if planets are formed from gigantic explosions of galaxys and cosmos, why are all planets formed in sphere and not some other shape like a square, rectangle or something like that?
209397
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.
415759
Our Solar System started out as a big amorphous cloud of gas and dust.  This cloud was spinning ever so slightly - probably imperceptibly so.  The cloud collapsed due to gravity, and our Sun formed at the center, where it was densest and hottest.  \n\nAs the cloud collapsed, it formed a disk (the accretion disk others have mentioned) because it was spinning - if there was no spin to begin with, it would have collapsed into a sphere.  Anything big that is spinning, as it gets smaller it spins faster (try it yourself - sit on a chair that swivels, spin around with your feet off the ground and your arms stretched out, then pull your arms in close to your body and you'll spin faster).  So that imperceptible spin of the original cloud became the much faster spin of the accretion disk.\n\nEventually planets formed in the accretion disk, that's why the planets all orbit the Sun in the same direction and in almost the same plane.  In fact, that's also why most of the planets spin on their axes in the same direction, and why most of the moons of the planets orbit in that same direction  - counter-clockwise.
385277
Of your choices, (e) is the best.  (a) is no good because it fails to account for the continuing generation of heat, as, for example, by the earth.  (b) is no good because they were all about the same temperature when they formed.  As the protoplanets grew larger, their interiors began to heat up.  (c) is no good because what matters more is density, and there's no reason to believe there are significant differences in the density of radioactive elements.  (d) is just plain false: Earth is farther from the sun than Mercury.\n\nThat leaves (e).  Surface area of a sphere is 4pr^2, where p is pi, and volume is (4/3) pr^3, so the ratio of surface area to volume varies as 1/r ... it varies inversely with radius, so the ratio for smaller planets is greater than that for larger ones.\n\nThe key connection, though, is internal <pressure>, which is a direct function of r.  Larger planets have more pressure at their centers, and that means more heat.  In fact, if the "planet" were big enough, it would become a star.\n\nConclusion:  Big means hot.
605145
They only hold solar system to it's size. A gravity phenomenom got all planetary make up from the left over gas, dust and ice of the sun's birth phase mould or mould and melt into shapes and form round by cosmic stellar dust wearing it to shape. The mass that gets formed by gravity clogs the core from ripping space or even time but gravity can still go through like a plugged bottle. Holding the planet together. Then reactions from the leftover chemicals of the planet form the ground from volcanism and life from static infection. So the planets just formed there, from out of the millions of solar systems in this galaxy alone one is bound to form life on that solar system's planets. So they just formed there, they don't require a need or if we need it. As we evolved to the Earth, not that the Earth evolved to us. Think of the different species on this Earth alone.\nA galaxy consists of billions and some less to millions of stars and stars' solar system of planets, planetoids, comets, asteroids, satellites and moons.
609219
So far, we can only detect fairly large planets around nearby stars in our own galaxy.  However, the number of planets we are discovering shows us that planets are very common.  There is no reason to think that other galaxies would be any different.\n\nWe don't have sensitive enough tools yet to detect Earth-like planets, but they probably exist. We don't yet know how common life, intelligent or otherwise, is in the universe, but considering how many galaxies there are and how many stars in each one, it seems as though there should be other intelligent beings out there somewhere.
127269
Yes, new stars and new planets can form from the remnants of dead stars.  We can see this process happening, although it is extremely slow, a process that take millions of years.\n\nEven our sun is thought to be a 3rd generation star.  When our sun dies, it will expand to become a Red Giant star, eventually cooling/contracting into a white dwarf, as this happens, a lot of stellar meterial will be released and will float off into space to make new stars in the future.
618256
They start out as an enormous gas cloud. Gravity causes the parcicles in the cloud to spin around the point of center mass, and the velocity of the spin subsequently flattens it into a disk. That's how galaxies, and on a much smaller scale, planetary systems form.
188540
The moon although is a sphere, but it is so far away that it appear a flat round disk. there is nothing strange about this and if people are not educated that moon is a spherical rock, they'd probably think it's a round flat disk.\n\nBut for me, I see it as a hole in the ceiling.
649563
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
276433
Our moon isnt the only spherical moon. The reason why some moons are spheres and others arent is that only when objects are massive enough do they become spheres due to their gravity acting on themselves. \n\nTake a look at some of the moons of the outer planets, like titan as the first person mentioned, many of those are spheres.
