# Main > General Discussion >  Extreme Planetary Features vs realism

## Naima

Hello , I wanted to know if there is a discussion , a topic , a thread , a subforum where can be posed questions , or informations about extreme planetary features and their possible realistic believability on a fantasy planet .


For example ....

It wopuld be interesting to have a collection of informations of features and what would those cause to the planet , if allow life , how would change a planet etc ....

Some could be to a Planet ....

* without moon ...* 

     what happens if our planet had no moon ? No sea rises, no stabilization of the Axis tilting that would extremely change with seasons go up and down even to the equator ?


* with two or more moon ...* 

     what happens if our planet had more than one moon ? how would change sea rises, stabilization of the Axis tilting?

*with a  large igneous province* 

   Just like the Siberian traps or others , would be possible to have anything like that and that life keeps going in the rest of the planet ? what    
   would change?

*woth Larger than Earth or Smaller than Earth size* 

  How would change gravity ? would be possible somehow to have the same Earthlike gravity?

*With No tectonics*

 Like Mars , woul dhave no orography , no mountains? what woudl happen? to Magnetic field perhaps? 

*With Ice Age or Tropical Age* 

  Like in our Prehistoric past , what causes this , woudl civilizations survive etc?

*Climatic distributions* 

 How does the climate change according to geographical features or Planetary formation? How would be a planet with all deserts and no plants, could be produced oxigen the same how woudl change life etc? Or if was mostly Icey ? 




Those are just some of the possible questions on the Planet features and how could influence not only the shape, the formations of mountains, the oceans etc but also how woudl life react , if could survive , what could be the conditions to make the civilization still be possible to survive etc etc ?

IS there a section like that or would it  be need to be created and if so Is it a good idea to have a place where to ask those questions?

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## Ghostman

> *woth Larger than Earth or Smaller than Earth size* 
> 
>   How would change gravity ? would be possible somehow to have the same Earthlike gravity?


If the planet's density is the same as Earth's, then larger size results in stronger gravity on surface, and smaller size in weaker gravity on surface. It is possible for the surface gravity for a different sized planet to be comparable to that of Earth if it's composed from more/less dense material, although this is only feasible to some limit.




> *With No tectonics*
> 
>  Like Mars , woul dhave no orography , no mountains? what woudl happen? to Magnetic field perhaps?


Mars does have mountains. It's largest mountain is much bigger than Mt. Everest.

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## Naima

Thanks , But the thread point is another .

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## Azélor

I saw a couple of topics here and there but no specific thread about this.

I can risk answering some of your questions:

1- that is the main argument we hear about it. Haven't done any particular research on this topic I still know that even with the Moon, the Earth axial tilt is not 100% stable and can be influence a little by powerful earthquakes for example. And a faster rotating Earth with no Moon to slow it down could mean a different weather pattern if it's spin fast enough. as stipulated in the Geoff cookbook :The Climate Cookbook

2 - with two or more moon ... : It depend on the size on the moons especially the closest to the planet. The other are much less important. To avoid having them colliding with each others, multiple moons need to have some distance between them. The bigger they are, the bigger the distance required. Larger objects are harder to keep in orbit, so they can't be too far. The farther you move away from the planet, the weaker is it's gravity pull. At that point, the gravity pull of the star become stronger and it becomes a planet or dwarf planet possible with an unstable orbit since it's so close to a much bigger planet and it's moon(s). 

So, with an earth-like planet, the possibilities are rather limited. You can add moons but they will be (usually) much smaller than OUR MOON. You could also make or moon close to make more space for others, or make it bigger. This will have a major impact as it will slow down the planet's rotation.

3- with a large igneous province 
I don't know how this is affecting the climate considering that Siberia's climate is already harsh. 

4- Gravity depend on the mass and not the size. It's possible the have a  bigger and less dense planet but it can't be stretched very far otherwise the planet could collapse. Having more water is an easy way to achieve this. 
A smaller planet with same gravity is also possible. 

5: no tectonic activity : and a cold heart in the center of the planet mean no movements on the surface. I didn't know Mars had no tectonic activity. I though there was a huge volcano on it's surface named mount Olympus...

No tectonic activity means no more earthquakes and no more mountains formation. With enough time the center of the planet will cool down completely. Normally, for a planet the size of earth, it would take billions of years to achieve that. The Sun would be gone for long or changed in a white dwarf. Either options mean no more life Earth. The final days of the Sun will turn the Earth into a blazing desert, then, into a cold place like Mars or even colder. Maybe it's not the end of life but it's definitely the end of civilization.  

