Why is the orbit of a planet around a star elliptical instead of circular?
398654
In the special case where centrifugal force exactly balances gravity, the orbit IS circular.  But say you have a planet in a circular orbit.  Give it a little extra speed in the direction it's going.  Now it has too much centrifugal force, and it begins to gain distance from the star -- but that means it's gaining potential energy and so must be losing kinetic energy.  So it slows down.  The detailed math shows that by the time it's gone 180 degrees around it has lost so much speed that, even though it is farther from the star, now it doesn't have enough centrifugal force to counteract gravity, so it begins to speed up.  Again, if you do the math, you see that it will return to the exact position it was at when you gave it the extra push, and with the exact same velocity it had just after that push.  So it will just keep repeating that orbit, which is highest (and slowest) at one point and lowest (and fastest) 180 degrees around; in other words, an elliptical orbit.
779660
The Earth and Mars do not orbit the sun in perfect circles, they are ellipses.\nSo sometimes the Earth is closer to the sun than other times, and sometimes Mars is closer to the sun than other times.\nSo if the Earth is farther from the sun than normal at the same time Mars is closest to the sun, and this happens when both planets are on the same side of the sun, then we are closer to Mars than any other time and therefore we can see it better.
150101
The small size and weird, distant orbit makes it feel more or less like a stray double-moon. I'm not sure why it's considered a planet really - maybe it's the smaller body that rotates it, Charon, that makes us class it as a planet, maybe it's because we looked for a reason for the deviations in the planets orbits and prematurely labeled it planet. As for me, I do - mostly because Pluto is always mentioned as a planet.
310923
It's orbit is closer to a circle than any other with an eccentricity of less that 1%. It also has a retrograde motion (it rotates from east to west instead of west to east). It always shows the same face to earth in the same way the moon does.
359420
Venus is closer to the sun than Earth so has a smaller orbit.\nMars is further from the sun than us, so has a bigger orbit.
1030
That might depend on wether the binary stars are very close (and spinning about each other) such that they "look" almost like a simple center of gravity relative to a planet far enough away to not be affected by lets say bumps in the gravity field.  Einstein told us that gravity distorts space and planets therefore merely follow the curved space layed out for them.  The earth senses the gravitational pull of both the sun  and say Jupiter but only the sun has a major affect because of proximity and mass.  The short answer to your question is that planets can exist in stable (though irregular) orbits around binary stars depending on the curvature of space in its orbit.  Hope that helps a little.
285187
The are round, in short, because it takes the least enegry to do it.\n\nGravity pulls the matter that will make up the planets/sun/... together.  All the forces are pointed towards to the objects center of mass, everything is being pulled inward toward the center.  With everything trying to get as close as possible to the same center of mass, the object develops a round/sphereical shape.  This shape minimizes the amount of potential energy the matter has above the center of mass.\n\nThe reason why some objects (like the Earth for example) as not totally sphereical is because they are roating around an axis.  Since the Earth is rotating on its axis (causing our days and nights), there is a bulge around the equator as the matter located there is rotating faster [tangential speed] than the matter located at the poles.  This faster tangential speed requires more centripetal force [not to be confused with the fictional "centrifugal" force] or order to keep it rotating.  In order to lessen the required force, the matter bulges out a little bid, increasing its radius away from the axis.\nCentripetal force = m(v^2/r),\nyou can see that a larger radius (r) away from the center means a smaller centripetal force is needed.
1036564
it is true, most of the visible star systems in our galaxy are binary (not 98%, more like 80% but that's still a lot). however, our sun is very unique in that it is just a single star system. We know this because the eccentricity (how much of a deviation from a perfect circle, with e=0 being a circle and 0<e<1 being an ellipse) of the planets within the solar system (sans Pluto) and really really close to e=0.\n\nAs you head to the outer planets, we see that the eccentricities increase substantially (but are still small, like e=.09 or something). This is obviously due to large bodies afflicting their gravitational pull. It was this theory that led to the discovery of Uranus, Neptune, and Pluto.\n\nNow we come to Pluto. Pluto's orbit is strange because a lot of energy came into play to put it in that orbit. The likelihood is due to the fact that Pluto was one of Neptune's moons or a rogue mass body from the early formation of the solar system. \n\nThe decision was reached to that because we know from endless observations that there is nothing close by that can produce enough of a gravitational pull to cause Pluto's orbit to be that offset. Even if it were dead, we would've still noticed it by now (if it were present) due to strong gravitational pulls from a single source. But none have been found. \n\nThe only reason why this belief is even more cemented now is due to the discovery of extrasolar planets within binary star systems that have insane eccentricities. This does not cement the theory, but adds a significant amount of evidence that our solar system is an anomaly in the vastness of space.
