Hundreds of AU, highly ecentric with the aphelion between the star and Earth, and the stellar radius of a supergiant.
It's not that bad, you're going to be in a relatively long period co-orbit with the Gas Giant to get it to approximate lunar tidal influences. Given that we are moving the Gas Giant into the Earth-Mars range of the solar system and the warmer climes should lead to a puffed up Jovian(+) primary that your terrestrial moon may only orbit a handful of times in the ~year-and-a-halfish it takes the system to orbit the parent GV star. Months is undoubtably a bit long for any orbits in those ranges,...but with a bit of eccentricity in both the co-orbit and the system orbit about the primary and you might well get "weeks." But the point was that such would strech beyond what I would consider "Earth-like" conditions, so the less we push the extremes the better the case for "Earthlike." Milankovitch would have a few more factors to consider.
I forget, is the orbital period based on distance of the satellite only, or does the mass of the satellite enter into it. In other words, would a heavier moon than Callisto in Callisto's orbit around Jupiter have to travel faster or would its period be the same? Suddenly, I can't remember.
Et tu BAUT? Quantum mutatus ab illo.
For an Earth orbiting a gas giant, being tidally locked can be more or less assured. Tides thus don't particularly affect the moon in the same way the Lunar tides affect Earth. Eccentricity would drive tidal heating though, so depending on where at in the system the planet is, that may be good/bad thing.
Yeah, I understand what you're saying. I certainly agree with you that such an Earth-sat will get tidally catapulted out of orbit at that distance, I'm just not sure what the relevance of this scenario is toward a planet being Earth-like. I don't understand why having the tides in the two scenarios approximately equal is needed to consider the planet Earth-like.
Last edited by Trakar; 2011-Feb-20 at 05:31 AM. Reason: * clarification/qualification
The so-called Hill "sphere" is best approximated by ratio of orbital periods. So, a satellite of a gas giant (or brown dwarf) orbiting a sunlike star as far as Earth should not have an orbital period longer than a month, or the star would heavily perturb the orbit. The geometric distance to the planet would be longer if the planet is more massive, but perturbations from the star would be equal.
I think he's saying if you increase the mass of the planet and keep the Earth-sat's orbital period constant, the Earth-sat will not be any more betrussled against perturbations from other objects (like the star), because the Earth-sat would still be the same depth into the planet's Hill sphere (as a fraction of the Hill radius).
Thanks guys. So my question is, why would we need to have similar tidal properies? I can understand that if we want it to be earth-like then it needs to be like Earth, but I'd ignore terra-specific genesis as long as could accept some sort of ecopoiesis. Will the tidal interaction with a fast rotating gas giant necessarily eject an earth-sized body in any orbit, or only in a more distant orbit (I'm not sure I followed what was said above)? Also, could the surface be tidally locked and the core still rotating to generate a dynamo? Also, would solar tides be plausibly sufficient for ecopoiesis? what about tidal interactions with other satellites of the gas giant?
Et tu BAUT? Quantum mutatus ab illo.
Carolyn Porco & Mike Shara Discuss Saturn and Cassini Mission. In the second half, Porco talks about models for formation of rings which adds the detail to this principle that the moon falling inside the Roche limit gets torn apart.
No. I am not the end all of planetary knowledge and mechanics.
I have looked across this thread and have decided that yes it is a possible argument.
No such planetary moon has been yet detected that could be so Earth like. Does not say there are none..
However unlikely it might seem. Its possible. That a Earth like moon could be found orbiting a gas giant.
That it be in the green zone or goldilocks region be a big ask, but still a possible scenario can be imagined.
Could it have given rise to life as we would find. Well yes because it could and no maybe not...
and I have not yet said and must make clear.. that not in this solar system will this be found... unless...
Until we find such a object or moon its just a possible discussion and nothing more.
Last edited by astromark; 2011-Jun-11 at 11:09 AM. Reason: added last line for clarity..
First the ratio of planet mass to moon mass while it might seem applicable to our solar system, there are two notable exceptions to it even in our solar system. Earth/Moon and Pluto/Charon, the latter which qualifies as a double planet system.
While the ratio might be a typical plantary-moon arrangement, it is most certainly not a hard and fast rule, or even a good guide to follow. We haven't studied enough other systems to know how they are arranged yet, we are probably about 20 years to a century away from being able to detect moons around extra-solar planets.
As to the habbitable moons, it's definately possible that a moon shielded by a gas giants magenetic field, and haveing techtonics driven by the gas giants gravity, could have a viable atmospere that is not stripped off by the sun. It would likely have a much different ecosystem if one developed, especialy if it was not tidally locked, as it would have it's normal day/night rotation, but a longer night cycle as it orbited through the shadow of the gas giant.
I think that if there are gas giants located in the habital zones of their stars, then some of those gas giants, will have large Earth sized moons and possibly life on them. I also think that double Earth sized planets, in orbit around each other, in their habital zones, probably exist. It's a big Universe, and the odds are good that a wide range of life bearing planets and moons exist. Giving the fact that we are just getting started in seeing the different types of extrasolar planets/moons; any planet to moon mass ratio based on our solar system, is probably not a universal rule. As has already been mentioned, even in our solar system, that "rule" is broken by the Earth/moon and Pluto/Charon. And it wasn't that long ago that we thought there was a "rule" that all the rocky planets in a solar system were located close to their stars, while the big gas giants were further away.
The two interesting moons in our solar system are Titan and Europa. My question is what would Titan have been like if Saturn orbited the sun in the life-zone and what would Europa have been like if Jupiter orbited in the life-zone?. Several gas giants have already been discovered orbiting quite close to their stars and just in this solar system there are at least two such moons with great potential for life. What would really be interesting is if there are gas giants with two or more such habitable moons.
Given that there are probably many more moons than planets in the universe, perhaps, most life is to be found on moons rather than single rocky planets like ours.