The orbit of mercury is different than rest of the planets. The reason for this seemed unaccounted for in Newtonian physics, and was later explained by Einstein's theory of relativity.

Here is a quick video that explains this easily.

For a while astronomers looked for a different planet/asteroid field structure to compensate for this, and therefore make it fit into the Newtonian physics prediction. My question is: what if they should have been looking inside the sun for this change in gravity?

If the sun had a center of density that wobbled in a certain way, could this result in the observable Mercury orbit?

I am not that interested in what ramifications are caused by challenging relativity, if I want to be called a heretic I will post this on the physics page. :)

Simply put, could the discrepancy between Newton's prediction and real world measurements of Mercury's orbit be a result of variations in the center of density of the Sun?

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    $\begingroup$ No, it won't work; or, better said, it won't work without affecting the orbits of the other planets. The problem with the abnormal perihelion precession of Mercury is that it is a phenomenon observed for Mercury and not for Venus, for example. $\endgroup$
    – AlexP
    Sep 10 '18 at 2:33
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    $\begingroup$ World Building isn't a good place for this question, and as the video explains, they were already looking for a planet, Vulcan. -- For science questions, World Building is for "If I change X, then what would Y, Z, and A look like?" Not for "If X is changed, what does X look like?" ... because we already know what X looks like, you just described it to us. We're here assuming that you're an artist (writer, game dev., etc.) establishing a setting, finding out what others see as reasonable, and when to use handwavium/unobtanium... not a curious non-artist satisfying an itch. $\endgroup$
    – Ghedipunk
    Sep 10 '18 at 3:26
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    $\begingroup$ I'm voting to close this question as off-topic because this is a straight physics (or astronomy) question and there are SE sites for that. $\endgroup$
    – StephenG
    Sep 10 '18 at 4:39
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    $\begingroup$ @StephenG, The moderators ruled that real-world questions, even if they have "more appropriate" SE sites, are on-topic. And the very suggestion that the Mercurian orbit may be due to variations with the sun is fundamentally a worldbuilding context. $\endgroup$ Sep 10 '18 at 8:18
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    $\begingroup$ @elemtilas, I understand, but the point I made for my part in the real-world question debate was that there's no practical difference between a real-world question with or without a WB context. If the only difference is some statement reading, "Oh, and I'm doing this for my fictional world Xebieus," then there's really no point to mandating a WB context. Worse, most of our sister sites don't/won't entertain fictional questions like this, leaving the OP nowhere to go. Thus (among other things) the reason for making real-world questions on-topic. $\endgroup$ Sep 11 '18 at 2:30

Historically, there were several alternative explanations of Mercury's orbit before general relativity came in.

The most popular hypothesis was the assumption of an additional inner planet (it was even named Vulcan or Zoe) that was almost impossible to see (essentially, it is only visible during a solar eclipse or during a transit).

Another possible assumption is a shape of the sun that diverges significantly from a sphere, a quadrupole moment of Newtonian gravitation can explain Mercury's orbit.

General relativity is the accepted reason because besides the orbit of Mercury it predicted other effects like bending the course of a ray of light passing the sun closely.


There's probably plenty of ways to get there from adding additional physics. Specifically ruling out relativity is probably extremely doable. That said, if you want to ignore other observed phenomena and focus on just this orbit without adding anything else... then possibly?

Since this isn't I'm going to throw a thought experiment at you.

If you imagined a redistribution of mass in or near the Sun to account for this orbit then you have the same question about this mass distribution:

"How did you get it with Newtonian physics acting on itself?"

Which you could maybe answer by saying there was some coupling on it of a similar nature. Which maybe Mercury or the new mass itself provides. If you're providing a closed loop of effects at some point then you're positing a "stable" system in a now extremely sensitive and chaotic environment; since it's an orbit not predicated on a single giant mass with negligible outside interference, but a "chain" of masses in concert. Granted it's less chaotic without things like gravity waves, etc. but I would... assume that orbits would decay faster than they already do for such a precisely orchestrated setup. Unless there was some self-reinforcing factor for this distribution of energy and mass.... which, under Newtonian physics, I don't recall there being any mechanism to achieve this in space. Stability under precise conditions maybe, self-reinforcing stability I don't recall though. Maybe balancing pressures (light, etc.)?

Conclusion, don't necessarily know enough off-hand and didn't do the research. But it seems like possibly with a decent dose of "I am suspending my disbelief for you". (Especially since any world building where mentioning this as a factor in-universe is going to have them thinking about ramifications of non-relativity anyway... ie readers are smart too)

  • $\begingroup$ The "chain" part is the most relevant piece of the thought experiment. The rest is gap and tangent filler. :3 $\endgroup$
    – Black
    Sep 11 '18 at 2:32
  • $\begingroup$ Most mass "chains" (moons, etc.) don't form closed feedback loops. Asteroid belt you could argue, but the entire mass of the belt is actually exceedingly small. To the point where it's basically non-existent. (I believe it also on rare occasion kicks asteroids? Which would be catastrophic on a "single body" orbit like Mercury) $\endgroup$
    – Black
    Sep 11 '18 at 2:35
  • $\begingroup$ well in my ignorant head, i was envisioning something contained within the Sun. Perhaps a shifting mass of matter (liquid mass churning)... or a cycle of matter getting dense then erupting etc. Obviously our capability of understanding the internal workings of the SUN are....limited lol $\endgroup$
    – user46322
    Sep 12 '18 at 0:59
  • $\begingroup$ Yeah that's how I would do too. You can get a bit hand-wavey with things nobody ever has a chance of seeing in a million years (but maybe not 2 million! :P). If the in-universe gets too close to the question you can have them just say "I don't know". Not ideal of course, which I assume is one of the things the Q is meant to handle. $\endgroup$
    – Black
    Sep 12 '18 at 20:23

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