The magnetic field would still be there but would be much weaker. But since we have a dying star, we don't need it as much. 

The other scenario is highly improbable and question the existence of the Big Bang but : An old planet (or just a very large rock) formed around a star but the star eventually died. Time passes and from the remnants of the old star come a new one, much smaller but more welcoming for life. The planet is not in the habitable zone of the star but is still a dead rock. Now, from here, in this improbable scenario, is it possible for life to settle on that planet? I have no ideas. and it seems too far from the question anyway. 

6 and 7: maybe you can find some answer here: World Dream Bank: PLANETOCOPIA  (Shiveria for cold world)

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## Naima

Hehe ok thankyou , but my thread is oriended toward the creation of a possible section where to discuss those topics so to have them organized and collected for future references.
So planetary building concepts that go beyond normal earthlike realities.

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## HBrown

I'll take a stab at these:

No moon -
    While the moon probably stabilizes the movement of the Earth's axis somewhat, it's not at all clear how essential that is to life.  Different authorities make different guesses, so take your pick.

    There is some thought that tides (and particularly tidal zones) were critical in allowing life to make the transition to living on land.  While that's probably true, lunar tides are not the only tides.  The sun also causes tides which, though not as large as the lunar tides, would probably be enough to do the trick

Multiple moons -
    Most moons in our solar system are nowhere near as large, relative to their planet, as the Moon is to Earth.  Our moon is thought to be a statistical outlier, an accident of the later period of planet formation.  The theory, which is supported by good evidence, has it that a Mars-sized proto-planet slammed into the proto-Earth late in formation, blasting a spray of debris out of the Earth's crust.  Some of that debris was lost to interplanetary space, some fell back to Earth, and some formed the Moon.

   This means that an Earth-sized planet is unlikely to have two large moons like our moon.  Other worlds in our solar system have large moons, notably Pluto, so the odds of finding one around another Earth are not impossible, but I would not expect it to be common.  Smaller moons, like Mars's two captured asteroids, might be common, and multiples of small moons might be common.  We have not data yet with which to make an estimate.

    If your world has multiple moons, they will interact with each other.  For long-term stability (geologic timescales), the moons will have well-separated orbits not in harmonic resonance with each other.  If your moons are similarly sized, they will appear as different sizes from the surface of your planet because of different orbital distances.

    Of course, in a fantasy world with magic and so forth, you can have as many moons as you like, as large as you like.


Large igneous province - by which I assume you mean a region of active volcanism
    Volcanoes affect climate in two ways.  They put a lot of gasses into the air and they put a lot of particulates into the air.  The gasses - methane, carbon dioxide, sulfur dioxide, and the like - are all strong greenhouse gasses.  They enhance the atmosphere's retention of heat.  The particulates, in the form of volcanic ash, act in much the same way as clouds do.  By reflecting a portion of the sun's energy back to space, they reduce the amount of heat available to be captured at the Earth's surface and thus have a cooling effect.  The particulates can be injected into the upper atmosphere and remain there for years.

    On the whole, the balance is probably tilted toward cooling.  The year after Penetubo (sp?) erupted was a cold and wet one for much of the northern hemisphere.  The year after Krakatoa erupted was known as the year with not summer, with snowfall recorded in the summer in many paces int he United States.  A prolonged period of eruptions by multiple volcanoes may allow greenhouse gasses to build up to a greater extent than these isolated eruptions did, but the amount of particulates would be correspondingly greater as well.

    Life has survived periods of volcanism before, and probably would again.  Life is marvelously adaptable.  It's not clear that the thing we call civilization would be as adaptable, dependent as it is on a small number of intensely grown crops, but assuming people survived (which seems likely), some kind of civilization would be established in the new world.


Different size of planet -
    A planet's gravity at the surface is dependent on the planet's radius and its mass: gravity = mass / square(radius).  It's mass is a function of its mean density and its radius:  mass = 4/3 pi x density x cube(radius).  If you put those two together you see that gravity is proportional to density x radius.  If you hold the density constant, then the gravity is proportional to the radius.  The bigger the planet, the higher the gravity.

