The eccentricity of Venus is almost 14 times smaller than that of Mars, and 2.5 times smaller than that of Earth, and 30 times smaller than that of Mercury. Is it just a pure fluke? Or is there some “physical reasons”? I realize those are the ratios as of now, and that they change over time: will the evolution of those eccentricities dramatically change those ratios? If not, then the question of a “physical reason” emerges again.
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$\begingroup$ Possibly related: astronomy.stackexchange.com/questions/28368/can-orbits-change $\endgroup$– Carl WitthoftNov 13, 2018 at 19:49
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$\begingroup$ Of course there is a physical reason, there is a physical reason for everything. It's more about the type of answer you're expecting: The answer, as illustrated by JamesK, is of the type "multi-planet interactions", and not "this combination of natural constants gives the eccentricity of Venus", which people sometimes assume is behind everything. $\endgroup$– AtmosphericPrisonEscapeNov 14, 2018 at 16:23
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$\begingroup$ @AtmosphericPrisonEscape I had the first kind in mind definitively but I hoped a narrowed answer akin to the resonances which explains that some periods are nearly commensurate. $\endgroup$– frapadingueNov 14, 2018 at 17:37
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1$\begingroup$ Ok I see, just wanted to clarify that. $\endgroup$– AtmosphericPrisonEscapeNov 14, 2018 at 19:51
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It has a low eccentricity, but there may not be a particular reason.
Image by Kheider on wikipeda using Gravity Simulator by Trevor Dunn
In a simulation of the solar system, both Earth and Venus had orbital eccentricities that were much below those of Mars and Mercury (note the two axes. Mars and Mercury are on a scale that is 10 times bigger). But in 25000 years, Earth will actually have a smaller eccentricity, and 20000 years ago Venus's eccentricity was much larger.
So there may not need to be a special explanation for this. Planetary eccentricities do change over 1000s of years due mostly to perturbation of other planets.
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$\begingroup$ So the hierarchy endures then, Mercury and Mars on one hand, and Earth and Venus on the other hand. More striking, Mars and Mercury have a remarkably stable eccentricity over this long time span. It feels that it needs an explanation but this might just be the classic mental bias to find causes! $\endgroup$ Nov 14, 2018 at 10:41
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$\begingroup$ @frapadingue Keep in mind, the variation on that chart for Venus and Earth (eyeballing it) is about 0.02 to 0.0025. The variation on Mars is about 0.08 to about 0.105, so Mars actually varies more than Venus and Earth in the chart above. It only looks smaller because the scale is shrunk by a factor of 10. But a question on Mars might be fun. It should be a separate question though. Stack guidelines. In a nutshell, as I understand it, Mars is more influenced by Jupiter, so it's orbital eccentricity swings are larger. They appear to be longer in time/period as well. $\endgroup$– userLTKNov 14, 2018 at 13:58