Center of Mass

What's this all about? Well, it has to do with why it's possible to detect a large planet orbiting a distant star by measuring how much the star wobbles. If we consider our own solar system, this idea could be expressed by saying: Jupiter doesn't orbit the Sun.

What, you say! Yes, this is the truth, and it's because Jupiter is the largest planet in the solar system, so large that all the other planets could fit inside it. In other words, it's a big boy as planets go.

When two objects are gravitationally locked in a dance we sometimes call an orbit, they don't actually orbit in the classical sense. They rotate around a center point. Now, for most of the planets in the solar system, this center point of rotation is well within the Sun and very close to the center. Not so, in the case of Jupiter. The center point is 1.07 solar radii from the middle of the Sun or 7% of the Sun's radius above the Sun's surface.

This makes the Sun and Jupiter do a dance in which both are moving and not stationary. This effect can be seen when a shot-putter swings a large metal ball around in order to build up momentum for a launch. The ball and the man wobble in this case.

In the case of a large gas giant planet orbiting a distant star, the closer the planet orbits, the greater the wobble, making it easier to detect, especially since the planet orbits the star in a few days instead of months.

You might think that the Sun's wobble would cause trouble for the Earth, but it's too small considering the distance the Earth is from the Sun. We would never notice it. On the other case, if Jupiter orbited the Sun inside the orbit of Mercury, we would notice it. It would affect our Earth's orbit around the Sun.

So, it's safe to say that you can tell people that Jupiter doesn't orbit the Sun. Now, you'll know why this is the case.

Thanks for reading.