How does a planet gain it's speed? Do all celestial bodies go in an elliptical orbit?

Answer 1

Excellent questions, Adam.

1. It turns out that the planets are not "gaining" any speed at all. The speed that they have now is, for the most part, the speed that they started out with when they were formed. To explain this, let me use a thought experiment that originated with Isaac Newton:

Suppose you rolled a cannon up to the summit of an extremely tall mountain, so tall that its peak was above the Earth's atmosphere and there was no air around you (this point will be important in a bit). Now suppose you fired a cannonball out of that cannon. The cannonball would travel in an arc as it fell toward the ground, and eventually it would hit the earth. Now, in keeping with the metaphor, suppose you used a bit more gunpowder and fired the cannon with more power. The cannonball this time would go further before it finally hit the ground.

But now suppose you fired the cannon with so much force that the arc made by the cannonball coincided perfectly with the curvature of the Earth. The cannonball's path would keep curving and curving as it flew along, but the Earth's surface would curve "out from under it", so to speak, and the cannonball would never hit the ground! Eventually the cannonball would go around the entire Earth and come right back to where you fired it from (and kill you if you weren't careful to step out of the way!). Of course, assuming there was no atmosphere to slow it down (which is why we assumed the mountain was above the atmosphere earlier).

The key thing to understand is that as the cannonball flies around and around the earth, there is nothing giving it any more speed. Whatever speed the cannonball has is whatever speed was imparted to it when it was first fired out of the cannon. It doesn't require anything to keep "pushing it along" in order to continue in its path.

With planets it is the same way. A planet's orbit is characterized by a certain amount of "energy" (which is related to speed). The planets are not being given any more energy by anything. They just keep going around the sun with the same speed they always had because there is nothing there to stop them.

The natural follow-up question to this is, "Where did the planets get their original speed from in the first place? What was the 'cannon' that fired them?" I'll refrain from answering that one for now to avoid being too long-winded, but I am happy to answer that if you are interested.

2. The notion that all celestial bodies go in an elliptical orbit is mostly true, but not 100%. Strictly speaking, an object only moves in an elliptical orbit if there is only one other object that is gravitationally pulling on it. In reality, this is never true, because everything in the universe gravitationally attracts everything else. However, the reason it is mostly true that the orbits are ellipses is because usually the effects of other objects are tremendously small.

For example, Earth is gravitationally affected not only by the sun but by the other planets in the solar system, but the other planets are so tiny compared to the sun that their force on the Earth is incredibly weak and the effect on the Earth's orbit is minimal. Similarly, Earth is gravitationally tugged on by other stars in the galaxy, but those stars, while massive, are so immensely far away that their force on the Earth is also incredibly weak. So, the Earth's orbit is nearly a perfect ellipse.