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Banquo's Ghost
2003-Jan-09, 01:41 PM
Could Earth-like planets exist around extra-solar gas giants? I'm thinking gas giants within the habitable zones.

kucharek
2003-Jan-09, 01:46 PM
You mean like in Star Wars?

Maybe the weather is more active, due to the sudden cooling when getting into the giant's shadow, so maybe being on a larger orbit than e.g. the jovian moons may be convenient.

I guess, the biggest problem is that the giant planets draw a lot of attention by comets and asteroids, so maybe big impacts happen too frequently.

Harald

Banquo's Ghost
2003-Jan-09, 01:51 PM
I suppose there would be a danger of getting in the way of comets being drawn to the larger body. I'm just wandering if such a world could exist. What would Titan be like if Saturn were closer to the Sun?

Avatar28
2003-Jan-09, 01:58 PM
I'm sure it could exist. Although they wouldn't TECHNICALLY be planets.

Banquo's Ghost
2003-Jan-09, 02:06 PM
okay- planet sized moon- picky, picky

traztx
2003-Jan-09, 02:24 PM
I don't see why not. Don't worry about the Sun. Io is kept very warm by Jupiter. Make it Earth-sized, and move it out a bit and maybe there is a life zone there.

Such a moon wouldn't have the same weather at all. The heat would come mainly from tidal forces so you don't have arctic and tropics. I don't know if there would be seasons. Maybe cycles based more on relative positions of other moons?

Maybe if it's made just right you can have liquid oceans with lots of volcanic islands here and there.

I'm just guessing... no expertise here. Comments welcome /phpBB/images/smiles/icon_smile.gif

Avatar28
2003-Jan-09, 04:56 PM
On 2003-01-09 09:06, Banquo's Ghost wrote:
okay- planet sized moon- picky, picky


/phpBB/images/smiles/icon_razz.gif

Glom
2003-Jan-09, 05:01 PM
It would be cool to live on Titan. Then you wouldn't need a telescope to see Saturn's rings. The Complete Cosmos had a gorgeous simulation of the vista on Titan.

John Kierein
2003-Jan-09, 05:07 PM
Europa is really earthlike. It is covered with water like the earth, but it is smaller and the water is deeper, but under the ice it is probably very much like being deep in the oceans of earth.http://www.geocities.com/CapeCanaveral/Galaxy/7827/

RafaelAustin
2003-Jan-10, 01:13 AM
What about the radiation? How far out does Jupiter's radiation extend?

I think for the planet (moon) to be stable enough to produce life like on Earth, the gas giant would have to be in an orbit similar to Earth and the moon would have to have a fairly wide orbit around the gas giant to avoid the extreme tidal and radiation disturbances.

Colt
2003-Jan-10, 04:33 AM
Isn't there a way that the moon would constantly be on the sun side of the planet but still rotate? I can't remember what it is called right now though. : -Colt

kucharek
2003-Jan-10, 08:19 AM
You're right. The body could be in one of the stable Lagrange Points (http://www.physics.montana.edu/faculty/cornish/lagrange.html) of the planet. But then, I guess, it's not longer a moon of the planet, but a co-orbiting planet of it's own.

Harald

NeptuneRise
2011-Jan-16, 06:22 PM
Well, Clement and Reynolds have used planets around blue suns. Both have a degree in physics and astronomy. Should count for something.

Hungry4info
2011-Jan-16, 07:49 PM
Isn't there a way that the moon would constantly be on the sun side of the planet but still rotate? I can't remember what it is called right now though. : -Colt

A sun-synchronous orbit. But it's not expected that natural satellites will be in such configurations
(and all moons rotate).

Hungry4info
2011-Jan-16, 07:56 PM
For Jupiter, Saturn, and Uranus, the total mass of their moon systems amounts to ~0.02% of the mass of the planet. For Neptune, after Triton was captured, Neptune probably lost much of its original system of moons.

If this is a rather common occurrence (Giant planet's moon systems being ~0.02% of the mass of their planet), then we can expect, at most, a Jupiter-mass planet can have a moon with a mass of 0.06 Me. For a Mars-mass moon, you'll need a gas giant of about 1.5 MJ. But Mars-mass bodies in the HZ may not be able to hold onto their atmospheres (see... Mars). For an Earth-mass moon, the gas giant will need to be around 15 MJ. This is in the mass range where it isn't exactly clear if the body in question is a planet or a brown dwarf.

