Conservation of energy for a planet

Question

I was reading Lectures by Feynman (which are soo good at any qualification!) and he talks about how energy of a particular planet in our solar system is constant at all times $ i.e, KE+PE= constant.$

But what if a rogue planet appears all of a sudden and, let's say, kicks out Mars from it's orbit and from the solar system. $(imagine!)$ Now the total energy of Mars is not the same as it was before. Am I missing out on something? (which I believe I do!)

Does this answer your question? Why is the gravitational potential energy defined as negative? I shamelessly draw your attention to the first answer to that question. — garyp, Jan 06 '21 at 12:44

score 2 · Answer 1 · answered Jan 04 '21 at 12:09

You've added a new planet to the solar system. You have to add the kinetic and potential energies of that object to the rest of the solar system. If you consider that enlarged solar system, you would find that KE + PE remains constant.

One troubling word in your question is "all of a sudden". One interpretation of that is the new planet appeared from nothing. That it was brought into existence suddenly. That is unphysical. Applying physics to unphysical situations leads to "solutions" that are meaningless.

Conservation of energy for a planet

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