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B Miscellaneous Trivia: Planets and Moons

  1. Apr 30, 2016 #1
    Hey everyone. A little introduction before I ask my questions. I enjoy playing Dungeons and Dragons a lot. For those that don't know, it's a tabletop fantasy game for supreme nerds like myself to enjoy and act out adventures filled with epic magic and battles and more of the such. Lately I've been building up my own campaign for my friends to play in. Now since it's a fantasy world there's a lot of things that obviously don't follow the laws of physics, on the other hand it just doesn't feel right to world build certain things without some semblance of realism to them. These are the things that I'd like to find out about which my questions will touch on. I've tried to Google some of these questions but tend to come up short in my searches.

    1. To start with, the most frustrating of which to me, is the subject of planetary rings around life-supporting terrestrial planets. Googling this subject merely turns up artist renditions of how said rings would appear in the sky when viewed from the planet's surface. Although these depictions are quite interesting to view, what I'm really looking for is what kind of effects these rings would have on the environment and ecosystem of a planet that supports life. The reason I ask this is because I'm aware our own Moon acts as a sort of shield for our planet against meteors and steroids, drawing them to it instead of us. So what would happen to the surface of the planet and what would life on the planet experience on a regular basis from a system of planetary rings in orbit?

    2. Next up is the subject of multiple moons or around a life-supporting planet. Tidal forces seem like an obvious effect to me, and I'd like as detailed a breakdown on this as possible without throwing numbers in. One of the things I've considered for my D&D world was two moons that are on the exact same orbital path and have the same orbital period around the planet, but are located on opposite sides of the planet. I really like the idea but at the same time it seems like a very unlikely scenario, but then again, if there an infinite number of possibilities in the universe then maybe this could be one of them. Anyway I'm interested in any other effects that multiple satellites would have on a planet that I'm likely unaware of.

    3. Continuing on the subject of moons, I also have a scenario where a planet's moon has the exact same orbital period around its planet as the planet does around its star. In other words, if one were to view the solar system from the top and draw a triangle connecting the planet, its moon, and the star, the triangle would retain the exact same dimensions at all points on the planets orbit. Just thinking about it, it does seem like a feasible scenario, but I'd certainly welcome any information and thoughts on the subject.

    4. Still on the subject of moons, another scenario in my mind is that of a satellite with terrestrial properties (if there are even any other types of satellites) capable of supporting life. This also seems like a very good possibility as I understand a few satellites in our own solar system have come under serious consideration for this scenario. So on this one I'm rather more interested in the environmental differences that would arise from orbiting a gas giant, like how life would adapt to the world passing through the gas giant's shadow regularly.

    5. Along similar veins as the first question, I remembered an episode from the TV series Stargate SG-1 where a planet skirted along the edges of an asteroid belt every hundred years or so, illuminating the sky with shooting stars during each such event. What I wanted was to use a similar scenario as a way for the planet's cultures to mark the long-term passage of time. Since it would be rather important for these "fire rains" as I called them (kinda cheesy, I know) to be consistent, I figured the asteroid belt should be the same distance from the star as the planet, but on a different inclination (like Pluto) so that they intersect only twice in the planet's year. It sounds like a good idea in my head but seems a bit far-fetched at the same time, I'd assume the planet's gravity would eventually just drain the asteroid belt of its constituents.

    6. Finally, I'm interested in basically a combination of the first two questions. Essentially, a terrestrial life-supporting planet with planetary rings and two or more moons. Obviously this is dependent on the answers to the first two questions. This is probably the scenario I desire most for my D&D world despite being such an extreme scenario. Am I being too greedy here, or is this a possibility? After all, though not terrestrial and devoid of life so far as we know, all four of our gas giants possess both rings and satellites, so, why not terrestrial planets? Infinite possibilities and all, right?

    So that's it for all of my questions. After seeing other threads, mine seems a bit paltry in comparison, but I am nonetheless very interested in what people might think about my scenarios. Thanks in advance!

    P.S.: I wasn't really sure what prefix to put this under so I just chose B by default lol.
     