    In the solar system, density is not constant.  Even if you build the planets out of the same initial ingredients, they will be more compressed in a larger planet and we would expect larger planets, in general, to have higher densities.  This is not always true.  Mercury, in spite of being a very small planet, has a density almost as great as Earth's.  And once a planet is large enough to retain hydrogen and helium in significant amounts, it's becoming a gas giant and its bulk density plummets.

    The primary determinant of density may be the region of the solar system where the planet was formed.  In our solar system, it is thought the planets were formed more-or-less in the orbits they inhabit today.  There is evidence that Neptune has migrated into a wider orbit over time and that harmonic interactions between planets have adjusted other orbits somewhat, but that doesn't affect this argument much.  The idea is that the inner planets formed in a region that was much hotter than the region the outer planets formed in.  Heat is a measure of how fast molecules are moving.  Hotter molecules are moving faster than colder ones and are harder to capture in a gravitational field.  Bigger molecules move more slowly than smaller ones at a given temperature and are easier to catch.  For this reason, the inner planets are systematically impoverished of the lighter elements, which means their densities are correspondingly higher.  In the outer regions of the solar system, water ice behaves much as rock does in the inner system, and the low densities of many of the moons of the outer planets is a reflection of this.


    What else would be different about different sized planets?  A larger planet, with its higher gravity, would likely have smaller mountains.  Mountains are limited by plastic deformation.  If you pile stone high enough, it crushes or squishes and can no longer support the weight of the stone above it.  It's no accident that the tallest mountain in the inner solar system is on Mars - it has a much lower gravity than does Earth.

   The atmosphere might be different too.  If the planet is too small, it can't retain an atmosphere.  The Moon has a negligible atmosphere and Mars's atmosphere is 1%-2% the pressure of Earth's.  A bigger planet, with higher gravity, can hang on to those lighter molecules longer.  A planet larger than Earth might well have a thicker atmosphere, which would retain more heat than does Earth.

   Venus is about the same size as Earth and has an atmosphere sixty times thicker.  What gives there?  One theory is that all the inner planets were originally burdened with the same heavy atmospheres composed primarily of carbon dioxide and methane and water.  The gravity of Mars was too low to retain the atmosphere, and over a billion or two years the atmosphere of Mars bled off into space.  On Earth, chemical reactions involving water and, later, the chemistry of life sequestered much of the carbon from the original atmosphere into the crust.  Venus was enough hotter than earth that its water was photodisected and the hydrogen bled off into space.  With no water, there was no mechanism to sequester the carbon, and Venus today is a balmy 750 Kelvins.

    Earth is, according to many estimates, on the inner edge of the Goldilocks zone.  Even five percent closer to the sun, it is thought, and Earth too would look like Venus does today.  If Earth was enough bigger than it is, it would have retained heat as if it were closer to the sun, and would not be habitable today.  So if you're going to have a much bigger planet than Earth, push it a little further from its sun to be safe - or give it a slightly smaller sun.


No tectonics -
    There are three forces that create mountains on Earth, the largest of which is tectonics.  The two minor forces are impact cratering and volcanoes.  Volcanoes are primarily driven by tectonics, so without tectonics there would likely be no active volcanoes today.  Most impact craters in the solar system were created in the first billion years of its existence.  Due to erosion, there are very few surviving craters on Earth's surface today.  Impact craters survive on the Moon because erosive forces are slight and because its lack of tectonics does not recycle its crust.

   There is an additional mountain forming process that is not seen on earth.  Planets formed when lots of smaller bits fell to a common center.  The heat of impact was not dissipated to space because other bits were piling on top and holding it in.  Eventually the heat rose high enough the rocks melted.  The earth's center is still molten with the heat of creation and of radioactive decay.  That heat drives plate tectonics.  The amount of heat originally produced is roughly proportional to the planet's volume.  The amount it can radiate to space is a function of its surface area.  Volume is proportional to the cube of a planet's radius but surface area is proportional to the square of its radius.  This is a long-winded way of saying that bigger planets cool more slowly that smaller ones.

    Because of faster cooling, smaller planets of the same age will have thicker crusts than larger ones.  When the crust is thick enough, its mechanical strength becomes greater than the currents driving continental drift, and tectonics cease.  But solid forms of a material are generally more compact than their liquid forms.  (Water ice is a notable exception to this.)  In a very small planet, a thick and rigid crust forms, but that's not the end of it.  The molten interior continues to cool and to shrink.  The crust, no longer supported, cracks and collapses inward, forming long scarps that run for thousands of kilometers.

