<p>Mercury appears to be on the boundary between being stable and unstable in the Solar System (e.g. <a href="https://iopscience.iop.org/article/10.1088/0004-637X/739/1/31" rel="noreferrer">Lithwick &amp; Wu 2011</a>). 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 <a href="https://ui.adsabs.harvard.edu/abs/1994A%26A...287L...9L/abstract" rel="noreferrer">Laskar 1994</a> and the picture below).</p>
<p>That is has a moderate eccentricity and is somewhat inclined (to the ecliptic, though less so to the solar equator) is not surprising.</p>
<p><a href="https://i.sstatic.net/Ury5p.png" rel="noreferrer"><img src="https://i.sstatic.net/Ury5p.png" alt="Orbital parameter variability in the Solar System" /></a></p>
<p><em>Numerical integration of planetary orbit parameters both forward and backward in time (from Laskar 1994).</em></p>

CC BY-SA 4.0
2023-10-04T14:52:10.603Z
+1 You may find this post interesting (although unanswered): https://space.stackexchange.com/questions/15231/is-mercurys-orbit-still-considered-potentially-unstable-in-the-very-long-term
Nilay Ghosh
CC BY-SA 4.0
2023-10-05T08:59:38.093Z
Isn't the question "*why* is Mercury on the boundary of stable and unstable?
thosphor
CC BY-SA 4.0
2023-10-05T09:07:23.793Z
@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.
ProfRob
CC BY-SA 4.0
2023-10-05T11:48:13.760Z
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.
DrZ214
CC BY-SA 4.0
2023-10-05T14:20:11.593Z
@ProfRob Indeed, the closest planet on average to every other planet in the Solar System is Mercury - the gravitational influences of other planets on Mercury tends to be higher than their influences on other planets.
Nuclear Hoagie
CC BY-SA 4.0
2023-10-05T17:26:33.107Z
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
CC BY-SA 4.0
2023-10-04T11:46:30.547Z
2023-10-04T11:46:30.547Z
ProfRob
<p>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. <strong>Edit:</strong> And to the invariable plane, it is inclined 6.35 degrees which is much greater than any other planet.</p>
<p>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.</p>
<p>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.</p>
<p>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.</p>
<p>Also note that Mercury has a 3:2 resonance in regards to its <strong>rotational</strong> period. This may or may not have anything to do with its orbital history.</p>

CC BY-SA 4.0
2023-10-04T09:56:51.567Z
The plane of the ecliptic is [inclined relative to the Sun's equator](https://en.wikipedia.org/wiki/Orbital_inclination#Observations_and_theories) 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.
notovny
CC BY-SA 4.0
2023-10-04T14:57:07.570Z
Please check these posts: https://space.stackexchange.com/questions/15231/is-mercurys-orbit-still-considered-potentially-unstable-in-the-very-long-term ... https://astronomy.stackexchange.com/questions/19126/exactly-how-elliptical-is-mercurys-orbit-visually-without-exaggeration ... https://astronomy.stackexchange.com/questions/32982/relativistic-mercury-orbit-around-the-sun ... https://astronomy.stackexchange.com/questions/24838/what-is-the-inclination-of-mercurys-orbit-w-r-t-the-suns-equatorial-plane
Nilay Ghosh
CC BY-SA 4.0
2023-10-04T18:06:41.047Z
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.
Michael_1812
CC BY-SA 4.0
2023-10-05T11:46:02.450Z
@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.
DrZ214
<solar-system><mercury>
Why is Mercury's orbit so unusual?
DrZ214
# Why is Mercury's orbit so unusual?

(Asked by: username_0 on 2023-10-04)

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.

**Tags:** solar-system, mercury
> **Comments:**
> **username_1**: The plane of the ecliptic is [inclined relative to the Sun's equator](https://en.wikipedia.org/wiki/Orbital_inclination#Observations_and_theories) 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.  
> **username_2**: Please check these posts: https://space.stackexchange.com/questions/15231/is-mercurys-orbit-still-considered-potentially-unstable-in-the-very-long-term ... https://astronomy.stackexchange.com/questions/19126/exactly-how-elliptical-is-mercurys-orbit-visually-without-exaggeration ... https://astronomy.stackexchange.com/questions/32982/relativistic-mercury-orbit-around-the-sun ... https://astronomy.stackexchange.com/questions/24838/what-is-the-inclination-of-mercurys-orbit-w-r-t-the-suns-equatorial-plane  
> **username_3**: 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.  
> **username_0**: @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.  

---

## 1 answers

### Answer by username_4 on 2023-10-04. Score: 17 | Accepted answer

Mercury appears to be on the boundary between being stable and unstable in the Solar System (e.g. [Lithwick & Wu 2011](https://iopscience.iop.org/article/10.1088/0004-637X/739/1/31)). 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](https://ui.adsabs.harvard.edu/abs/1994A%26A...287L...9L/abstract) 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.

[![Orbital parameter variability in the Solar System](https://i.sstatic.net/Ury5p.png)](https://i.sstatic.net/Ury5p.png)

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

> **Comments:**
> **username_2**: +1 You may find this post interesting (although unanswered): https://space.stackexchange.com/questions/15231/is-mercurys-orbit-still-considered-potentially-unstable-in-the-very-long-term  
> **username_5**: Isn't the question "*why* is Mercury on the boundary of stable and unstable?  
> **username_4**: @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.  
> **username_0**: 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.  
> **username_6**: @ProfRob Indeed, the closest planet on average to every other planet in the Solar System is Mercury - the gravitational influences of other planets on Mercury tends to be higher than their influences on other planets.  
> **username_7**: 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.  

---