So, I won't say Earth-mass moons around gas giants are impossible, but I'm willing to bet they are very infrequent.

Ilya
2011-Jan-16, 09:08 PM
Necro thread alert! :)

But now that it is here...

I don't see why not. Don't worry about the Sun. Io is kept very warm by Jupiter. Make it Earth-sized, and move it out a bit and maybe there is a life zone there.

Such a moon wouldn't have the same weather at all. The heat would come mainly from tidal forces so you don't have arctic and tropics. I don't know if there would be seasons. Maybe cycles based more on relative positions of other moons?

Maybe if it's made just right you can have liquid oceans with lots of volcanic islands here and there.

If you "move Io out a bit", it won't be warm any more. Tidal forces drop off as a cube of distance.

Bobunf
2011-Jan-16, 09:28 PM
Maybe the weather is more active, due to the sudden cooling when getting into the giant's shadow, so maybe being on a larger orbit than e.g. the jovian moons may be convenient.

I don't understand the concern about eclipses by gas giant. The diameter of Jupiter is about 143 thousand kilometers.. The circumference of Ganymede's orbit is about 6.7 million kilometers. Even if the Sun/Jupiter/Ganymede axis were always perfectly aligned, the eclipse would only last for 2.1% of the time or about 3 hours every seven + days. It seems unlikely that a 3 hour eclipse in the middle of an 86 hour day would have too dramatic an effect; especially considering that the night would be 86 hours long.

It would make the moon a little cooler, but it hasn't been decided how hot it was to start with.

Hungry4info
2011-Jan-16, 10:19 PM
Agreed.

Remember, half of Earth is always in the shadow of a planet (itself) without causing terrible problems.

swampyankee
2011-Jan-16, 10:55 PM
Time to search arxiv.org. I found these (http://arxiv.org/find/all/1/all:+AND+habitable+moon/0/1/0/all/0/1) pretty quickly, although only a couple are relevant to the OP's question.

neilzero
2011-Jan-17, 04:21 PM
Exactly Earth like is very improbable, with the probability improving as more differences are allowed, 24 hour rotation with respect to the sun cannot occur except approximately, as the rotation about the gas giant adds to the day length half of the time and subtracts half of the time. At least 1/10th Earth mass is likely necessary to retain an oxygen atmosphere. More than three Earth mass likely means more than 1.5 g gravity which is likely the long term upper limit for healthy and happy humans. I suspect even Neptune mass planets can have up to three Earth mass moons, even though this does not occur in our solar system. Multiple sun systems are common, so why not multiple planet systems, with at least rare geometries? Four 3 month seasons will not occur, and reasonably close is likely rare, perhaps none except in the polar regions. Most of the moons in our solar system are tide locked, but none of the planets are tide locked, so I don't know why tide licked is expected for moons, other than most moons in our solar system are tide locked. Tide Locked means only a narrow band of the planet has comfortable temperatures and all the water is likely trapped in a great icecap on the always dark side. Tide locked moons don't have an always dark side, but the dark side may last an earth year. The color temperature of most suns is cooler or hotter than our sun, so extremes are not very earth like. Photosynthesis will produce free oxygen, but free oxygen is thought to be rare by other mechanisms. A very advanced civilization can manufacture Earth like planets and moons to tight specifications. There is a slight possibility that Earth was manufactured, but no compelling evidence. Neil

Romanus
2011-Jan-19, 11:37 PM
A few years ago there was a study theorizing that a common ratio between gas giant mass and combined satellite mass in the Solar System (~1/10,000) could put constraints on the mass of any"exojovian" moons.

link:
http://www.spacedaily.com/reports/Gas_Giants_Consistently_Larger_Than_Their_Moons.ht ml

For my part, I think they exist, and may even be common, but--strictly IMO--are not nearly as favorable environments as isolated terrestrials in the habitable zone. If the host planet has a powerful, Jupiter-like magnetosphere, trapped particles will erode the atmosphere and lovingly bake the surface in ionizing radiation. If the moon is in an Io-like resonant orbit, it could be subject to volcanic activity that would make Archean Earth look like Kansas. If it's deep in the planet's gravity well, impacts will be far more energetic (and common) than they would be on Earth; if it's far from its primary star, the atmosphere would have to be very dense for an Earth-like environment. In sum, short of a Europa or Enceladus-type environment I think the odds are longer against a hypothetical "Pandora" or "Blue Moon".