   So even without tectonics, you can still have mountains.  On a planet with an atmosphere and especially with water, those mountains will be eroded away fairly quickly (geologically speaking).  Continents may not disappear, but they would appear to our eyes flat and featureless.  The weather patterns on such a world would be very different.  Mountains form rain shadows, for example.  Water-laden clouds can't get over the mountains.  If they are pushed high enough to get over, they cool enough they drop most of their water as rain.  When the Indian subcontinent slammed into Asia and raised the Himalayas, it change the world's weather patterns.  I don't know enough to predict how weather would be different in a mountainless world, but we can be sure it would be very different.


Ice Age or Tropical Age -
    We don't really know what causes an ice age.  One theory that has pretty good traction is that the Earth's climate, a chaotic system (chaotic in the mathematical sense), has regions of relative stability.  One of them is a warmer regime more or less like the historical norm, and another is an ice age regime.  This theory is bolstered by the cyclic nature of ice ages.  A 'tropical age', if there is such a thing, might be another stable regime.

    Climate changes over time and we are far from having a complete understanding of it.  But we do know that it's primarily a heat engine.  The more heat that's in the atmosphere, the more active the climate is.  People are often surprised at how close those climactic regimes are.  Cool the Earth by 4.5 Kelvins and we'll have an ice age.  Warm it by the same amount and the polar ice caps will be all gone.  Warm it another 4.5 K and we'll have palm trees at the poles.

    As to how well civilization would survive, that's an ongoing experiment.  My guess is not too well.  Our civilization is dependent on a small number of cereal crops.  The best soil for growing those crops is found in the mid-latitudes of the northern hemisphere.  Fortuitously, the temperatures at these latitudes are also suitable for these crops.  If we shift the band of suitable temperature north or south by very much, which would be the case in an ice age or a tropical age, the best land for growing would no longer have a good climate for growing.  

    If the shift in climate occurred slowly enough, we might be able to develop new crops that could a) grow in the climate now found in mid-latitudes (but not if they're under ice), or b) grow well in the soils where the climate was now suitable.  My guess, given how well we are currently cooperating on climate change, is that it would not happen without a major disruption.

    There's another problem.  Consider an ice age.  Ice now comes down to, say, the forty-fifth parallel.  How many people are displaced?  Much of Europe is frozen, most of Russia, and the northern tier of states in the United States.  Where will all those people go?  Will they be welcomed with open arms?  If they are accepted into southern countries will the ideas they bring be acceptable or disruptive?  How will all those people be fed if the world's food supply is impacted?

    Perhaps it's fortunate that the current experiment is with global warming and not global cooling.


Climactic distributions -
    You have several topics lumped into this one.  I'll start with the easiest to answer, the one about a desert planet with no plants.  Even that one is really two questions.  First, for much of its history, Earth was a desert planet.  Life got along fine in the oceans and didn't venture out of the mother.  But of course, there were plants, just not on land.  If we define a plant as an organism that can capture sunlight and turn it into energy, and then postulate a planet with no plants, what would it look like?  It would be dead.  If the planet has plate tectonics, there might be bacterial life that metabolizes sulfur compounds near undersea vents, but that's about it.  Without plants, there can be no animals.  Animals get their energy from eating plants or from eating animals that eat plants.

    Such a planet would not have an oxygen atmosphere.  Oxygen is a highly reactive element.  Free oxygen (O2) in the atmosphere is ephemeral.  Sooner or later it will all react with something and pass out of the air.  We have oxygen in the air because we have living beings (plants, mostly) that give off oxygen as a waste product and constantly replenish the oxygen that is lost.  If we ever find a planet with free oxygen in the atmosphere, it will be a planet with life on it.


    The climate of a location on Earth is determined firstly by water and latitude.  The latitude determines how much solar energy you get and hence the temperature.  Water determines if you're living in a desert or not.  How much water you get depends on ocean currents and geography.  Google climate zones and you'll see how they are distributed on Earth.  Currents in the northern hemisphere move in clockwise gyres; they go counterclockwise in the southern hemisphere.  Places with similar latitudes and similar relations to ocean currents have similar climates.  Britain and the Pacific Northwest have climates with much in common.  So do the eastern coast of North America and the coast of China.  Southeast Asia and the Caribbean Basin are both prone to hurricanes and cyclones.  The seaward side of mountain ranges is always wetter than the inland side.

