Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: Glitterguts on 05/12/2013 12:18:55
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Heyoo!
So I'm working on creating a believable fictional planet with two moons and a ring system. I have some questions regarding this and any information would be deeply appreciated. I'll give you the properties of this fictional world first. Note too that it is a little bit older than Earth, but life started around the same time, and it must be able to support lifeforms.
Sun: G0 star
Terrestrial equivalent orbit: 1.22 AU
Day length: 40 hours
Year length: 1.3 earth years. It takes 474.5 earth days to orbit its sun. 284.7 planet days to orbit its sun.
Properties: : 4.2 Density (vs Earth's 5.5). 1.74 Earth radius. 1.33 Gravity. 3.6 Earth mass
Moons: 2 moons, both relatively small- a radius of .30 if Earth's is represented as 1.
The questions I then have about the moons and ring systems are as follows:
A.
- If the two moons were to be of high density in comparison to their host planet (say a density of 8), would it be possible for them to be captured objects from deeper within the galaxy?
- Is there a way to have the two moons stabilize each other’s orbits, so they don’t float away gradually like our moon does? I want moons that appear bigger in the sky than ours does, whilst being roughly the same size.
- How would the moons influence the oceans? How would I "measure" how much they influence the oceans- as in, what factors play the biggest part in this?
B.
- If the planet had rings made of dust and rock debris, could this be caused by old moons and/or captured objects?
- Old moon/s: how much would extra moon/s effect life on the planet, since the rings would have to be from a “recent” event? Recent as in after life started.
- How many extra moons could be used as an explanation for the rings realistically while still allowing planet life?
- Captured objects: perhaps asteroids etc? Would that be plausible, and keep the rings alive over time if they kept replenishing them?
C.
- If the planet had rings made of dust and rock debris, how would it effect the planet in general?
- Would it block sunlight to certain places or influence other weather patterns?
- Would it reflect sunlight, like the moon does, and shine at night?
- Would it be visible in the day sky like the moon is sometimes?
I hope this doesn't break any forum rules (I can't see how it would, buuut I apologize if I missed something.) I know there's a lot of science based forums that forbid posts which require speculation. Hopefully that wasn't too many questions either- I tried to keep them to the point and labelled so it makes replying easier. If you note anything at all that wouldn't work in the details of the planet itself, please let me know about that too!
Thank you. :)
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Is there a way to have the two moons stabilize each other’s orbits ...
A trojan moon is a moon residing at the trojan point of another (larger) moon.
https://en.wikipedia.org/wiki/Trojan_%28astronomy%29
But one moon has to be much bigger than the other for that arrangement to be stable.
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You've researched your "feasible fiction" quite well...
Here are some fictional answers based on current theories. Bear in mind that despite recent discoveries of exoplanets, we still know almost nothing about exomoons, so our knowledge is based on a sample size of 1 planetary system.
Anyway, this might give you some more ideas...
A1. If the two moons were to be of high density in comparison to their host planet (say a density of 8),
The dense rocky core of differentiated planets and meteorites tends to be a nickel/iron alloy.
Iron has a density of 7.9, and Nickel 8.9, so an overall density of 8 is just possible, if it had almost no lighter components, like silica rocks (silicon/oxygen). Forming it naturally would be a challenge, as you would need to take a differentiated body (where it was once a molten balll, and all the heavy elements sank to the center) - say an asteroid the size of Ceres, and carefully chip away the outer layers of silicates.
Slightly less dense (around 7) would be more believable.
would it be possible for them to be captured objects from deeper within the galaxy?
Based on current theories, iron is produced when a heavy star fuses all its hydrogen to iron; heavier elements do not produce more energy when fused. Iron and Nickel are released when a supernova blasts this material into space, irradiating the debris with neutrons. Even heavier elements can be produced when neutron stars collide. These could seed a protoplanetary disk with heavy elements, but getting all of the heavy elements into one place, and all the light elements elsewhere is a challenge.
When moons come from elsewhere (like Jupiter & Saturns outer moons), they tend to be in tilted orbits, sometimes backwards (retrograde). To dissipate enough energy to be in a stable, close orbit probably requires formation in-place, or a collision (which would mix up the composition).
