From wikiWHYfiles
[edit] Introductory Note
A planet is generally considered a massive spherical natural object that orbits around a central star. Planets are part of our solar system. There is sometimes a bit of a controversy as whether an object is planet.
For example Pluto is sometimes not considered a planet.Most recently Pluto has been designated a dwarf planet, so it is a kind of planet but not in the same sense as the other larger main planets.It is now widely believed from good astronomical evidence that there are planets around other stars. Although many of these planets are very large, sometimes 2-3 times bigger than Jupiter. These large planets are sometimes called failed stars or brown dwarfs because they do not produce their own fusion reactions like typical stars.
[edit] Reason
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 shape of small objects (like people and houses and mountains and small asteroids) are determined by their mechanical properties. You can take a rock and cut it into a particular shape and it will pretty much stay that way.The larger the object, though, the stronger its gravitational field. Imagine that you want to build a really tall building. You have to make sure it has a really strong foundation, or the foundation will be crushed by the weight of the building and the building will fall. If there was anything really big sticking up on a planet or a star, gravity would pull it down. If a planet was like a cube, the corners of the cube would be higher than the rest of the planet. Since planets and stars are so big, you cannot build a "foundation" strong enough to hold up those corners! Anything you built it out of would be too weak to hold them up. Gravity would eventually pull them down. Even solid rock will flow like a liquid, although very slowly, if it is pulled by a very strong gravitational force for a very long time. Corners on a cubical planet or star would eventually just squish down. Since gravity pulls toward the center of the planet or star, everything gets pulled down into a sphere. However, planets and stars are not really perfect spheres. They spin, so they bulge out a little around the equator.
[edit] Related Articles
- Firstly, ALL mass, down to the level of a neutrino, has gravity. Comets, being smaller than planets, have LESS gravitational pull, but they still have some.
- Secondly, all systems tend to go to a state of minimal energy UNLESS something prevents that from happening. Thus, a block will fall to the ground unless something holds it up. A stack of blocks will TEND to fall to the ground unless stacked just right (as any two-year-old finds out). A HUGE pile of blocks -- let's say, the size of a planet -- will TEND to fall into each other unless some other force prevents it.
- When you do the calculations (requires calculus), the lowest level of energy for something pulling itself by gravity is a sphere. Thus, over time, something that starts out NOT as a sphere (say, an oblong) will, unless something else is happening, become a sphere.
- Thirdly, solids, by their very nature, do not like to crushed together. They will resist doing so unless the force is too large for them to resist it.
- For a pile of rocks the size of a planet, the amount of gravity overcomes this resistance to being forced together. The rock, even though it would rather not do so, becomes a sphere because the gravity is so strong.
- But for a pile of rocks the size of a comet or asteroid, however, the gravity is NOT strong enough to overcome this resistance to being crushed. The rock stays non-spherical, because the gravity is not strong enough to overcome this resistance.
- Thus, planets become spheres while comets stay non-spheres.
- Be aware that our Sun is NOT a perfect sphere. Its spin around its axis causes a bulge at its center, much like runny clay on a potters wheel will droop away from the center.
[edit] References
1. The Planets
2. Why are planets all spherical?
3. Why are planets round
