Physics Van 3-site Navigational Menu

Physics Van Navigational Menu

Q & A: the big new planet

Learn more physics!

Most recent answer: 01/20/2016
Q:
I need to start out why I came up with the question to see where I am coming from. I was reading a news article about a hypothesized 9th planet by Brown and Batygin. Then I started thinking about the gravitational force of the Sun. We are locked in our orbit because of this force pulling on us. Along with all the other planets and objects in our solar system rotating round and round. Then comes this 9th planet idea that a planet exits billions of miles further away than Pluto with an orbit of 10-20,000 years. My questions are. Do we know how far the Sun's gravity reaches out and does that force change for bigger planets like Jupiter which is pretty close? Are there any other know objects in orbit past Pluto? It is probably hard to find this giant Neptune sized planet. Wouldn't it be easier to find any object big or small in orbit? Would the force of gravity be able to hold onto this large planet that far away?I could ramble off so many questions but I'll hold off. Trying to get an idea answered from an internet search that lead me to a site with a question and answer section is pretty cool in my book.
- Paul R (age 30)
Lake Tahoe, California USA
A:

The Sun's gravity falls off smoothly as 1/R2, 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)

Follow-up on this answer.