A2. Is there a way to have the two moons stabilize each other’s orbits, so they don’t float away gradually like our moon does? I want moons that appear bigger in the sky than ours does, whilst being roughly the same size.
The Moon is drifting away slowly because the orbital period (almost a month) is much longer than Earth's Day. Tidal drag is slowing the Earth's rotation, and that angular momentum is being transferred to the Moon.
So if one of the hypothetical moons had an orbit close to the period of a day on the fictional planet (40 hours), it would produce a static tide, and would not drift away.
This orbit would be much closer than Earth's Moon, and so would look much larger in the sky. You need to ensure that this moon is outside the Roche limit, or it would crack along any fault lines, and the pieces would drift apart from each other.
A second moon would be in a stable orbit if it were about 50% further from the planet - so the orbital periods were not a simple ratio like 2:1 or 3:2.
In such a configuration, the closer moon would not produce changing tides, but the further moon would produce quite large tides, and would tend to drift away. Earth's moon is really quite large compared to Earth, and a smaller second moon would be more normal.
A3. How would the moons influence the oceans? How would I "measure" how much they influence the oceans- as in, what factors play the biggest part in this?
The tidal force (http://en.wikipedia.org/wiki/Tidal_force#Mathematical_treatment) is based on the mass of the moon & planet, and the distance between them. The impact increases rapidly as the distance decreases (inverse cubic function).
That's all the time I have at present...
Of course, anything is possible in fiction!
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RD:
Thank you heaps for the link! Much appreciated. I'll do some further research on that. :D
Evan_au:
Yes, I've had my head in astrophysics for the better part of a week, so I'm glad it shows lol.
Thank you for all the detailed info; I think I'll settle for making the moon a density of 3 or so- like our moon. I suppose if they were both to be slightly varying sizes, with a density of 3, it is feasible for them to have been parts of this planet once that got blown up from an impact from space? Or, just formed at the same time as my planet did and just happened to be within the same vicinity to get caught in orbit? Either works for the purpose of the story, I just wanna be thorough.
With all this new info to consider I'll get to working on the moons now. Then the technicalities of the ring/s should come easier. As for the anything is possible with fiction thing- very true! But I'm weirdly anal retentive about planetary specifics. It's all speculative however so there is a *little* breathing room for artistic licence. Buuut personally I do believe the best artworks are the well researched ones, and I always want to learn new things so...what better opportunity than this, haha.
Thanks again to the both of you!
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Time for a few more speculations...
Astronomers still debate theories for the rings of Saturn (https://www.google.com.au/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1256&bih=823&q=rings+saturn&oq=rings+saturn&gs_l=img.3..0.1643.3874.0.5662.12.9.0.3.3.0.172.530.7j2.9.0....0...1ac.1.32.img..2.10.207.GAAKUDRRBow) - the best example known at this time (Uranus (https://www.google.com.au/search?hl=en&site=imghp&tbm=isch&source=hp&biw=1256&bih=823&q=rings+saturn&oq=rings+saturn&gs_l=img.3..0.1643.3874.0.5662.12.9.0.3.3.0.172.530.7j2.9.0....0...1ac.1.32.img..2.10.207.GAAKUDRRBow#hl=en&q=rings+uranus&tbm=isch) has some more subtle rings).
In particular, there is debate (http://en.wikipedia.org/wiki/Rings_of_Saturn#Formation_of_main_rings) about whether they represent a short-lived phenomenon (in astronomical terms) following the disruption of an icy moon which strayed inside the Roche Limit (http://en.wikipedia.org/wiki/Roche_limit), or whether they are a long-lived remnant of Saturn's formation.
C1. If the planet had rings made of dust and rock debris, how would it effect the planet in general?
Icy rings would evaporate if they were closer to the Sun, so a planet with an Earth-like temperature could only sustain rocky rings.
Because the rings are all around the planet, they would not create a visible daily tide.
Any moons of the planet would have a big effect on the rings - the debris in the rings will be swept out of any orbits which have a resonance with the orbit of a moon. This is the cause of the many ringlets seen in closeup images from the Cassini spacecraft.
