Why are planetary orbits elliptical and not circular?
Answers
Answer:
Imagine it this way: a planetary object soars by the Sun at a high speed; at this point, it only has its own velocity that it gained during the explosion when it was first created. As it passes near the sun, a new force i.e. the gravitational force of the sun acts on the object and starts to pull it in its direction. But as it falls towards the sun, a new component gets added; this is the velocity because of acceleration due to gravity. This component, combined with the initial velocity that a planet has, keep it from falling into the sun and give rise to an elliptical orbit.
In short, a planet’s path and speed continue to be effected due to the gravitational force of the sun, and eventually, the planet will be pulled back; that return journey begins at the end of a parabolic path. This parabolic shape, once completed, forms an elliptical orbit.
Inertia and gravity must combine in impressive fashion for any orbit to occur, and given how many other factors can affect the velocity and path of an orbiting object (e.g., other sources of mass/gravity), a circular orbit is just highly unlikely.
However, if you decide to become an astrophysicist, perhaps that can be one of your career goals… finding as many perfect circular orbits as you can!
Because orbits are general conic sections. Why this is true is another fascinating question in and of itself, but for now I'll just assume it. The point is that circular orbits are special examples of general orbits. It's perfectly possible to get a circular orbit, but the relationship between the bodies' velocities and separation needs to be exactly right. In practice it rarely is, unless we plan it that way (e.g, for satellites).
If you threw a planet around the sun really hard its path would be bent by the sun's gravity, but it would still eventually fly off at a tangent. Throwing it really hard would make it almost go straight, since it moves by the sun so quickly. As you reduce the speed, the sun gets to bend it more and more, and so the tangent is flies off on gets angled more and more towards moving backwards. So general hyperbolas are possible orbits. If you move it at the right speed, then it'll be just slow enough that other tangent points 'exactly backwards', and here the motion will be a parabola. Less than this and the planet will be captured. It doesn't have enough energy at this point to escape at all.
A key realization here is that the path should change continuously with the initial speed. Imagine the whole path traced out by a planet with a high velocity. An almost-straight hyperbola, say. Now as you continuously lower the velocity, the hyperbola bends more and more (continuously) until it bends "all the way around" and becomes a parabola. After this point, you'll have captured orbits. But they have to be steady changes from the parabola. All captured orbits magically being circles (of what size anyway, since they have to start looking like parabolas at some point?) wouldn't make any sense. Instead you get ellipses that get shorter and shorter as you get slower. Keep doing this, and those ellipses will come to a circle at some critical speed.
So circular orbits are possible, they're just not general. In fact, I'd say the real question is why the orbits are often so close to circular, since there are so many other options!