Living in a earth-like moon orbiting a planet

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Copper Bezel
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Re: Living in a earth-like moon orbiting a planet

Postby Copper Bezel » Tue Aug 06, 2013 5:04 am UTC

Yeah, it's an interesting question, although it's really equivalent to asking what the likelihood of the main proposition is. Alphas exist in droves (we have two, though not in the appropriate orbit around the right sort of star.) Beta itself is already an extremely unlikely capture event. Gamma is just (extremely unlikely)^2.
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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Tue Aug 06, 2013 2:27 pm UTC

Elmach wrote:Let me rephrase my earlier question:
1) Is it possiblelikely for another 'large' body to be in orbit around Alpha? ('large' meaning big enough to have an appreciable effect on Beta's orbit)
2) If so, what would be the consequences of the second large body?

Yeah, I'll have to echo Copper on this one. Most rocky-moon/double-planet systems (Earth-Moon, Pluto-Charon) result from a collision event, where enough rocky material is ejected from the primary to form into a terrestrial secondary, or by coalescing out of a single protoplanetary disk. But Alpha is a gas giant, so Beta would have had to have been captured rather than ejected. A capture trajectory is incredibly rare; captured moons are almost always tiny and irregular with highly eccentric orbits.

The only large rocky captured moon with a circular orbit in our solar system is Triton, a roughly moon-sized body orbiting Neptune. Its orbit is retrograde and highly inclined from the ecliptic. The probability of a low-density gas giant capturing an Earth-mass moon in a circular prograde non-inclined orbit is extremely low; the probability of a low-density gas giant having two such satellites is even more vanishingly miniscule.

Triton has cleared most of the Neptunian moon system. Any other moons of Alpha couldn't be too close to Beta or they'd either be ejected, accreted, or knocked into Alpha. The 1:2:4 resonance of Io, Europa, and Ganymede is only possible because of their small size in comparison to Jupiter; since Alpha is only eight times heavier than Beta, such an orbital resonance with any body of significant mass would be rather unlikely.

Copper Bezel wrote:Even the ones that do make use of IR don't exactly have infrared vision per se. The visible spectrum for most animals apparently starts no higher than 300 nm and goes not much lower than our lower limit of 700 nm - for the range around 8 µm where "room temperature" infrared radiation happens, you apparently need very different apparatus.

Yeah. The reflectory properties of most nocturnal animal eyes gives them enough night vision that infrared light isn't really that important. We haven't evolved reflectory eyes despite having fairly dark nights, so there's no particular reason to suppose that the inhabitants of Beta would.

The gas giants in our solar system have an albedo averaging 3.7 times greater than that of our moon. Given that Alpha would have an area 1300 times larger than our moon, the intensity of Alpha's nighttime reflectance onto the near side of Beta would be 4800 times higher than the intensity of a full moon on the equator. That's less than five percent of the sun's illuminance, of course, but it's somewhere between twilight and an overcast/cloudy day. So nighttime on the near side would never be very dark at all; you'd never need artificial lights to read with at night. The only truly dark period would be the 1 hour or so at noon.

How would life adapt to that?

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Re: Living in a earth-like moon orbiting a planet

Postby MrY » Tue Aug 06, 2013 4:51 pm UTC

davidstarlingm wrote:The only truly dark period would be the 1 hour or so at noon.


MrY wrote:Image


The eclipse occurs for every people in Beta at the same time, but not the same time of the day!
For people at the nearest point, Alpha is at zenith, like the sun.
But for other people, Alpha also hide the sun, but not at zenith!

For people on west, the eclipse occurs every day at the same hour in the morning.
For people on east, the eclipse occurs every day at the same hour in the afternoon.
For people on the outer side, Alpha hide the sun yet, but below the horizon, so this is the night infact.

Therefore we can deduce the longitude of a place depending on the time of the eclipse!

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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Tue Aug 06, 2013 6:36 pm UTC

MrY wrote:The eclipse occurs for every people in Beta at the same time, but not the same time of the day!
For people at the nearest point, Alpha is at zenith, like the sun.
But for other people, Alpha also hide the sun, but not at zenith!

