Re: Jupiter and definition of planet
Edit: the following is entirely wrong, there seems to be a valid issue with both the definitions of "orbit" and "planet".
Wonder if this will be brought up in the next IAU conference.
The actual definition of a planet is that it orbits the system's
barycenter - which is the point around which two or more bodies orbit (technically speaking - every orbit is actually several bodies that go around a common barycenter - we just say that one is orbiting the other if that barycenter is within the physical body of the other). So if you average out the barycenter of the entire system, Jupiter still orbits that.
Another example would be a binary system, where you have two stars that orbit a common barycenter:
Any planet in this system would also not orbit any single star because its barycenter with both stars would likely be around the stars' own barycenter, but for binary system we have different definitions of "orbit".
As for Jupiter, it does NOT in fact orbit the sun - just because it is so massive that it skews the sun's own orbit around its own center of gravity enough to create a common barycenter between them that is outside the sun's body, and thus causes it to act like a very extreme binary star system, and so the sun and Jupiter orbit a common barycenter:
This would be an actual orbit:
In that regard, just like two stars in a binary system don't "orbit each other", or one doesn't orbit the other - Jupiter doesn't orbit the sun (nor does the sun orbit Jupiter), but rather they both orbit a common third point - their barycenter.
