and a few other planets that we haven't discovered yet; then what about large objects like the asteroid Ceres? The decision was made that planetary formation was a requirement for a planet. Ceres is a fragment and it can't ever be a planet.

Saturn, Jupiter and Uranus all have rings. The rings are now collections of dust and fragments. At one time they were larger fragments, even moons, possibly dwarf planets. They got too close to the gravity field of the huge planets and the tidal stress broke them down into the rings that we see today. Gravity is a strong force in astronomy, and while we don't understand how it is created or even conducted, the reactions of it are well known.

If Earth's moon isn't careful it could get too close to the earth and be broken up into a ring (currently though it trying to escape, but not doing a very good job of it). It is also slowing down the rotation of the earth. Dinosaurs may have had a 25 hour day. All of these things are caused by gravity.

Gravity is at its most efficient when it creates a sphere. It can't pack that much mass into a smaller or more efficient space. A black hole is a point, but its action is spherical. Its attraction is the same in all directions because of the gravity. A black hole destroys whatever gets too close by simple gravity. Things don't get sucked into a black hole; they get broken into pieces and pulled into strands and are finally ground down to dust or energy before they reach the black hole.

Answer 3

Planets are formed from gas and liquid. Gravitational attraction and other forces between the molecules forms them into an approximate sphere.

For the same reason raindrops are also spherical. This property was used in the making of musket balls. Molten metal was dropped from a tall tower and would form a solid metal sphere by the time it hits the ground.

The first link shows a movie of a sphere of water forming from a tap.

Answer 4

Assuming that the planet has significant size, it then must have a fair mass, and mass has this quality called gravity.
Gravity tends to pull things into the center, especially the heaviest things. And that is how things kind of shape themselves into a ball. As the mass gets bigger and bigger, all that weight tends to press down on the center (because of gravity) and make the center of the core very hot, even molten liquid metal. When that occurs it is easy to see how things become pretty round.

In the case of smaller space rocks which are cold (no molten core) they could be and are almost any shape. Naturally as things bang into them flying around out there in outer space, they tend to get sort of rounded off a bit.

Answer 5

The more mass you have the greater the gravity pulling that mass towards the centre of the whole lump.
Planets (and many of the moons in the solar system) have so much mass (compared to asteroids or comets for example) that the gravity pulling all the mass towards the centre makes it take on the smallest shape that can hold all the mass.
The sphere is the geometric shape that contains the most volume within the smallest surface.

And indeed one of the "rules" for being defined as a planet by the IAU is that it is massive enough to have formed a sphere.

Answer 6

Ever tried to spin a square spinning top?

Ever tried to bowl a cubic bowling ball?

Wouldn't cube shaped raindrops look silly?

Answer: sphere's are the perfect shape - they keep the largest mass into the smallest space.

Therefore gravity will try to pull matter into a sphere, and in the case of liquids, surface tension will try to make perfectly little spherical drops (surface area to volume increases with the anything as it gets smaller - that is why rain drops are round, and bath water is not).

Although planets are very nearly spherical, Mars is known to have a "gravitational anomaly" - a bit of terrain that unbalances the planet - like a spinning top with a small lump of lead stuck on one side.

That unbalances the planet somewhat. It is a very tiny amount, but tells you why a planet must be nearly spherical to be in equilibrium.

Have a look at some asteroid pictures on the internet. Most are too small for their gravity to have pulled them into the perfect shape, so most look like giant potatoes. Consequently, they tumble and roll through space like an out of control skiier on a mountain slope.

PS - Can those who have given my reply a thumbs down please email me with an explanation of why they did so.

Answer 7

When a planet is first formed it is mostly a big mass of molten stuff. Like water in outer space it takes the shape of the sphere because of surface tension and equal potential surface. (that is to say that there is no gravitational down hill on the surface of the molten planet) Also, since there is some inherent rotation, the equator is slightly larger than the spin axis diameter.

Answer 8

assuming for a moment that a planet was cube shaped (which would be impossible due to basic physics), after time, the corners would wear down and fill in the faces, so that it would become sphere.

the reason everything is sphere shaped to begin with, is gravity pulls all matter toward a larger mass. the easiest way for all that matter to get as close to the center of where gravity pulls it is to become a sphere.

Answer 9

In a word - GRAVITY!
Once an object is of sufficient size, it will eventually sucumb to its own gravity and form a shape whereby all the molecules are attracted to its centre by the force of gravity. the resulting shape is spherical. The cut off for this effect is, I think, a few hundred miles so objects of this size and lower will not have sufficient gravity to influence its constituent molecules and therefore may be irregularly shaped.

Answer 10

How about Hyperion? Hyperion is a moon of Saturn discovered by William Cranch Bond, George Phillips Bond and William Lassell in 1848. It is distinguished by its irregular shape, its chaotic rotation, and its unexplained sponge-like appearance. It was the first non-round moon to be discovered.