# The big new Planet

*Most recent answer: 01/20/2016*

- Paul R (age 30)

Lake Tahoe, California USA

The Sun's gravity falls off smoothly as 1/R^{2}, where R is the distance to the center of the Sun. There's no special distance where that law quits working, at least on the range we're discussing here. Newton figured that out (ok, probably Hooke figured it out first). Newton used the calculus he invented to figure out what would happen then to a planet in that gravitational field. It will make elliptical orbits with the Sun at one focus, just as Kepler had already described. Gravity keeps a distant object in orbit, but with a long orbital period, also as Kepler had described.

The interesting part that gives evidence for the big planet comes from the gravity effects that aren't part of Kepler's laws. That is, planets and smaller things orbitting the Sun are affected not just by the Sun's gravity but also by gravity of other planets.

On your other question, Pluto is one of the objects in the Kuiper belt (). I think most of the time it's actually a little closer to the Sun than most of the other Kuiper objects. Long-period comets come from even farther away, probably a cloud of such objects: .

Mike W.

posted without vetting until Lee returns

*(published on 01/20/2016)*