418864
Stars are lumps of matter massive enough to sustain fusion reactions in their cores and produce light and heat. If planets were big enough they would ignite too and become stars: Jupiter would be a star if it were just ten times more massive than it is - lucky for us it isn't.\n\nStars spend most of their lives with hydrogen as fuel. When that runs out they turn to heavier elements, until finally there is nothing left and something has to give. Depending on the mass of the star, it could collapse into a white dwarf, planet-sized but super dense and still shining for a very long time. Heavier ones  become neutron stars after exploding into novas or supernovas, so dense a teaspoonful might be as massive as a mountain. Still heavier ones can collapse into black holes (which like global warming were once theoretical but now no one doubts their reality).\n\nIf a star is extremely massive it will burn its nuclear fuel quickly - these are the blue giants, which may last only a million years or so. Our sun is right in the middle with a lifetime of about ten billion years, enough time for humans to evolve and wonder what it's all about. The sun is now halfway through its lifespan, but more than halfway through its life-sustaining period; it will eventually expand into a red giant out to the orbit of Mars, vaporizing all the inner planets.
617446
No, planets and stars are round because of gravity (to be more accurate, gravity is the curvature of space-time). This is a caused by a different phenomena than surface tension, which is due to the electromagnetic attraction between water molecules. The force of gravity and the electromagetic force are two different forces.  \n\nSurface tension does not actually make the drop round; air pressure does that.  Surface tension simply gives the drop an outer wall like the wall of a balloon.  \n\nThe reason that water drops form into spheres is the need to equalize the pressure inside the drop. Air pressure tries to squeeze the drop, and the internal pressure inside the drop pushes back  - creating the internal pressure of the drop. This puts tension on the wall of the drop (Where the wall of the drop is caused by surface tension.). The geometric shape of the sphere minimizes the wall tension (http://hyperphysics.phy-astr.gsu.edu/Hbase/surten2.html).  A shape with corners, such as a cube, would allow forces to be concentrated at the corner, which would smash the corner flat, or else cause the area around the corner to expand. A sphere allows for an even pressure distribution.\n\nThe pressure on a planet is caused by its own weight pulling its own matter inward.  A sphere allows for an even distribution of gravational 'pressure'.  \n\nIn other words, a sphere resists being crushed better than any other shape, so objects under internal or external pressure tend to become round (irregardless of what is causing the pressure).
495658
1. Its mass, its age, its energy state\n\n2. The earth's atmosphere makes stars appear to twinkle owing to their vast distances from us. They appear as points of light and so the light rays are bent differently by small ongoing variations in the earths atmosphere and so cause minor movements and fluctuations. However planets are disks rather than points (even though our eyes do not perceive their size). They are large enough to not appear to move like stars do as the position variations caused by bending of light by the atmosphere are 'within' the size of the planetary disk. Also the visible planets are very very bright compared to most visible stars so variations in light intensity are not so obvious.\n\n3. This is a difficult one. For example:\n\nIs it hydrogen, the primordial element of the universe??\nIs it energy as energy is what makes it all happen??\nIs it gravity as gravity causes clouds of hydrogen to condense into stars??\nIs it fusion as without fusion of hydrogen and then other elements nothing works??\n\nNo I cannot answer it. I put this one in the "too hard" basket.
328777
The sphere has the smallest surface area among all surfaces enclosing a given volume and it encloses the largest volume among all closed surfaces with a given surface area. For this reason, the sphere appears in nature: for instance bubbles and small water drops are roughly spherical, because the surface tension minimizes surface area.\n\nA sphere surface can also be defined as a surface formed by rotating a circle about its diameter.
541044
yes there are straight lines. what you describe is according to the earth and the solar systems that appear round due to the force that acts from all directions creating a universal container (the sphere) which is round. But look at the BIGGER picture a straight line has to exist even if it is as small as an atom (universe - meter scale). also there are other geometrical forms. infact the only reason why round is soo common in all universal shapes (objects). well think of all forces acting on a body from all sides then reason the kind of shape it will produce when all forces level out the object.
511701
Look up "the nebular hypothesis".  you also are confusing the Milky Way (a disk shaped system of a hundred billion stars or so of which our sun is a part) and the Solar System (our sun and its surrounding planets and other assorted detritus.)\n\nhttp://en.wikipedia.org/wiki/Solar_nebula
44610
Gravity pulls everything to the center. The larger the object the greater the gravity the more perfect a sphere. Jupiter has a smoother surface than the earth. The earth is smoother than the moon. Asteroids are even smaller and don't always have enough gravity to overcome the structural properties of the mineral it is made of, so smaller asteroids have more unusual shapes and are not always very spherical.