Glom
2011-Jan-20, 07:37 PM
A few years ago there was a study theorizing that a common ratio between gas giant mass and combined satellite mass in the Solar System (~1/10,000) could put constraints on the mass of any"exojovian" moons.

link:
http://www.spacedaily.com/reports/Gas_Giants_Consistently_Larger_Than_Their_Moons.ht ml

For my part, I think they exist, and may even be common, but--strictly IMO--are not nearly as favorable environments as isolated terrestrials in the habitable zone. If the host planet has a powerful, Jupiter-like magnetosphere, trapped particles will erode the atmosphere and lovingly bake the surface in ionizing radiation. If the moon is in an Io-like resonant orbit, it could be subject to volcanic activity that would make Archean Earth look like Kansas. If it's deep in the planet's gravity well, impacts will be far more energetic (and common) than they would be on Earth; if it's far from its primary star, the atmosphere would have to be very dense for an Earth-like environment. In sum, short of a Europa or Enceladus-type environment I think the odds are longer against a hypothetical "Pandora" or "Blue Moon".

Damn you. I wanted to make the first Avatar reference.

Anyway, what about the possibility of an Earth-ish body orbiting a bone fide brown dwarf, orbiting a star? Imagine what Paganism would be like.

Hungry4info
2011-Jan-21, 07:18 PM
Anyway, what about the possibility of an Earth-ish body orbiting a bone fide brown dwarf, orbiting a star? Imagine what Paganism would be like.

On our planet, or on theirs?
I'm not really sure what it means, either way though. Are you talking about how their explanations of their star system? (planet-centric / brown-dwarf-centric / star-centric?)

neilzero
2011-Feb-05, 09:07 PM
The brown dwarf could look and appear to move exactly like our sun, or could be much different. This is because a brown dwarf does not have to orbit a main sequence, or any other kind of "star".
Young brown dwarf's are thought to have higher photosphere temperature than our sun at present, and they cool slowly = 20 degrees c in 5? billion years. Neil

Ara Pacis
2011-Feb-05, 11:12 PM
If we can have a giant impact creat our moon despite the astronomical likelihood of its occurance, maybe a smashup in moon orbits around a gas giant is plausible. Maybe if it's a gas giant that migrates closer in that jupiter has, there would be a higher incidence of smash-ups or captures of larger planets to become moons.

I'm not sure I understand the idea that a tidally locked moon would have a night as long as Earth's year. Ganymede's period is about a week long and Callisto is a little over two weeks long. It might be possible that a bit of heat is gleaned from sunlight reflected off the gas giant's clouds and heat radiated out from the atmosphere. After all, the size of the parent planet in the moon's sky is likely to be around 400 times larger than Earth's moon appears in our sky.

BigDon
2011-Feb-07, 05:42 PM
For Jupiter, Saturn, and Uranus, the total mass of their moon systems amounts to ~0.02% of the mass of the planet. For Neptune, after Triton was captured, Neptune probably lost much of its original system of moons.

Okay, I learned there was a limit from earlier reading on this forum. But this is sort of a different issue. What would happen to the (speculated) original moons? Did they end up in Neptune, Triton or get ejected from the system?

Hungry4info
2011-Feb-07, 08:38 PM
Probably ejected.

Hernalt
2011-Feb-08, 03:31 AM
A common mass scaling for satellite systems of gaseous planets (http://www.nature.com/nature/journal/v441/n7095/full/nature04860.html)
"General implications. When more than a few per cent of a gas planet's mass in solar composition material is processed through a circumplanetary disk, one or more generations of inflow-produced satellite systems are likely9, 23, with earlier satellites doomed to collision with the planet. Today's observed satellites are then the last generations that formed as inflow to the planets waned, implying that they formed very slowly in low-pressure, 'gas-starved'37 disks."

