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In this alternate universe, we still have the gas giants of Jupiter, Saturn, Uranus and Neptune. However, each planet has its mass increased $3.5$ times.

How would this change affect the gravity of the rocky inner planets, especially Earth? Also, how would it affect the asteroid belt?

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    $\begingroup$ Increased its mass by a triple and a half means 4.5 times the original? $\endgroup$ – HDE 226868 Aug 5 '15 at 20:45
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    $\begingroup$ I don't think this question is answerable - the n-body problem is notoriously difficult. $\endgroup$ – Rob Watts Aug 5 '15 at 21:18
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    $\begingroup$ @HDE226868 Triple-and-a half means 3.5. $\endgroup$ – JohnWDailey Aug 5 '15 at 21:51
  • $\begingroup$ Would this change happen instantaneously, or would it have happened when the planets formed? $\endgroup$ – HDE 226868 Aug 5 '15 at 22:45
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    $\begingroup$ Increased by 3.5 is a total of 4.5. $\endgroup$ – JDługosz Aug 6 '15 at 4:32
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Oddly, some effects will not be very noticeable if the planets are stipulated to be in the same orbits. The gaps in the asteroid belts are an effect of orbital resonance rather than gravitational pull, and the Jovian "Trojan" asteroids are going to be in the same spots because the L-4 and L-5 points are determined by the relative positions of Jupiter and the Sun.

On the other hand, massive giant planets will have a much greater retinue of moons, rings and orbiting debris because of the greater gravitational pull. Jupiter will probably be a "Brown Dwarf" at 4.5X its current mass, so will also be a very strong source of infrared radiation, and moons like Europa might not have an ice covering (in fact, the water covering may have evaporated away since there would be no ice cover and the gravitation of the moon would be too small to prevent the water from evaporating away). Saturn may also become a brown dwarf, with similar effects on Titan.

Spacecraft will be in grave danger trying to carry out gravitational slingshots since they will be entering a system with thousands or millions of small bodies in orbit, making the risk of collision quite high.

Ancient peoples will also have been able to see the giant planets with the naked eye from antiquity, so there would have been 8 visible planets, with the changes to mythology and astrology that that would cause.

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    $\begingroup$ 4.5 M$_{\text{J}}$ is nowhere near massive enough for a brown dwarf. $\endgroup$ – HDE 226868 Aug 5 '15 at 22:38
  • $\begingroup$ The mass needed to become a brown dwarf is about 13 times the mass of Jupiter. Jupiter wouldn't become a brown dwarf in this scenario. Saturn wouldn't, either. $\endgroup$ – FihanoLeSugg Aug 5 '15 at 22:38
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    $\begingroup$ Depending on your sources, the minimum size of a Brown Dwarf can be as low as 1Mj (i.e. the mass of Jupiter): Nomenclature: Brown Dwarfs, Gas Giant Planets, and ? Authors: Boss, A. P., Basri, G., Kumar, S. S., Liebert, J., Martín, E. L., Reipurth, B., , Journal: Brown Dwarfs, Proceedings of IAU Symposium #211, held 20-24 May 2002 at University of Hawaii, Honolulu, Hawaii. Edited by Eduardo Martín. San Francisco: Astronomical Society of the Pacific, 2003, p. 529 So it is quite possible to classify a 4.5Mj object as a Brown Dwarf $\endgroup$ – Thucydides Aug 6 '15 at 0:18
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    $\begingroup$ @Thucydides The paper is 12 years old and out-of-date. I also cannot find anywhere in it that makes that claim. In fact, on the second page, it states that anything below 13M$_J$ is a planet, not a brown dwarf. Page 529 merely states that there is uncertainty as to the classification of some objects in the range. It contradicts what you say. $\endgroup$ – HDE 226868 Aug 6 '15 at 0:28
  • $\begingroup$ @Thucydides Triple-and-a-half is 3.5. $\endgroup$ – JohnWDailey Aug 6 '15 at 0:44
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The question how the change would "affect the gravity of the rocky inner planets" doesn't make much sense to me. Do you mean affecting the orbit, as most other answers and comments assume, or do you mean affecting the mass and hence the surface gravity?

Larger gas giants might mean a higher likelihood that even more material is sucked into them. Less material remains to coalesce as the inner planets, which are indvidually smaller, and hence a lower surface gravity.

Also fewer visible craters on the moon, if there is a moon, and fewer impacts on Earth.

  • One possible outcome would be a civilization descended from dinosaurs because they were not wiped out.
  • Or a smaller Earth which lost all atmosphere and water early on, much like Mars.

Are you getting anywhere with your alternate Earths? I fail to see how your questions lead to useful stories.

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  • $\begingroup$ The comments were correct. I do mean affecting the orbit. How could I not make sense when the questions were so easy and straightforward? $\endgroup$ – JohnWDailey Aug 6 '15 at 15:34
  • $\begingroup$ @JohnWDailey, because you asked about the gravity and not the orbit. As Thurcydides pointed out, the orbits won't be affected much. $\endgroup$ – o.m. Aug 6 '15 at 16:53

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