For people on west, the eclipse occurs every day at the same hour in the morning.
For people on east, the eclipse occurs every day at the same hour in the afternoon.
For people on the outer side, Alpha hide the sun yet, but below the horizon, so this is the night infact.

Therefore we can deduce the longitude of a place depending on the time of the eclipse!

Ah, yes. Very good observation.

The daily simultaneous eclipse would make clock-setting and synchrony a great deal easier. Same with navigation. People would work out time zones and whatnot really quickly.

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Re: Living in a earth-like moon orbiting a planet

Postby Quizatzhaderac » Tue Aug 13, 2013 7:52 pm UTC

davidstarlingm wrote:Yeah, I'll have to echo Copper on this one. Most rocky-moon/double-planet systems (Earth-Moon, Pluto-Charon) result from a collision event, where enough rocky material is ejected from the primary to form into a terrestrial secondary, or by coalescing out of a single protoplanetary disk. But Alpha is a gas giant, so Beta would have had to have been captured rather than ejected. A capture trajectory is incredibly rare; captured moons are almost always tiny and irregular with highly eccentric orbits.
Couldn't the rocky beta-gamma pair form and then be captured by alpha? I'm sure that there's a respectable chance alpha's capture of beta would screw up gamma's orbit, but if I saw a system alpha-(beta-gamma) I'd think beta-gamma forming and then being captured is more likely than nested capture event.

For a story taking place on the dual world: I suggested the forerunners did it as a way of their species itself reproducing, to create new species with novel minds. Artifacts left behind would ensure fitness advantages to have certain types of intelligences, making evolution of sapience quicker and more predicable.

Also, another wild idea: Alpha the gas giant, is an air giant. Mixture of oxygen, water, carbon dioxide, and some buffer gasses. Microbial life (contaminates from the inhabited planets) lives on aerosols. Settlers from beta live in dirigibles and get their power from turbulence; I can't think of a reason why they'd settle on alpha, but I think we can agree that'd be awesome.
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Re: Living in a earth-like moon orbiting a planet

Postby davidstarlingm » Fri Aug 16, 2013 2:46 pm UTC

Quizatzhaderac wrote:Couldn't the rocky beta-gamma pair form and then be captured by alpha? I'm sure that there's a respectable chance alpha's capture of beta would screw up gamma's orbit, but if I saw a system alpha-(beta-gamma) I'd think beta-gamma forming and then being captured is more likely than nested capture event.

I think there's more than a respectable chance alpha's capture of beta would screw up gamma's orbit. Unless gamma is in an incredibly tight orbit and beta comes in at exactly the right angle, gamma is going to be shot out like a bullet.

On a related question: While we're engineering solar systems, is there any way to create a network of orbiting planets such that a stable trajectory for hypervelocity gravity-assist spacecraft launch exists?

For a story taking place on the dual world: I suggested the forerunners did it as a way of their species itself reproducing, to create new species with novel minds. Artifacts left behind would ensure fitness advantages to have certain types of intelligences, making evolution of sapience quicker and more predicable.

Hmm....so not quite so malevolent as originally supposed.

Also, another wild idea: Alpha the gas giant, is an air giant. Mixture of oxygen, water, carbon dioxide, and some buffer gasses. Microbial life (contaminates from the inhabited planets) lives on aerosols. Settlers from beta live in dirigibles and get their power from turbulence; I can't think of a reason why they'd settle on alpha, but I think we can agree that'd be awesome.

That's quite awesome indeed.

Is it feasible though?