    The reason latitude is important is twofold.  First, he further poleward you go, the lower the angle of the sun in the sky.  A higher percentage of the light reaching Earth reflects off the atmosphere at higher latitudes and the light that reaches the ground is attenuated through more atmosphere.  Second, axial tilt changes how great the seasonal changes are.  In the tropics, seasonal changes are not very significant.  In the polar regions, the difference between a winter with no sun and a summer with no night can be extreme.

    Changing the axial tilt on your world will change how latitude interacts with climate.  A lesser axial tilt will push the regions of negligible seasonal difference poleward but will not eliminate the other regions: they will be compressed instead.  You'll still have a land (or sea) of the midnight sun, but it won't be as extensive.  It will only disappear if your axial tilt is very small.

    There is a lot more to climate than I've touched on, and a lot more than I'm qualified to speak to, so I'll stop while I'm ahead.



A topic you did not touch on is how the star around which your planet orbits will affect the planet, but perhaps that's a topic for another time.

--HBrown

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## Naima

Thanks that side is indeed interestig .

Still I would propose some new ideas possibilities .


*Nebula proximity* 

 How would work a world with a Huge big nebula visible in their sky ? Woudl have any effect like radiation , killing lifeforms or instead would have mostly no effect and allow the civilizations to rise?

*Nested moons*
 Coudl a moon have a secondary moon orbiting it ? What consequences coudl be on the Earth?

*Moon of a Larger Giant gas* 
 Coudl a Earth develop under the influence of a giant Gas ? What coudl prevent Radiation to kill anything on the surface? what are the conditions to
 support an Earthlike planed with a Big Saturn or Jupiter in the Night Sky?

*Twin suns*
  How would work Earthlike planets with a double sun in the sky shining on civilizations? Woudl those rise ?

*Twin Earths ...* 
   Could perhaps two earths orbit one near the other and form a double minisystem with both supporting life?

*Ring in the Sky*
   What could cause to have a Earthlike planet a Ring of detrites all around the planet just Like Saturn?


Btw on the 

*Large igneous province* 
   I meant a huge land of Lava , not only some Volcanic activity in a region ... Its a pretty like a sea of Lava or a lava lake on 
   the open surface .

*Axial Tilting periodical movements* 

   Could be possible small Axial tilting movements that bring the axis of the Earth much more down during a period and 
   upper in others so changing drastically the Climate based on very small periods ( at least geologically ) like for example 
   every 10 years or every 50 years or even less? Would that affect much the flora and fauna of the regions ? Perhaps 
   adapted climatic plants? Or massive migrations? 

*A mount Olympus* ( from Mars ) Volcano on Earth 
     would air be breatheable still on its top ? how woudl be even gravity?

*Large Chasms*
    I think we all have seen a lot of Large Chasms in the soil on most fantasy maps , but are those actually possible in RL    
    and what woudl be their origin ? how massive they could be ?

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## Azélor

*Nebula proximity*
yes, some nebula are visible from earth, so life is possible

*Nested moons*
Yes but it seems it's really rare. You need to meet different conditions

the smaller moon need to be in the inside the Hill sphere of the larger moon : Hill sphere - Wikipedia, the free encyclopedia
the smaller the second moon is, and the larger the larger moon is, the easier it is to make it fit in the sphere. The Hill sphere will be larger if it's far from the planet. 

so the consequences are limited because the moon can't be too big or too close to Earth. To test more specific scenario you could try Universe sandbox, there is a free trial: Universe Sandbox


*Moon of a Larger Giant gas*
I don't know enough to answer 


*Twin suns*
Yes if the second star is far enough. The closer it is, the hotter it becomes. Until the planet become a desert. 
You can have 2 stars orbiting close to each other near the center of the system. The impact on the temperature could be small if you put the planet farther from them.
You can have the planet orbit 1 star and having a second star (smaller) orbiting around the main star but very far away. Or have the closest star orbit the other star is also possible.  

*Twin Earths ...*
I don't know, they could become tidal locked to each other. Some parts of each planet would never have direct sunlight. 

*Ring in the Sky*
having a moon that orbit too close to the planet. If the moon is small enough and if it's closing the distance slowly (and not crashing)  it will disintegrate and create a ring. If the moon is too big, it could fall on the planet instead and killing pretty much everything. 

