# Why is Mercury's orbit so unusual?

Mercury's orbit has the most eccentricity, 0.2, of all the planets. And this orbit is also the most inclined relative to Earth, 7 degrees. Edit: And to the invariable plane, it is inclined 6.35 degrees which is much greater than any other planet.

Why is this? A body that close to the Sun should have tidal interactions that circularize the orbit much quicker than other planets. It is also hard to fathom why the orbit should be so inclined... an accretion disk should not naturally form that way, especially so close to the Sun.

Are there any theories on this? All I can think of is some relatively recent impact that greatly disturbed its orbit, but the surface of Mercury does not exactly show evidence of it.

Note: This has nothing to do with Einstein's relativity creating an anomaly in the orbital period. Whenever I search this question on the web, the results are always overwhelmingly talking about that anomaly.

Also note that Mercury has a 3:2 resonance in regards to its rotational period. This may or may not have anything to do with its orbital history.

• The plane of the ecliptic is inclined relative to the Sun's equator by about 7.1°, and the current angle between Mercury's orbital plane and the Sun's equator is about 3.33°, less than half that of Earth. Commented Oct 4, 2023 at 9:56
• Commented Oct 4, 2023 at 14:57
• Indeed, the 3:2 spin-orbit resonance (SOR) of Mercury is a direct result of a relatively high eccentricity. Direct computation shows that, for a physically reasonable rheology, a tidally Mercury despinning Mercury is virtually doomed to get stuck in one of the higher-than-synchronous SOR`s, the probability of 3:2 end-state being maximal. Tell me if you need references. Commented Oct 4, 2023 at 18:06
• @notovny True but looking at it from the perspective of the invariable plane, Mercury is inclined 6.35 deg which is much greater than any other planet. I should've put that in the OP, will edit. Commented Oct 5, 2023 at 11:46

Mercury appears to be on the boundary between being stable and unstable in the Solar System (e.g. Lithwick & Wu 2011). Its orbit can be highly perturbed by interactions, principally with Venus and Jupiter, and its eccentricty and inclination are thought, in contrast to all the other planets (except Mars), to be highly variable (see Laskar 1994 and the picture below).

That is has a moderate eccentricity and is somewhat inclined (to the ecliptic, though less so to the solar equator) is not surprising.

Numerical integration of planetary orbit parameters both forward and backward in time (from Laskar 1994).

• +1 You may find this post interesting (although unanswered): space.stackexchange.com/questions/15231/… Commented Oct 4, 2023 at 14:52
• Isn't the question "why is Mercury on the boundary of stable and unstable? Commented Oct 5, 2023 at 8:59
• @thosphor that's a reasonable question. Short answer is it comes out that way when you do the analysis. A longer answer is probably that it is the least massive planet and has the shortest orbital period that allows it to interact in lots of complex ways with the orbits of the other planets. This allows it to excite lots of resonances, some of which, can lead to instability. Commented Oct 5, 2023 at 9:07
• Ty for those amazing charts, but out of curiosity, are there such charts that include Ceres? I'd really like to see how the main belt compares to Mercury and Mars in those charts. Commented Oct 5, 2023 at 11:48
• The stabilizing effect of the Sun tidal forces could be part of the reason how Mercury has been able to remain in its almost-unstable orbit for billions of years.
– jpa
Commented Oct 5, 2023 at 17:26