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Re: Living in a earth-like moon orbiting a planet

Postby Quizatzhaderac » Fri Aug 16, 2013 8:26 pm UTC

davidstarlingm wrote:I think there's more than a respectable chance alpha's capture of beta would screw up gamma's orbit. Unless gamma is in an incredibly tight orbit and beta comes in at exactly the right angle, gamma is going to be shot out like a bullet.
I'd agree that the absolute probability of alpha-(beta-gamma) isn't great, but compared to (alpha-beta) capturing gamma is such a way that beta captures gamma? We may also want beta and gamma as close as possible anyway. While not likely, this still isn't the most complex stable orbital system; as I understand it Gliese 644 has a star (1) orbiting a system of a star (2) orbiting a system of a star (3) orbiting a binary system (5), and maybe planets, moons and brown dwarfs we can't see.
Also, another wild idea: Alpha the gas giant, is an air giant. Mixture of oxygen, water, carbon dioxide, and some buffer gasses. Microbial life (contaminates from the inhabited planets) lives on aerosols. Settlers from beta live in dirigibles and get their power from turbulence; I can't think of a reason why they'd settle on alpha, but I think we can agree that'd be awesome.
That's quite awesome indeed.

Is it feasible though?
Honestly I say probably not, but I'm not 100% sure since I doubt it's a simple system. Algae live in terrestrial clouds, but I have no idea about the rates they need to be reintroduced from the ground/ocean, or what the settling rates are for aerosols.
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Re: Living in a earth-like moon orbiting a planet

Postby Sockmonkey » Sat Aug 17, 2013 3:09 pm UTC

With the heating and cooling being so regular yet so circumscribed, what would that mean for wind patterns?
I can imagine making use of such daily shifts in sailing vessles, assuming the place has sufficient water for any sapients to do so.

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Re: Living in a earth-like moon orbiting a planet

Postby Fizban » Mon Feb 23, 2015 4:41 pm UTC

Hello. An amazing conversation! I am currently building a sci fi exploration game and I've been looking for something about habitable moons.

I have a question though (and forgive me if I've missed an earlier response); Wouldn't the days be incredibly long? Considering Beta is tidally locked to Alpha, the day/night cycle wouldn't be ruled by revolutions of Beta, but instead by orbits. Wouldn't this make one of their days the equivelant of 50 (ballpark figure) of our days?

Would this also cause some pretty bad weather at sunrise?

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Re: Living in a earth-like moon orbiting a planet

Postby SDK » Tue Feb 24, 2015 10:08 pm UTC

Thanks for bumping this! A very interesting thread.

The 24 hour orbit was an assumption from the beginning. davidstarlingm nailed the masses and distances you'd need in this post (quoted below). It's relatively simple physics to calculate all this yourself. If you wanted to change the length of the day, just figure out how much further out (or closer in) you'd need the moon to be.

davidstarlingm wrote:Alpha: 8 Earth masses, 10 Earth radii, 18 degrees of angular diameter (appears 36 times larger in diameter and 1300 times larger in area than the sun or moon).

Beta: Orbits every 24 hours; has a tangential orbit speed of 29 km/s, takes 1 hour and 11 minutes to pass through Alpha's shadow at high noon every day. 404,000 km away from Alpha, or just 5% farther than our moon is from us (though the distances from surface to surface is much less, only 334,000 km, or 89% of the Earth-moon surface separation).
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Re: Living in a earth-like moon orbiting a planet

Postby sevenperforce » Tue Feb 24, 2015 11:03 pm UTC

SDK wrote:Thanks for bumping this! A very interesting thread.

The 24 hour orbit was an assumption from the beginning. davidstarlingm nailed the masses and distances you'd need in this post (quoted below). It's relatively simple physics to calculate all this yourself. If you wanted to change the length of the day, just figure out how much further out (or closer in) you'd need the moon to be.

davidstarlingm wrote:Alpha: 8 Earth masses, 10 Earth radii, 18 degrees of angular diameter (appears 36 times larger in diameter and 1300 times larger in area than the sun or moon).

Beta: Orbits every 24 hours; has a tangential orbit speed of 29 km/s, takes 1 hour and 11 minutes to pass through Alpha's shadow at high noon every day. 404,000 km away from Alpha, or just 5% farther than our moon is from us (though the distances from surface to surface is much less, only 334,000 km, or 89% of the Earth-moon surface separation).

I think this calculation was made to maximize the angular size of the primary planet in the sky of the earth-like moon; if the primary is heavier, the orbit will be faster at a given distance, meaning you'd need a larger orbit (and a thus a smaller angular size) to hit that 24-hour "day". Gas giants don't really get much larger in terms of radius as their mass increases because they just get denser and denser at the core.