Compare Wiki Ganymede (#3) and Callisto (#4), particularly differences in differentiation despite similar composition.

420/candy: Revolutionary Concepts for Human Outer Planet Exploration (HOPE) (http://www.nasa-academy.org/soffen/travelgrant/bethke.pdf)

whimsyfree
2011-Feb-08, 07:10 AM
Could Earth-like planets exist around extra-solar gas giants? I'm thinking gas giants within the habitable zones.

It's a big universe so they no doubt exist but current satellite formation theories disfavor moons as large as the Earth. Capture during migration is another mechanism by which an Earth sized planet could find itself orbiting a gas giant. Radiation induced by the gas giant's magnetosphere is another problem, as is the very likely synchronous rotation of the giant moon.

Trakar
2011-Feb-10, 11:59 PM
Could Earth-like planets exist around extra-solar gas giants? I'm thinking gas giants within the habitable zones.

It's a potential, but to remain as close as possible to truely Earthlike as possible, it would have to orbit so far from the gas giant that you would approximate lunar tidal effects and impacts. The increased co-orbit would lengthen the month but this isn't a significant issue from my perspective. Even at the greater distance the much less dense gas-giant would be several times the apparent size of the Moon, and unless you are talking about a very close orbit around a much dimmer, smaller star you are going to get many more eclipses that last for longer periods, weeks to months (which would, IMO, start strongly deviating from Earth-like),...but if the co-orbit were inclined to the stellar orbit (complex tidal pattern, not sure of long-term stability of such arrangement) you might avoid most of the problem except for twice a year and a couple weeks 2x a year wouldn't stretch it beyond my rather stringent qualifications of "Earthlike." Particularly if it were a longer year - and with the added insolation from reflected primary light off of a larger, brighter companion, Mars orbit and more might suffice for closely Earthlike.

An impressive setting, particularly as one of many moons, a fertile ground for encouraging spaceflight and expansion beyond the cradle.

Ara Pacis
2011-Feb-16, 10:57 PM
It's a potential, but to remain as close as possible to truely Earthlike as possible, it would have to orbit so far from the gas giant that you would approximate lunar tidal effects and impacts. The increased co-orbit would lengthen the month but this isn't a significant issue from my perspective. Even at the greater distance the much less dense gas-giant would be several times the apparent size of the Moon, and unless you are talking about a very close orbit around a much dimmer, smaller star you are going to get many more eclipses that last for longer periods, weeks to months (which would, IMO, start strongly deviating from Earth-like),...but if the co-orbit were inclined to the stellar orbit (complex tidal pattern, not sure of long-term stability of such arrangement) you might avoid most of the problem except for twice a year and a couple weeks 2x a year wouldn't stretch it beyond my rather stringent qualifications of "Earthlike." Particularly if it were a longer year - and with the added insolation from reflected primary light off of a larger, brighter companion, Mars orbit and more might suffice for closely Earthlike.

An impressive setting, particularly as one of many moons, a fertile ground for encouraging spaceflight and expansion beyond the cradle.

What kind of orbital period and semi-major axis are you thinking of that would generate eclipses with durations of week to months?

Hungry4info
2011-Feb-17, 01:48 AM
Hundreds of AU, highly ecentric with the aphelion between the star and Earth, and the stellar radius of a supergiant. :)

Trakar
2011-Feb-18, 05:35 AM
Hundreds of AU, highly ecentric with the aphelion between the star and Earth, and the stellar radius of a supergiant. :)

A blue supergiant at that!

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.

Ara Pacis
2011-Feb-19, 08:23 AM
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.

chornedsnorkack
2011-Feb-19, 10:05 AM
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.

Very slightly faster. What matters is the combined mass of primary and satellite, and the orbital period is proportional to cube root of combined mass. Since Callisto is about 15 000 times less massive than Jupiter, a satellite of small mass in place of Callisto would have about half a minute shorter period.

Trakar
2011-Feb-19, 06:31 PM
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.

The main factor is the radius of the orbit. The issue for the senario I laid out is approximating lunar strength tidal impacts with a Jovian mass influence, which means it is going to have to be much further away. In fact, looking at a very crude approximation, such may be undoable as the distance required to duplicate lunar tidal impact may actually put an Earth-sized body into an unstable Jovian orbit...I'll go over this in more detail sometime this weekend to make sure I didn't fat-finger a calculation somewhere.

