On July 9, I posted a gif of Pluto and Charon orbiting their mutual center of mass, located in a point of space between them. I said this about the video:
We usually think of a satellite orbiting its primary. In this case, it would be Charon, the tiny dot, orbiting Pluto, the larger dot. But we’ve always known this is not really the case. The two objects orbit each other. More accurately, they both orbit their common center of mass (or center of gravity).
Here’s a simulation from Bad Astronomy showing the same principle. This represents a planet and a star instead of a moon and a planet, but they work exactly the same way. The center of mass of the system is called the barycenter. Phil Plait, the “Bad Astronomer,” describes it this way:
As you can see in the animation, the planet makes a big circle and the star makes a small one. And if you watch closely you’ll see they’re always on opposite sides of the barycenter; when the planet is on the left of its orbit, the star is 180° around on its right.
This wobble in the star’s movement was used to detect most of the first exoplanets, or planets of other stars than our sun. The first two were found in 1992. It wasn’t until three years later, in 1995, that the third was discovered. Now we’ve identified almost 2,000 for sure, ranging in size from smaller than Mercury to several times as large as Jupiter. About 3,000 more probable exoplanets are waiting for verification.
There may very well be more planets in the universe than there are stars.