425798
Our planet earth and for that matter all planets, moons and stars are generally spheres, not cubes or cylinders or odd shapes. Some small galactic bodies such as asteroids comets do have odd shapes, but planets, suns and stars and even moons look like spheres. That's because, when they were formed millions of years ago, they were essentially molten liquid. Gravity and meniscus forces (surface tension) cause the object to take on a rounded shape--and it cools in that position. Do you remember seeing astronauts squeezing out juice or other liquids, they immediately become round in space.  It is the nature of gravity too pull uniformily toward the center--as the distance from the center increases, so does the gravitational force. You can think of gravity as a force that points inward toward the center of all things that have mass-- so that every part of the surface (especially liquids)is pulled evenly toward the center, resulting in a spherical shape.\n\nPlanets are not perfect spheres because there are other forces acting on the planets too which are trying to stretch them--rotational forces, etc. Because they were once hot moltent liquid, they buldge in the middle for these secondary centrifigal forces. Earth and the moon reflect this phenomina. Although gravity keeps planets close to spherical, there are other forces that cause deviations from the basic spherical shape. For example, the rotation of the earth once every 24 hours, causes an apparent centrifugal force which creates a bulge at the equator. In fact the earth's diameter at the equator is 7,926 miles while the diameter between the poles is only 7,900. \n\nNote that planets and moons had to be a liquid or they would not have been able to escape the gravitational force of the nearest larger body. Remember that liquid will seek the path of least resistance as it moves. In otherwords, an impact on the earth would cause a splash of moltent material that could escape the earth's atmosphere at the exact angle (path of least resistance) to be able to exit the planet's atmosphere without burning up. Once it exits into space, the gravitational pull of the larger body will pull it into orbit. It will stay in orbit until it is acted upon by a larger force. Fortunately, we haven't seen any larger forces since the earth was created and therefore, the conditions will remain for millions of years into the future.
505925
You could also talk about the tension on the ropes or chains and explain how the force is perpendicular to the direction of motion and therefore causes a circular or arced path.  It seems obvious because the ropes have a set length, but this is the exact same phenomenon that keeps the earth in orbit.  The gravitational force is perpendicular to the diretion of the earth's movement (on average, of course.  Since the actual orbit is elliptical, it's not a perfect analogy).
994578
Technically, it is ... just slowly ... the orbits that the planets make around the sun are actually the planets spiralling inward toward the sun ... they will collide with the sun eventually.
167191
What you are asking is essentially, are the other planets in the same plane as the earth's orbit, known as the ecliptic? No, they are not, but they are pretty close. The angle between their orbits and the plane of the ecliptic is called the inclination:\n\nMercury     7°\nVenus      3.39°\nEarth  0°\nMars      1.85°\nJupiter1.3°\nSaturn2.49°\nUranus0.77°\nNeptune1.77°\nPluto      17.2°\n\nAs can be seen from this list, most of the planets have orbits in the same plane as the earth, more or less. \n\nIt is also interesting that all of these planets orbit in the same direction, in more or less the same plane, and also spin in more or less the same direction (counter clockwise when seen from the celestial north pole, looking down on the solar system). The only two planets which have their rotional axes different are Neptune (tilted on its side) and Venus (a retrograde rotation; that is, it is rotating clockwise when seen from the celestial north pole). These spins also agree with the spin of the sun, which has a huge amount of mass and therefore angular momentum by comparison. It is thought that these spins are all so similar because the solar system formed out of one big rotating ball of material that coaelesced into the orbiting, spinning planets and the spinning sun.
337086
The planets are actually trying to move in a straight line at a high speed due to the intertia from some force long ago.  When two objects are moving by each other, their gravity interacts and pulls them toward each other, bending the path that would otherwise be a straight line.  If they are too massive and too close, the gravity pulls them so much that they collide.  If things are balanced just right, the path is bent into a complete ellipse.  It's an ellipse instead of a circle because the sun is actually also being pulled a tiny bit toward the earth, as well.\n\nYou could consider gravity to be sort of a "perpendicular inertia", and the planets would revolve around the sun FOREVER if there were no other forces involved.
265249
I would presume from other galactical planets that are further from the sun than earth that their dormant not motile state is due to the hardening of their planet crust.  Also, as you fill a tire with water and spin, it takes more energy to rotate than that without, hence, slower process at least.
370
The Earth is round because it is a large body, and gravity tends to make large bodies round because that is the gravitational efficient configuration for the matter to be in.\n\nThere are a few reasons you can encounter non-round bodies in space:\n\n1) the object is small (for example asteroids), so it's gravity has less say proportionally on the shape\n\n2) It has experienced a catastrophic collision with a large body\n\n3) It is under very strong rotational or tidal forces\n\n4) It is expelling matter/energy in the form of cosmic jets, nova ejecta, etc.
66192
It is due to the slight orbital eccentricity of Earth rotation around the Sun. Since the orbit is not perfectly circular the distance between the two bodies varies.
664542
All orbits are elliptical (note that circles are subsets of ellipses.)  Exactly how Pluto go to be where it is (and where also includes its coordinates in a velocity space, in addition to xyz space) is not known.  One interesting thing about it is that it is in a resonance with Neptunes orbit, so that whenever it is close to Neptunes orbit, Neptune is on the other side of the sun. Neptune's mass can't perturb Pluto.  In other words, Pluto is in a special orbit that is stable. \n\nI doubt it had its origin as a moon of Neptune, or else it likely would have been flung into a very radical orbit by an encouter with that planet. More likely it is one of MANY objects that found itself orbiting in similar orbits; all the others have long ago been removed from the picture by Neptune encounters.