Hungry4info
2011-Feb-19, 07:50 PM
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.

Trakar
2011-Feb-19, 11:55 PM
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.

Only if the Earth-like moon orbited in tight orbit around the gas giant. If the Earth-sat was formed at a distance that even approximated early Earth-lunar scales, so long as you have an equivilantly high rate of spin, you'd maintain rotation. The only problem, which I alluded to before, was that there would be a big difference in orbital dynamics between what we see between the Earth and the moon, and between what we would see between Jupiter and the Earth if we seperated them sufficiently to match the Earth-Moon tidal/gravitational interactions. The two big differences being that 1) Jupiter is much more fluid in its response to tidal effects than the Moon (ever was or ever will be), and 2) Jupiter rotates much faster and with enough mass that any slowing due to drag by the much less massive Earth is going to be negligible. To quick and crude calculations it looks like an Earthlike moon at sufficient orbital radius to get the mass of Jupiter to approximate the mass of the moon in its effects upon the Earth, is going to put the Earthlike moon on a path to complete orbital ejection/escape within fairly short timeframes. I'd have to do more detailed analysis to better qualify how long that might actually be, but in the Earth-Moon system there is a mutual braking process at play reducing Earth rotation reduces the rate at which the Earth's tidal bulge accelerates the moon lifting it into higher less interactive orbits. In a Jupiter-Earth situation the Jupiter tidal bulge is going to be whipped forward at a much greater rate, and isn't going to slack down appreciably, meaning a steady and long term significant radial acceleration away from Jupiter, and without the various influences of a gravitationally significant co-orbiting body, I'm not sure that Earthlike remains a valid qualification.

Hungry4info
2011-Feb-20, 12:43 AM
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.

Trakar
2011-Feb-20, 05:18 AM
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.

They are an important primary influence and formative process in the development of the Earth as we recognize it today, shed or significantly alter them, particularly early in our planet's history, and we are unlikely to generate a very Earth-like planet. * It isn't so much that the conditions must precisely match, but rather that we should generally and broadly approximate all of the conditions and influences as much as we can if we are looking for generally similar conditions and outcomes.

chornedsnorkack
2011-Feb-20, 07:11 AM
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.

Trakar
2011-Feb-20, 09:10 AM
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 not sure I understand what you are trying to communicate, please explain and clarify your comments.

Hungry4info
2011-Feb-20, 06:27 PM
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).

Trakar
2011-Feb-20, 07:14 PM
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).

Could be what he is trying to say, but it doesn't really fit with anything we've discussed so far, so I'm unclear as to the relevence? Why would you keep the Earth-sat's orbital period constant?

chornedsnorkack
2011-Feb-20, 08:03 PM
Could be what he is trying to say, but it doesn't really fit with anything we've discussed so far, so I'm unclear as to the relevence? Why would you keep the Earth-sat's orbital period constant?

Because it is the disturbances from the star that set the upper limits to the orbital period of the satellite.

Ara Pacis
2011-Feb-21, 10:56 PM
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?

Hernalt
2011-Jun-11, 04:46 AM
A common mass scaling for satellite systems of gaseous planets (http://www.nature.com/nature/journal/v441/n7095/full/nature04860.html)
"General implications. When more than a few per cent of a gas planet's mass in solar composition material is processed through a circumplanetary disk, one or more generations of inflow-produced satellite systems are likely9, 23, with earlier satellites doomed to collision with the planet. Today's observed satellites are then the last generations that formed as inflow to the planets waned, implying that they formed very slowly in low-pressure, 'gas-starved'37 disks."

Compare Wiki Ganymede (#3) and Callisto (#4), particularly differences in differentiation despite similar composition.

Carolyn Porco & Mike Shara Discuss Saturn and Cassini Mission (http://www.youtube.com/watch?v=2Vm6mW1zlRQ). 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.

astromark
2011-Jun-11, 11:04 AM
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.

dgavin
2011-Jun-11, 04:40 PM
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.

Trantor
2011-Jun-13, 03:42 PM
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.

Paul Wally
2011-Jun-17, 04:33 PM
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.