You could also imagine a scenario in the future where the rings are made either of space junk or orbital infrastructures. 




*Large igneous province*
Lava does not erupt in large quantity so it usually cools down pretty fast. That said, there was a time where earth was mostly made of lava. So it's possible but the planet could not host lifeforms without advanced technology.  

*Axial Tilting periodical movements*
dunno
*
A mount Olympus ( from Mars ) Volcano on Earth*
according to my knowledge some species could survive (in the lower stratosphere which is lower than the Olympus)but not humans. there is not enough oxygen. 

*Large Chasms*
There is plenty of Chasms on Earth! The grand canyon and the rift in east Africa are some of the best examples. Usually, chasms get filled with water when they get big enough unless they form in the desert. The biggest one would be the Atlantic ocean.

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## xpian

*Nebula Proximity* - Let me share something with you...something I'm using in my novel's (fantasy) setting.

One of the most well-known and easily visible nebulae in Earth's sky is the Orion nebula. It's about 1500 light years away from us. It's that little glowing smudge under Orion's belt.

But there are much larger nebulae out there in the universe. For instance, the LMC is a nearby dwarf galaxy visible in Earth's southern skies. It has a large nebula complex called the Tarantula nebula. The LMC is 180,000 light years away...yet the Tarantula nebula within it is still visible to the naked eye (the bright parts, anyway). This means that the Tarantula nebula is incredibly bright and titanically vast--it's one of the largest star-forming regions in our whole local group of galaxies.

Here's the kicker: if you were to take the Tarantula nebula and put it where the Orion nebula is, near Earth...it would FILL HALF THE NIGHT SKY. It would be so bright that it would cast shadows. Imagine looking up into the sky every night (for half the year, anyway) and seeing a vast, frozen inferno of filamentous glowing gasses. Yet it would still be more than a thousand light years from the planet, meaning that life would be relatively safe from radiation, supernovae, etc. (Relatively safe. If I were a god planning on making a home for life forms, I would probably not choose to locate their planet on the edge of an active star-forming region. If you want to make your planet safe for life in such a situation, just make sure it has a big, strong magnetic field and is orbiting a powerful sun capable of clearing a bow shock through those nebular outskirts.)

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## xpian

*Moon of a Large Gas Giant* 
I think most physicists believe this to be entirely possible. A gas giant can get very large before it ignites to form a star. Some estimates are that it would take TEN Jupiter masses before nuclear fusion could ignite. So Jupiter is already huge, but you can get a lot bigger and still be a gas giant planet. A big gas giant can have very large moons: Ganymede is larger than Mercury or Pluto. Something the size of Mars or Earth could easily orbit a large gas giant.

As for radiation, the inhabitants of such a moon would be even better off than they are on Earth. Jupiter has a massive magnetic field that deflects a great deal of solar and interstellar radiation. And an earth-size moon could easily have its own strong magnetic field adding to that protection. Even Ganymede has its own magnetosphere.

As for warmth, it's true that Jupiter is quite a ways out in the solar system, and thus gets a tiny fraction of the solar radiation the Earth does. This means that Ganymede, Callisto, and Europa are all frozen solid on the surface. But we now know that jupiter-like gas giants don't need to be far from their star. Our planetary surveys have shown many "hot jupiters" orbiting nearby stars at many distances, from the baking zone of Mercury and Venus out to the goldilocks zone (liquid water) of Earth. It's entirely possible to have an inhabited gas giant moon at a comfortable range. Plus, the gas giant itself is likely radiating some heat. Jupiter does this: it puts out more heat into the universe than its taking in. We're not entirely sure why...we know it's not fusion going on, but must be some other process deep within the planet.

And while its true that Io is super hot due to jupiter's tidal forces heating it up, that wouldn't necessarily happen to any inhabited moon. It's all a function of the orbital distance.

Atmosphere is no problem for such a moon. Titan, for instance, has a very thick atmosphere with wind, rain, etc.

