Why are the inner planets solid, and the outer ones gaseous?
353300
This question goes all the way back to the formation of the solar system.  Back then, when the Sun was a very young star, it was surrounded by what's known as an 'accretion disk'.  This disk was simply just a lot of gas and dust that never got sucked into the sun.  Eventually, particles started colliding with each other and they started to clump together.  Eventually the clumps started sticking to other clumps, so on and so forth.  However, there was a difference starting to show up.  The sun, being young as it was, was really hot, and it had pretty strong solar winds.  These winds were able to blow lower mass particles (like hydrogen and helium) out of the inner solar system.  So planets like the Earth and Mars were never able to collect these gases.  But out by, say, Jupiter, the winds had calmed down.  And it was a lot colder.  Cold enough, in fact, that water ice could form on the surface of the new planets.  This water ice added to the mass of the outer planets.  Once this ice was added, there was enough mass for the outer planets to incorporate hydrogen gas.  Hydrogen and helium are both incredibly light gases.  If a planet don't have a strong enough gravitational field, then it won't be able to hold on to the H and He.  With the ice and the cooler temperatures, this was able to happen.\n\nWow, I kind of rambled there, but I hope it helps!
58992
Scientists believe that the outer core is a liquid made out of an iron-nickle alloy, and the inner core is a liquid that behaves like a solid, and is mostly made out of iron.
50650
From an astronomy standpoint...\n\nThe Sun and the planets formed out of the same protostellar material.  This protostellar material was mainly hydrogen and helium with trace amounts of heavier elements produced by past generations of stars.  Something like a shock wave of other disturbance in space causes the material to compress and become gravitationally unstable.  The contraction of the protostar begins.\n\nThe majority of the material formed the Sun the rest of it was essentially stretched out in a disk.  Local gravitational unstabilities over time coalesce to form clumps of matter in the disk.  These clumps of matter are the protoplanets.  Again over time these clumps sweep out material in the disk of the solar system and build to large masses that are the present day planets.   \n\nThat's planet formation in a nutshell...at least that's how astrophysics sees the formation of Earth like planets.
190863
Jupiter, Neptune, Saturn, and Uranus. Venus has a lot of gas on it, but it actually has a inner core and crust just like the earth does. Pluto is thought to be either an astroid or comet caught by the suns gravity. Something else that's cool is Neptune and Uranus both have rings around them just like Saturn does.
62719
Venus, Mars, Mercury, Pluto and that new one are big rocks just like Earth, whereas Jupiter, Saturn, Uranus and Neptune are balls of gas.\n\nMars and Venus are the most comparable in mass (with Venus very close and often called our sister planet).  Mars has polar ice caps.
290836
Solid. Most elements at standard atmospheric temperature and pressure are solids. Some of the non-metals are gases, but only Mercury and Bromine are liquids at STP.\n\nEDIT: Gloo-gal, why answer if you don't know or don't care?
330346
As of right now, scientists believe that planets are by-products of the formation of stars. A star is formed when particles in a swirling cloud of dust and gas (a nebula) get closer and closer together and begin to combine hydrogen atoms to form helium atoms. This produces lots of energy (which is why stars, like the Sun, shine). Planets are the leftover "clumps" of dust and gas that do not produce their own energy.
1069030
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.
981269
It's from radioctivity and friction.  The gas giants also create more heat than they recieve from the sun.  The sun is nowhere the near the only source of energy in the solar system.
626769
The 'surface' (and I use the term loosely) of gas giants like Neptune is defined to be where the atmospheric pressure is equal to the sea level pressure of Earth.\n\nThis means that the surface is actually gaseous.\n\nFurther down though there is probably a solid core and liquid ocean.
747063
Mercury actually does have an atmosphere but the planet's small size makes it hard to retain.  It is not stable.
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.
391206
All the planets and the sun formed about 5-billion years ago within a huge cloud of interstellar gas and dust. Some regions of this cloud were more dense than others, and it's there that mutual gravitation formed the material into spherical objects that continued to grow as more material was drawn in to each. One of those clumps of material became our sun, and the others grew into the earth and other planets.\n\nGod should be spelled B-I-G B-A-N-G\nPBrax@netscape.com
953138
Gravity, because the sphere is the most economical shape in the universe.  It holds the most mass in the smallest area.  It is why planets have their shape as well, I was not aware that Nuclear fusion had any effect on the shape of the sun...\n\nmay you forever learn and forever ask
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.
102507
Saturn's interior is similar to Jupiter's, having a rocky core at the center, a liquid metallic hydrogen layer above that, and a molecular hydrogen layer above that. Traces of various ices are also present. Saturn has a very hot interior, reaching 12000 K at the core, and it radiates more energy into space than it receives from the Sun. Most of the extra energy is generated by the Kelvin-Helmholtz mechanism (slow gravitational compression), but this alone may not be sufficient to explain Saturn's heat production. An additional proposed mechanism by which Saturn may generate some of its heat is the "raining out" of droplets of helium deep in Saturn's interior, the droplets of helium releasing heat by friction as they fall down through the lighter hydrogen.
268015
The matter that made up the sun and planets existed at one time as a very large disk. As the matter clumped together, it was already on the same plane. That's the theory, anyway.