Why are planets such perfect spheres?
413253
They're actually not perfect.\n\nBut check this link out:\n\nhttp://www.pa.msu.edu/~sciencet/ask_st/031198.html\n\nAnd this:\n\nhttp://www.sciam.com/askexpert_question.cfm?articleID=000D93E1-778C-1C72-9EB7809EC588F2D7&catID=3\n\nThey retain a spherical shape because of gravity.
1008299
It has to do with the initial formation of the solar system. A star and solar system forms basically of leftovers of stars that have gone nova or supernova, just star gas and leftover material. These begin to collect and become more dense as gravity pulls on them. This gas then become more solid in some places and the particles or planetesimals will have speed, initial speed from the formation process as the object swirls. Then, over millions of years some of the solid objects collide and become bigger, eventually forming planets. Since the initial swirling gas gave all the materials speed in the same direction then this does not change generally when they become planets.
1102975
Because gravity pulls equally in all directions.\nAs soon as the materials that the earth is made of began to gather together under self-gravity, the center pulled equally in all directions.  As heat from friction and radioactivity made much of the early earth molten, gravity pulled it into the most efficient shape -- a sphere.  Because of the earth's rotation and centrifugal force, it's not quite a perfect sphere -- it bulges a bit at the equator and is slightly flattened at the poles -- but it's simply gravity doing what it does naturally.  It's why all large bodies are in the shape of a sphere.
44602
Why are any of the planets round?
623455
The most basic stable physical shapes of revolving and rotating masses is spherical-note not a perfect sphere, because the shape tends to be flatted on the ends. Hence even the sun bulges along its equator and is flattened at the poles, as is the earth-this happens because of centrifugal forces caused due to rotation which throw material outwards, while gravity holds it and pulls it inwards.\n   \nThere must also be enough  mass too to generate enough gravitational force so that a spherical shape is formed. Astronomical bodies which are only a few kilometres across are irregularly shaped-eg. Ceres, an asteroid
791325
Most are actually more ellipsoid in shape (i.e. not perfect spheres).\n\nI'm no astronomy/physics expeert, but I think it has something to do with gravity and rotational forces.
124146
Stars are like the sun, except they are very far away.  Although made from elements that are gases on earth, the stars are not made of gases.  Their gravitational mass compressed the elements to the point of thermonuclear fusion (just like a huge hydrogen bomb).  This fusion reaction give off photons of light (which is why stars shine and the sun “feels” hot).\n\nPlanets are similar to the earth.  Made of different components, some solid, some usually liquid or gaseous.  Planets do not shine of their own light, but reflect the light from the sun.  Planets tend to travel in somewhat fixed orbits around stars.  In addition, some planets have moons circling them.
463058
Planets aren't directly formed from explosions and whatnot, they are formed from gravitational accumulation of mass in dust clouds. They are approximately round for the same reason anything that is approximately fluid that experiences a central force is round. Central forces have circular potential energy wells.\nConsider a cubic planet. A mass on the face of such a planet would be experiencing less force than a mass near a corner of such a planet (The corner has a greater amount of non-cancelling mass "behind" it. If you know the divergence theorem, this can be seen as well.). Since less force is exerted on points on the faces than points near the vertex, the vertex will slowly collapse if the mass is great enough, easily pushing the points on the faces outwards until an equilibrium is reached. This thought experiment works for any polyhedron, with the sphere being the limiting shape. Once the shape is approximately spherical, it is a well known proof that the force experienced by a particle outside the sphere will be the same as the force if the sphere was a point mass at the center of the central force, giving us back the circular potential energy map.
530735
two reasons, gravity and angular momentum.  Gravity tends to pull matter into one centralized spot, but angular momentum must be conserved.  That mass that escaped being part of the sun attracted mass to centralized locations, what became the planets, flattening things out to a disk shape
789855
I go with the Big Bang theory and that our planet just happened to be in the right place at the right time. If our planet grew life then there is the possibility that other planets in other galaxies grew life as well. We just happened to get the right amount of heat and light from the sun.
948233
They have solids cores.  Other than that the are mostly gas, due to the distance from the suns gravity.  When our solar system was formed most of the "heavier" elements stayed close to the major gravity pull, the sun. once out any other heavy elements that made it out that far condensed to gether to form cores.  Now; the farther away from the sun you go the coder it gets.  Now the gases that are out that far basically freeze and get "solid".  hope this helps!!\n\nBy the way- ALL planets need a core.  even though it might not be of same rocky material as other planets (Jupiter, saturn)..think of my explaination above.  If none of the solids came out that far out, they could get together to form a core, but something has to be in the center of a planet.
80
Largely because of the massive amounts of gravitational force involved.  A sphere is the only sustainable shape when talking about forces of that magnitude.  Stars are incredibly dense towards their core, and over billions of years the matter will coalesce in a sphere like shape.
1141942
some asteriods have gravity.billions of years ago,some rocks gathetred together to form the planets.when two asteroids colide to one,small particals are blast out but still controlled by the gravity of the astroid.when millions of astroids form to one,the small particalls get more.soon planets are born.but every planet dont have moons yet instead they have rings even earth.the small particalls eventually will collide again to form moons.even saturns ring will eventually form moons and you can see no rings
780074
Gravity and the spinning of the earth on it's axis... which gives you the one shape out of all possible shapes which allows for the greatest amount of surface area... the sphere.
699589
Kepler figured out the laws that describe planetary orbits.  This enabled much more accurate planetary positions (which Kepler published), and set the stage for Isaac Newton to explain that gravity  was the force that held the planets in their orbits.\n\nUp until Kepler, everyone was sure that planetary orbits must be some form of perfect circle. His discovery that they are actually elliptical was an important turning point in the history of science, away from the Aristotelian method of applying philosophical concepts to physical reality, and towards modern empirical science.
308046
Gravity. A sphere is the shape which allows every part of the planet to be as close as possible to the centre. Planets which spin are flattened slightly, so that they are not exact spheres, the faster the spin the more they are flattened. Earth is slightly flattened at the poles. If a planet was not a sphere, the parts which stuck out further than average would effectively be "mountans" and any loose parts would roll down under gravity, thus the planet becomes more spherical
384998
Because gravity acts pretty much in a straight line.  As the swirling dust clouds collected into the earth, the particles acted on each other to form clumps, and the clumbs attracted to each other, then you end up with a nucleus of matter imparting a gravity field radially out from the centre.  Things will be colliding with the nucleus from all directions with an equal probability and thus a sphere.