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I have just found out that the Milankovitch Cycle, a machination responsible for the creation of the Pleistocene ice ages, has its part played partly by orbit from the entire solar system. As a result of the orbit we already have back home, the cycles--on average--work as follows:

  1. Eccentricity (orbital shape): Varying between 0.000055 and 0.0679 over the course of 100,000 years.
  2. Obliquity (axial tilt): Varying between 22.1 and 24.5 degrees over the course of 41,000 years.
  3. Axial precession (change in the orientation of the rotational axis on a rotating body): Polaris being the North Star for a total of 26,000 degrees.

In this alternate solar system, the changes are as follows:

  1. Mercury is twice as wide as Earth and eight times as massive, orbiting the sun from a distance of 5.5 million miles
  2. Venus is 175% the width of Earth and 5.5 times as massive, orbiting the sun from a distance of 65 million miles
  3. Earth stays right where it is, orbiting the sun from a distance of 93 million miles, but the moon is a different story. It is now 3200 miles in diameter, has only 14% of Earth's gravity and orbits Earth from a distance of 330,000 miles.
  4. Mars, now a waterworld, is 2.6 times as wide as Earth and seven times as massive. It orbits the sun from a distance of 141.6 million miles.
  5. Jupiter, Saturn and Uranus have each doubled in diameter. Their distances from the sun are 500 million, 900 million and two billion miles.
  6. Neptune and Pluto simply don't exist.

With this list, I was told that such changes wouldn't have any dramatic effects on gravitational pertubations.

With that in mind, how would these changes affect the durations and extent of Earth's Milankovitch cycles?

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  • $\begingroup$ I think you'll find out in the same simulations you run to find out whether that situation is really stable or not. $\endgroup$ – JDługosz Jul 6 '16 at 16:02
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    $\begingroup$ For all the other planets to be that different would imply that the protoplanetary disk they evolved from would be very different. Which seeing as it would be the same disk Earth evolved from would mean that the odds of Earth having same size and orbit as now would be very close to zero. Same goes for the orbits of other bodies. This is one of those things you can ignore in fiction though. Nobody is going to run the simulations to prove your scenario can't happen... Also the effects of "minor" gravitational perturbations are cumulative and add up to major changes after few billion years. $\endgroup$ – Ville Niemi Jul 6 '16 at 16:17
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    $\begingroup$ @JDługosz That depends. It's theorized that when Jupiter first formed it was a lot hotter, thus it was actually about twice its current size due to all of that hydrogen having more energy. But with the temperature it is now...you couldn't get it back up to twice its normal size by adding mass without turning it into a star. (In fact at about 1.6x its current mass, Jupiter would actually start getting smaller as the increased gravity outpaced the buoyancy of hydrogen) $\endgroup$ – guildsbounty Jul 6 '16 at 17:48
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    $\begingroup$ After days of looking up information and listening to lectures on Planetary formation... this isn't really possible. The size of Jupiter is at the cap for Jupiter-esque situations. A bigger Jupiter means that it wasn't stopped by Saturn and the inner Solar System would not have formed the way it has. Generally we're left with about 3 inner planets. Hot Jupiters you might get 1 planet closer than mercury and way far out about earth size. Other gas giants might form further than 5 AU out but it's doubtful if something hasn't formed to stop Jupiter... $\endgroup$ – Durakken Sep 19 '16 at 5:23
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    $\begingroup$ Also to do with Jupiter sizes... The most massive we've found is like 11 Jupiter Masses, but the largest planet is .8 Jupiter Masses and like twice the radius something like that. Jupiter itself is thought to have been twice as massive at formation, but lost it, because big planets radiate away their material... and also Jupiter itself is about as big as you get in terms of radii/mass because the mass starts crunching it in more. $\endgroup$ – Durakken Sep 19 '16 at 5:27
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I'm afraid the short answer is that's it's probably impossible to tell unless you set up your new solar system and actually do N-body orbit integrations to see what happens over time. Things like eccentricity, obliquity (tilt), and precession variations can be chaotic (or not) depending on the orbits and masses of the planets. It's unlikely you'll have the same Milankovitch cycles we do now, because you've changed the interactions of the planets by changing their masses and orbits (changing their diameters makes no difference).