You can also have a 3:2 spin-orbit resonance, where the moon spins three times for every two orbits around the planet. I don't, however, know what sort of eccentricity would be required in order to make this resonance stable. To have a 24-hour "day", the orbit would need to take 36 hours, which would require a greater orbital radius and make the angular size of the planet in the sky decrease. Presuming a prograde rotation, the planet would pass overhead every four hours, causing three solar eclipses each day and three "lunar" eclipses (transits, actually) each night depending on your longitude and the time of year. There would be a wide range of maximum and minimum tides on a 6-day cycle.

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Re: Living in a earth-like moon orbiting a planet

Postby Himself » Wed Feb 25, 2015 6:06 am UTC

As far as the appearance of the eclipse goes, I suspect the darkness might not even be complete, since we'd get refraction/scattering through Alpha's atmosphere.
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Re: Living in a earth-like moon orbiting a planet

Postby sevenperforce » Wed Feb 25, 2015 3:49 pm UTC

Himself wrote:As far as the appearance of the eclipse goes, I suspect the darkness might not even be complete, since we'd get refraction/scattering through Alpha's atmosphere.

Well, here's an eclipse of Saturn.

Image


I don't think the refraction/scattering around Alpha's edge would let much light through, especially because we'd be much closer to Alpha than Cassini was to Saturn.

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Re: Living in a earth-like moon orbiting a planet

Postby Himself » Wed Feb 25, 2015 5:29 pm UTC

Another thing I've been thinking of in regard to atmosphere is the atmospheric circulation of Beta. Since we are talking about an object with the same mass, radius, and rotation as Earth, it seems likely that it would have similar wind belts. In other words, the atmosphere would be largely dominated by an E-W circulation of air. This, it seems to me, might help even out temperature differences caused by the E-W insolation gradient.
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Re: Living in a earth-like moon orbiting a planet

Postby Neil_Boekend » Wed Feb 25, 2015 6:02 pm UTC

Tides would be huge. I'd imagine the prevailing winds would be tidal, assuming an axial rotation of 24 hours or so.
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Re: Living in a earth-like moon orbiting a planet

Postby SDK » Wed Feb 25, 2015 6:11 pm UTC

Pretty sure in this case, since the moon is tidally locked, the only tides are going to be a result of any eccentricity in the orbit.
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Re: Living in a earth-like moon orbiting a planet

Postby sevenperforce » Wed Feb 25, 2015 7:07 pm UTC

In the case of full tidal locking, the tides would depend solely on eccentricity and be directionless -- that is, you wouldn't have any prevailing current resulting from them. You would have a tidal maximum at the near/far radial poles when you come nearest to Alpha, but this would be the tidal minimum of the equatorial points perpendicular to the radial direction. At the farthest distance from Alpha, the radial poles will have their tidal minimum and the perpendicular equatorial points will have their tidal maximum (eccentricity and tidal bulge greatly exaggerated; not to scale):
tides.png
tides.png (8.74 KiB) Viewed 2773 times


The time of day at which low tide and high tide take place (for each point on the planet) as well as their intensity, will change with respect to Beta's apsidal precession and Alpha's orbit around the star, just like our spring and neap tides; however, this effect will be on a much longer cycle than the approximately 7-day cycle of Earth. The lines show the flow direction (flood and ebb tides) for each point on Beta.

On the other hand, if you have a 3:2 resonance, the tides will vary on a six-day cycle and have a constant ocean current moving around Beta.

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Re: Living in a earth-like moon orbiting a planet

Postby gladiolas » Thu Feb 26, 2015 3:25 am UTC

http://www.centauri-dreams.org/?p=32448

This is an enormous ring system. How massive can the shepherd moons be, and how long can the rings last?

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Re: Living in a earth-like moon orbiting a planet

Postby Neil_Boekend » Thu Feb 26, 2015 6:44 am UTC

SDK wrote:Pretty sure in this case, since the moon is tidally locked, the only tides are going to be a result of any eccentricity in the orbit.

Doh! Of course.
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