Gravity holds the universe together

Gravitational force, why planets have orbits, and how we know that black holes exist

When you think of gravity, you might think of the force that makes apples fall on people’s heads (talking about you, Newton!). But it’s also why planets have orbits! Any two objects in space have a gravitational force between them. Kepler’s and Newton’s laws come together to describe characteristics about orbits, and also why things orbit.

Enter….. the Universal Law of Gravitation, discovered by #1 gravity enthusiast Isaac Newton! (Inspired by Kepler, of course). This is how it works: Every mass attracts other masses due to the gravitational force. The strength of the gravitational force between 2 objects is directly proportional to the product of their masses, and decreases with distance between them.

This explains why planets orbit! The gravitational force between two objects holds them together. However, the crazy thing is that Newton discovered unbound orbits. This is when an object escapes the force of gravity and has a parabolic/hyperbolic path, rather than an ellipse.

One common misconception is that smaller mass orbits around the larger mass. But what really happens is that objects orbit around their common center of mass, which is located much closer to the larger mass! This is why the Earth orbits around the Sun, because the Sun is x times more massive than the earth. The center of mass is basically inside of the Sun:

The Sun is so massive! (Image Credit: Kaiser Science)

Binary star systems are another great way to see this phenomenon. When the stars are around the same mass, it looks like they are both moving around the point in between them!

A binary star system and orbits visualized! (Image Credit: Parnika Singh)

The great thing about these equations from Newton and Kepler are that future astronomers can use them and figure out unknown quantities. For example, if you know the orbital period and average distance of some object, you can calculate the mass of the other object, by using Kepler’s 3rd Law (Newton’s version), which included the masses and gravitational force! If one mass is way bigger then we can just pretend the smaller mass isn’t there. That’s how we know the mass of the sun! And that’s also how we know that black holes exist, because we have observed stars orbiting around “nothingness”.