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  3. May 1, 2016 #2

    Janus

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    any protection provided by the Moon is quite minimal. It's gravity is just 1/81 that of the Earth, so there is no real reason for objects to be drawn to it rather than the Earth. While on occasion, the Moon would get in the way of an object that would otherwise hit the Earth, this happens for quite a small percentage compared to the number that do hit the Earth. As Far as a ring system goes, since these generally don't consist of a great deal of total mass, I don't really expect one to have a significant effect on planetary ecology.
    Two moons orbiting like this are not stable. Even if you started with such a situation, the slightest disturbance would cause them to drift out of position. You might consider a "horseshoe orbit" situation. In it, you have two moons at about the same orbital distance from the planet. Because the gravitational interaction between the moons you get the following situation: One Moon chases the other, The gravity between them pulls forward on the trailing moon and backward on the leading Moon. The forward pull on the trailing moon causes it to climb to a higher orbit and the backward pull on the leading moon causes it to fall into a lower orbit. Lower orbits are faster than higher one, so at some point the leading moon is moving faster than the trailing one and pulls away from it. Eventually, its faster orbit causes it to start chasing the other moon, so it becomes the trailing moon and process repeats with the roles of the moons reversed. This cycle keeps repeating with the moons alternating chasing each other.
    such a situation is not possible except in very special cases. One is where the Moon is at a particular point on a line joining the Star and planet, another is when it is at a particular distance on the exact opposite side of the planet from the star and the last two are points 60 degrees ahead of or behind the planet in its orbit around the star. The first two are not truly stable and a moon placed there will eventually drift away. The last two, while stable are as far away from the planet as the planet is from the star, so you can't say that an object there is orbiting the planet, but rather sharing the planet's orbit around the Star. There are also some limiting factors in terms of the ratios of star mass to planet mass and planet mass to object. The four points mentioned here, plus the one from the last question (objects orbiting on opposite sides of the primary are called Lagrange points.[/quote]

    4. Still on the subject of moons, another scenario in my mind is that of a satellite with terrestrial properties (if there are even any other types of satellites) capable of supporting life. This also seems like a very good possibility as I understand a few satellites in our own solar system have come under serious consideration for this scenario. So on this one I'm rather more interested in the environmental differences that would arise from orbiting a gas giant, like how life would adapt to the world passing through the gas giant's shadow regularly.
    [/quote] It depends on the orbit around the gas giant. The closer the orbit, the shorter the period between eclipses and the longer percentage of time of each orbit it spends in eclipse. However the closer the orbit, the more likely that the moon will be tidally locked to the gas giant, so the planet's "day" will be as long as its orbit around the gas giant. So let's say that we have a Jupiter sized gas giant with an moon orbit of 24 hrs. The far side of the Moon would have a 24 hr day of 12 hrs light and 12 hrs dark, the gas giant facing side would have the star eclipsed for about 1.8 hrs of the day light hrs. ( assuming the moon orbits in the same plane as the gas giant's orbit around the star. It you move the moon out far enough so that it is not likely to be tidally locked to the gas giant, you'd be looking at an orbital period maybe 250 days, of which the star would be eclipsed for a bit under 1/2 a day. You could come up with a whole bunch of scenarios, and the exact behavior would depend on the exact parameters.[/quote]

    5. Along similar veins as the first question, I remembered an episode from the TV series Stargate SG-1 where a planet skirted along the edges of an asteroid belt every hundred years or so, illuminating the sky with shooting stars during each such event. What I wanted was to use a similar scenario as a way for the planet's cultures to mark the long-term passage of time. Since it would be rather important for these "fire rains" as I called them (kinda cheesy, I know) to be consistent, I figured the asteroid belt should be the same distance from the star as the planet, but on a different inclination (like Pluto) so that they intersect only twice in the planet's year. It sounds like a good idea in my head but seems a bit far-fetched at the same time, I'd assume the planet's gravity would eventually just drain the asteroid belt of its constituents.
    [/quote]A number of the periodic meteor showers that we see on from Earth are caused by the Earth passing through the debris left behind by old comets. These can last a good number of years as the periods of the debris and the Earth are different and we pass through different parts of it at different passes. If the debris and the Earth were equal distance from the star, they would have equal periods and it would pass through the same part of the belt on every pass. This would tend to sweep this region cleaner faster than otherwise
    It's hard to say, we have just one star system that we can closely examine. There might be reasons why terrestrial type planets don't tend to have ring systems.
     
  4. May 5, 2016 #3
    Unless they are replenished (provided with additional material from an outside source, e.g., collisions of other moons, or another moon providing material - such as Saturn's moon Enceladus), rings are relatively short-lived (≈100 million years). Rings with sufficient material will eventually aggregate into a moon if they are outside of the Roche limit of the planet and have sufficient density. Otherwise the tidal forces of the planet will prevent a moon from forming. Depending on the material of the rings, they can be depleted over time through evaporation and/or meteor bombardment. In either event, they would typically form around the equator of the planet and have very minimal effect on the ecosystem of the planet.
    As Janus stated above, two moons that have the exact same orbit, but are on opposite sides of the planet, would not be a stable orbit. They would either collide with each other, or one of the moons would be flung away from the planet, or into the planet. If one moon is doing the "pushing" while the planet does the "pulling" and an orbital resonance can be achieved, it is possible to obtain a stable orbit. Under certain circumstances orbital resonance can be stable and self-correcting. The Galilean moons around Jupiter are a fine example of stable orbital resonance. The moons around Mars (Phobos and Deimos) are examples of when a stable orbital resonance is not achieved. Phobos is slowly spiraling in towards Mars. When Phobos gets within the Roche limit of Mars it will be ripped apart by tidal forces and likely to briefly form a small or partial ring before eventually colliding with Mars. Deimos, on the other hand, is slowly spiraling outward, away from Mars, like our moon is moving away from Earth.
    I am not sure I understand. Using the Earth and our moon as an example, are you suggesting giving the moon a 365 day orbit around the Earth? If so, the moon would no longer be in orbit around the Earth but rather fall into orbit around the sun instead. The orbital period of an object is determined by the mass of the object and its distance from its parent. If I misunderstood and you are referring to Lagrangian Points 4 or 5, then an object in such a position could prove stable (depending upon its mass), but it would not be in orbit around the planet. Like the planet the objects in Lagrangian Points 4 or 5 would be in orbit around its star, and would not be considered a moon.
    There are two possibilities to consider: 1) Either the moon is tidally locked to the gas giant; or 2) The moon is far enough away from the gas giant to have its own rotational period.