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## xpian

*Olympus Mons on an earth-like planet*

Mars's Olympus Mons (and the three other Tharsis Montes) are very big volcanic mountains. They bulge from the surface to a ridiculous degree--Olympus Mons is three times the height of Mt. Everest, and that's on a planet that's half the size of Earth. Gravity? of course there's gravity. It would be a bit less up there, but not so much that you'd notice. Any point that's further from the planet's center of mass, including the top of Mt. Everest, has less gravity. It's just negligible. I think the confusion comes from the idea that there's zero G just a little further up, in space...which isn't true. There's gravity up there as well. The reason people on the ISS are weightless isn't because the Earth's gravity has stopped, it's because they are in free fall (they are orbiting the planet so swiftly that they are continuously "falling" around it.) You can't be in free-fall whilst standing on a mountain top--even one as tall as Olympus Mons.

Air would be very thin up there, however, as Azelor has mentioned. Only enough for the tiniest life forms, and certainly not enough for any animals like humans. That's if you're being very accurate with your planet's atmosphere.

The other thing about Olympus Mons to remember is how SHALLOW the slope of the mountain is. Seriously. You'd start walking up one side of the mountain and you wouldn't really be able to see the top. It would just be a gently rising slope in front of you. You'd keep walking, day after day, trudging over lava flows, and the air would just get thinner and thinner as you got higher up. Maybe the winds would get really nasty at certain levels, but you still wouldn't see the top of the mountain--just more slope rising in front of you. And you'd die long before you reached the top.

Which is not to say that extreme verticality is impossible. There are cliffs on Mercury that are kilometers high. It's conceivable that you could have sharp, tall, spiky mountains taller than the Himalayas on an earth-like planet. Extreme materials, extreme geology, extreme plate tectonics...it's certainly possible to have crazy-tall mountains that you *would* see the tops of as you were climbing them...as you were dying on the slopes of them...

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## Azélor

Jupiter has a massive magnetic field  but that magnetic field is not dangerous unless you are very close or far enough. But if it's too far, you lose the possible benefits of it. That is solving the radiation issue.

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## Naima

Just got Universe sandbox , its pretty cool to simulate all the bodies, I tested two moons on earth and they orbit fine a Luna and a Titan aroung earth at a roper distance, though I dunno what effects could have on earth .... Adding a ring of debris around the moon gives special strange phenomena around earth and moon like trail of stuff going in and out orbits ... so far so fun ... 

but I hope they will implement some biology related habitats simulations as well ...

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## groovey

Really interesting thread and great info and thoughts on some topics I had wondered about, so thank you to those of you taking the time to share it with the rest.

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## Naima

If you have some experiments in mind about the systems I can perform them in the simulator .

For the rest if you have ideas on weird natural or "innatural" features vs Realism please post them here and so lets have a whole thread about those arguments .

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## Naima

Just found this other interesting program ....

Extreme Planet Makeover

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## Naima

Another one .

Space Engine - Home page

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## Azélor

the universe sandbox team is working on version 2. It will come out in alpha between thursday and someday.

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## Naima

> the universe sandbox team is working on version 2. It will come out in alpha between thursday and someday.


Do u know if who has it can freely upgrade? And what features will be in it?

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## Azélor

Universe Sandbox | faq

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## Naima

well I have steam version 2 .2 .0 ... what does that means then ?

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## Azélor

actually the version we have is Universal sandbox 2 but it's called just Universal sandbox
The next version is the third but they call it Universe sandbox²

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## Naima

Ok back to some weird new Extreme planetary features : 


Half emisphere Illuminated
    A planet with only one side under sun and the other under night and an eternal twilight zone .

Red sun 
  As title says what woudl change , how woudl be the size of sun insky , how atmoshpere colors etc? Any influence on 
  possible civilizations?

Long/short day cycles 
  What woudl happen toEarthlike planets with a too short day cycle , like 6 -12 hours or longer like 48 - 96 hours? How would 
   life adapt?

Long/Short Seasons
   Hw woudl change life on earthlike planets if the orbit around the sun was extreme slow or faster? 

Increased mineral presence 
   How woudl be a planet gravity if its Iron mass was extremely more abundant than our planet or the opposite ?
   How would 

Acid Lakes
  Are possible Acid like supersized lakes or even seas?

Binary/Trinary Star systems
  how would life adapt and survive?

Gliese 667 Sunrise

----------


## Ghostman

> Long/short day cycles 
>   What woudl happen toEarthlike planets with a too short day cycle , like 6 -12 hours or longer like 48 - 96 hours? How would 
>    life adapt?