For example, changes in the Earth's eccentricity currently depend primarily on the gravity of Jupiter and Saturn. Since you haven't changed the masses of those planets and have barely changed their orbits, their influence would be roughly the same... except that you've made both Venus and Mars several times as massive as the Earth, and since their orbits are much closer to Earth's than is Jupiter's, this probably would have an effect. (E.g., the gravitational force of Venus on Earth at closest approach is about half that of Jupiter's; in your new system, it's about three times stronger than Jupiter's.)

It's a little hard to tell what you've done with the Moon: "has only 14% of Earth's gravity" -- does that mean its surface gravity is 14% of the Earth's (as compared with the current lunar surface gravity, which is 16.5% of the Earth's)? Assuming that plus the change in the Moon's size, your new Moon is actually almost twice as massive as the old one. Since it's further away, the change in the gravitational influence of the Moon almost cancels out. However, the tidal effects will be a little different: crudely speaking, the new Moon's tidal effects will be about 75% as strong, making the Sun's influence relatively stronger. You could guess that the periods for the obliquity (tilt) and precession variations would be a little longer than they are now (since the general history over the past several hundred million years is for the periods to get longer as the Moon moves further out in its orbit) -- if everything else in the solar system were unchanged.

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  • $\begingroup$ Titan is larger than our moon, yet its gravity is 14% that of Earth's. Is having a Venusian gravity triple as strong as a Jovian gravity a bad thing? And for the obliquity period and precession variations, how much longer? $\endgroup$ – JohnWDailey Sep 20 '16 at 1:11
  • $\begingroup$ @JohnWDailey: having Venusian gravity be stronger makes the situation more complicated. In our Solar System, you can ignore the effects of Venus and Mars for some calculations (Milankovitch's original calculations used just the Sun, Moon, Earth, Jupiter, and Saturn). In your new system, you would also have to include Venus and probably Mars, since their gravitational effects are significantly stronger. $\endgroup$ – Peter Erwin Sep 20 '16 at 11:47
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Kind of seems like there are a lot of variables for my uneducated brain to reconcile. But I'd like to give it a shot if I may.

The combined added mass off these planets would tug the sun a great deal more. Because of this, the Earth's orbit would change accordingly and result in a different eccentricity. Perhaps less elongated at the aphelion phase but further away during the perihelion phase. The axis tilt may not be as affected unless the ice ages last longer/shorter depending. More solid ice may tilt the earth further.

Finally the precession may lessen due to the moon being less dense and not affecting the tides and currents as much. It stands to reason that the less tidal activity in the oceans, the less wobble the earth will have and thus less extreme seasons. Also the moon wouldn't have as much impact on plate techtonics, which could also lessen the wobble.

I want to state for the record that I have never heard of this topic and everything I said is from me googling and extrapolating what I think are plausible outcomes from the parameters stated. My apologies if I'm completely wrong and have only proven my ignorance on the subject, but I thought I'd have a crack at it. On the other hand, I hope I helped even if it's only in a limited manner.

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  • $\begingroup$ Hi Dave, welcome to Worldbuilding. If this is a result from some googling, you might consider addind links to what you have found. That would improve the overall quality of your answer, as well as give a chance to the OP to check for themselves. $\endgroup$ – clem steredenn Sep 19 '16 at 5:41
  • $\begingroup$ Hi, thanks, and sorry for my lack of experience in this forum. I know better for next time! $\endgroup$ – Ledav Sep 19 '16 at 5:44
  • $\begingroup$ No worries, everyone started ignorant. And as a first answer, I've read worse. That being said, don't give up on your answer just yet. You can edit it anytime. $\endgroup$ – clem steredenn Sep 19 '16 at 5:55

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