    If the moon is tidally locked and you are on the side facing the gas giant, then you would have two brief periods when you could see the sun. Almost half of your orbit around the gas giant would be in eclipse, and almost the other half would only be reflected sunlight from the gas giant. You would only see the sun briefly just before moving into eclipse with the gas giant, and again briefly as you move out of eclipse with the gas giant. If you were on the opposite side of the moon, then your day would be equal to your night, and you would never see the gas giant the moon is orbiting.

    If the moon is far enough away from the gas giant to have its own rotation, then things get more interesting. I do not pretend to know what the effects of either orbit might have on life or its evolution. For that you will need to consult an astrobiologist.
    Such a situation could only occur if the asteroid belt were somehow being regularly replenished, or if the orbits of the planet and/or asteroid belt were changing over time. Eventually, after enough passes by the planet in question, it will have consumed all of the objects with in the asteroid belt within its gravitational range. Even the cometary material Earth regularly passes through, that Janus mentioned above, will eventually be depleted after enough orbits.
    It is certainly possible to have a terrestrial planet capable of supporting life with a ring system and two or more moons. A moon or some other object that gets within the Roche Limit of the planet will be broken apart by tidal forces, and could easily form a ring system (at least temporarily). Two or more moons are also very possible, as long as they are not in the same orbit. Just be sure to give them a stable orbital resonance.

    Sources:
    Origin and Evolution of Saturn's Ring System - Chapter 17 of the book ''Saturn After Cassini-Huygens'' Saturn from Cassini-Huygens (2009), pp 537-575, ISBN 978-1-4020-9216-9, (arXiv free reprint)
    Roche Limit - Wikipedia
    Orbital Resonance - Wikipedia
    Lagrangian Point - Wikipedia
     
    Last edited by a moderator: May 7, 2017
  5. May 5, 2016 #4
    How much of the incident light do they intercept?

    Janus should know better. He is repeatedly on the opposite side of Epimetheus, and does none of the three.
    And yet they have been on spiral for some time.
    "Briefly"? Mimas is eclipsed for 2 hours or so... leaving 9 hours day and 11 hours night.
     
  6. May 5, 2016 #5

    Janus

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    Janus and Epimetheus do not maintain positions that are constantly opposed to each other, they actually follow the "horseshoe" orbit profile I mentioned in my post. By stating that such constantly opposing orbits are not stable it is meant that bodies placed there will not maintain their respective positions. They will drift out of this arrangement. One of the possible results of this is settling into a horseshoe scenario.
     
    Last edited: May 5, 2016
  7. May 5, 2016 #6
    That would depend on what the rings are made from and their density. In either case, the rings would not intercept, or reflect, enough light to make any appreciable difference to life on the planet.

    Nevertheless, Janus is correct. Two identical moons in the exact same orbit on opposite sides of the planet is not stable.

    "Some time" is relative. In the span of time since the solar system was formed, both Phobos and Deimos are only brief visitors in Mars' orbit.

    Mimas has an orbit of 22.6 hours with an eccentricity of 0.0196 and an inclination of 1.574°, which means it is eclipsed by Saturn for much longer than just "2 hours or so..." Furthermore, since Mimas is tidally locked with Saturn, when the moon is on the daylight side of Saturn, half the moon will only receive reflected sunlight.
     
  8. May 6, 2016 #7

    The winter hemisphere would, in addition to low Sun angle, have Sun eclipsed by rings. The summer hemisphere would, in addition to high Sun, receive sunlight reflected by rings.
    Then when did they form, and how?
    Um. How?
    Mimas´ orbital radius is 185 000 km. Saturn´s diametre is 120 000 km.
    Thus Saturn spans something like 0,65 radians in Mimas´ sky, right? Out of the 6,28 radians.
    Oh yes - the near side has half the time night.
    Simply speaking: ground spans 180 degrees of sky equator. Saturn, less than 40 degrees. For far side of Mimas, Saturn is underground, so 180 degrees of daylight. For near side, night is still 180 degrees of ground, eclipse is under 40 degrees, so still leaving over 140 degrees of daylight.
     
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