# Can orbits change?

I was reading about how the planet Mercury changes its orbit. I was wondering if farther away planets, particularly ones with elongated orbits, could do that as well? Planets and objects such Halley's comet, Pluto, Sedna, the dwarf planets, or even planet Nine (if there is one). Could the same happen to those objects, and if so how great are those changes? Can other things change as well like the speed of the orbit, or the size?

• After reading this answer physics.stackexchange.com/a/137399/37754 I'd ask them if the outer solar system is chaotic. Given a long-enough time span, and something like if Mercury gets tossed into a highly elliptical orbit), it seems quite reasonable that one could get pulled out of it's orbit. Nov 13 '18 at 1:04
• Venus and Jupiter elongate and deelongate the orbit of the Earth in cycles of 700,000 years. Nov 13 '18 at 1:18
• Halley's comet must change its orbit due to its outgassing near the Sun. Nov 13 '18 at 22:11

# Precession

I assume you're talking about the precession of Mercury, one of the famous tests of general relativity. One reason Mercury is a good planet to test the relativistic predictions of precession is that its precession is more dramatic than the other planets. We can calculate the shift in the longitude of perihelion by $$\frac{\mathrm{d}\varpi}{\mathrm{d}t} \propto n^3a^2$$ with some other (important!) factors multiplied in. $$n$$ is the mean motion (where $$n\propto 1/T$$) and $$a$$ is the semi-major axis. Over one orbit, the net change is approximately $$\Delta\varpi=\frac{\Delta\varpi}{\Delta t}\Delta t\approx\frac{\mathrm{d}\varpi}{\mathrm{d}t}T\propto n^3a^2T=\frac{a^2}{T^2}\propto a^{-1}$$ Orbits with larger semi-major axes should have smaller $$\Delta\varpi$$, meaning that Mercury's shift is the largest in the Solar System; therefore, the other planets' orbits do precess, but by smaller amounts.

# Planetary migration

theRiley mentioned this in their answer. Planets' orbits can change over time through a process called planetary migration. This often occurs by one of two mechanisms:

• Disk-driven migration, where interactions with a protoplanetary disk change a planet's semi-major axis. This is the main theory for the formation of hot Jupiters.
• Scattering, where interactions with other planets or planetesimals causes drastic orbital changes. This is thought to have happened in the Solar System several billion years ago (see the Nice model, the Nice II model, and variations).
• The relativistic precession of Mercury, 42 arcseconds/century, pales in comparison to the ~500 arc seconds/century Newtonian precession of Mercury caused primarily by Venus, Jupiter, and the other planets. Nov 16 '18 at 14:21

Yes, especially early in a system's life-cycle, when things are less sorted. Resonances between neighboring planets' orbits can arise, inducing eccentricities which can enhance their mutual interactions, resulting in altered orbits, sometimes catastrophically of course, as with Theia and the proto-earth.

There is a theory that the Late Heavy Bombardment of the earth (4.1 - 3.8Ga ago) was caused by disruptions in the orbits of Saturn and Jupiter, which further disrupted asteroids in the asteroid and/or Kuiper belts. This is called the giant-planet migration hypothesis.