C2. Would it block sunlight to certain places or influence other weather patterns?
Saturn's icy rings can be seen to cast a shadow on the surface of the planet. Presumably, rocky rings would cast an even darker shadow on the planet. Regions near the equator would have sunlight dimmed during the day (for perhaps a quarter of a year), resulting in lower daytime temperatures in the winter. The rings would radiate some heat during the night, making the nights slightly milder.
C3. Would it reflect sunlight, like the moon does, and shine at night?
Yes, for much of the planet near the equator, the rings would form a spectacular arch in the sky before dawn and after sunset, as sunlight reflects off them down to observers who are still in darkness.
Depending on the angle relative to the Sun, the rings could enter the planet's shadow near midnight, creating a break in the rings. However, if the rings were at a sufficient angle, at midwinter & midsummer, even this gap would be illuminated, forming a complete arch at nighttime.
C4. Would it be visible in the day sky like the moon is sometimes?
Yes, they would be visible, provided they were made of a material that is fairly light in colour.
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What about diffusion in the atmosphere? would there be defined shadows from the particles defining a 'ring'? Also, giving a planet a different density should have a impact on what materials it's made of, and it is from those materials composition you get life if you want to be really picky, so you would need to define what type of life you're referring too, carbon based?
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If the inner edge of the ring system extended down to the outer edges of the planet's atmosphere, dust and rubble on the inner edge of the rings would experience friction and would spiral down out of orbit, producing:
- Frequent "shooting stars" below the rings as grains of dust enters the upper atmosphere and burn up at high alitutude
- Sometimes sonic booms as meteorites pass through the atmosphere, and explosions as they disintegrate in the lower atmosphere
- Occasional rocks and pebbles striking the ground below the rings, as larger boulders enter the atmosphere; most of it would burn up in the atmosphere, leaving a smaller core to strike the ground
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RE: evan_au "A second moon would be in a stable orbit if it were about 50% further from the planet"- Do you think maybe going with a trojan moon would be a good idea, or would one moon being further away work better, and why? Also, is it that the further a moon is, the greater the effect on the ocean?
Having my moons right on their roche limit makes their orbit work out to be something ridiculous like two hours which doesn't seem right, I'd imagine moons travelling that fast would be a bit of a danger? I'm not sure why I feel that way about it but the size of the planet vs the short moon orbit seems a bit fishy to me. I can always place them outside the roche limit and just explain it as them having drifted off slightly, but any input on the effects that a fast orbiting moon/s would have would be appreciated.
Thanks for all the info on the rings-- how would they be sustainable? As in, last for a really long time/continue to thrive? I think introducing more moons during the planet's development might prove hazardous for the development of life so any alternatives would be really appreciated (or correct me if I'm wrong.) I was thinking maybe waaaay back when life forms were just emerging there was a third moon that got too close and got ripped apart, but for the rings to keep replenishing after that...that's the tricky part.
Would the shadowed areas from the rings in the equator mean that for the large part of the year, that area is not actually a tropics zone? That the rings would nullify the direct sunlight effect?
I really like the idea of the small rock showers actually haha but that'd make it even more of a pressing issue, to sort out how these rings can remain for so long without disintegrating/how they replenish themselves.
RE: yor_on Carbon based life, yes. I assume there'd be less iron on my planer due to its larger size than Earth and smaller density.
Thanks so much!
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Want to make it a exciting system? How about meteors, asteroids or whatever, colliding on a ongoing basis? Like a fairly stable system in itself but with the 'rings' constantly replenished by other stuff? Would make it riskier, possibly, for spacecrafts but maybe you can have some genesis happening close by :) like some wandering planet that at some time can get caught, naturally with its own 'old' artefact's near by, or it just passing by.
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I was actually thinking that, like having asteroids etc be in close proximity frequently, I'd have to think of ways for space travel to be doable for them tho. I'd imagine just launching NOT NEAR THE RINGS would work best hahaha. >.>
I gotta do some research on asteroids and stuff next- ty for the inspiration/pointers!
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np, might become a book I will read with interest, don't let 'hard science' boggle down your imagination, though :)
Take it where it wants.