The Earth itself may be used as an example, since it's rotation has been slowing down and continues to slow down. 620 million years ago the day was ~22 hours. It may have been something like only 4-5 hours at the time when the Moon formed. Life would probably adapt to a shorter day/night cycle quite easily. A longer cycle might be more challenging in arid climes due to the temperature changes (deserts get very hot during day and very cold during night; if both day and night last longer then there's more time for heating up and cooling down.)




> Long/Short Seasons
>    Hw woudl change life on earthlike planets if the orbit around the sun was extreme slow or faster?


How long it takes to complete a near-circular orbit mostly just effects the length of seasons on the planet.

If the orbit happens to be very elliptic then it's duration becomes much more relevant because the planet's distance to the sun changes significantly throughout a year. Seasons might be more extreme for either the northern or southern hemisphere, and milder for the other. If for example the planet happens to be closest to the sun during the winter of northern hemisphere, then winters will be mild in the north and extremely cold in the south, with summers being cool in the north and extremely hot in the south.

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## Naima

So if the axial tilt is stuck vertically on the elliptical traje tory , the seasons would be always eternal and with slight ups and downs due to the distances ?

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## BlackChakram

I can contribute some details here as well.

*Moon of a Larger Gas Giant* 
While gas giants do have huge magnetic fields that deflect a lot of radiation, they also create tons. Jupiter's moon, Io for example is in a radiation belt strong enough to give a human a lethal dose in a very short timespan. I can elaborate on xpian's ideas about heat generation as well. Jupiter generates heat because it's still contracting from its formation. That gravitational energy is radiated out slowly. However, this energy does little to warm anything except Jupiter itself. 

Additionally, unless your moon is a captured asteroid, it's almost guaranteed to be composed primarily of ice. Most of the gas giant moons are. This has advantages, though. The tidal forces that stretch Io are also suspected to be stretching Europa and Callisto, possibly creating a liquid ocean beneath a surface of ice. Sooo, if you want an aquatic race, gas giant moons are excellent. But xpian is right. You could easily have something like an earth-sized planet an acceptable distance from a gas giant to support life. With a thick enough atmosphere, you could even keep it relatively warm.

*Twin Earths* 
Entirely possible. In fact, Earth and its moon are considered to be a "double planet" because they're so close in size (astronomically speaking). This likely would cause tidal locking, but wouldn't necessarily mean some parts of each planet get no sunlight. The moon is tidally locked to earth, but because its axis is tilted, it doesn't create an eclipse every time it orbits.

*Axial Tilting Periodical Movements*
Yes. Theoretically possible. What you're describing is an effect called "precession". Earth does it at a small angle on a 50,000 year cycle. Think of it like a spinning top that starts to wobble. We don't see this mess with the seasons on Earth because that 50,000 year period is far slower than the 365 day period for a year. But IF you had a planet that was precessing much faster than it was orbiting, you could get some wonky effects. However, any planet precessing that fast would sort of even itself out. So the only way you could do this would be to bend reality and have the planet precess very quickly but have a very far orbit. Science *fiction*, right!  :Wink: 

I can add a few other crazy ideas, too.

*Interesting Exosolar Planets*
Some of the planets we're theorizing have some crazy properties. Planets tidally locked with their parent stars so that one side is permanently dark, the other bright. This creates turbulent wind patterns at the terminator (the border between light and dark). You could then have a planet with permanent areas of twilight with permanent violent storms.

We've also theorized that there are "carbon planets" composed almost entirely of carbon. These planets could literally have volcanoes that erupt diamonds.

*Rogue Planets*
It's hardly impossible to have planets that get flung away from their parent stars and sent into deep space. These rogue planets would literally just wander between stars, almost permanently dark and frozen. But you could have life living deep under the surface, surviving on heat radiating from the core. I think there was a Star Trek Voyager episode that did something with a rogue planet.


If your axial tilt is stuck vertically (which, if I understand what you're getting at would mean NO tilt), then you pretty much won't have seasons. You'll get a planet with weather the same all year round. Farther north you go, the colder it gets, but that'd be about it. If your trajectory is elliptical enough, you will get seasons, but they'll affect the entire planet at once, rather than the north-south hemisphere opposites we get on earth.

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## Naima

Thankyou , how many different abitable worlds you can Imagine? or Astronomy can define as abitable? from positions in the system to size, features, etc etc ... I really can think only of a few but I bet there are dozens of other combinations I Can't think of .

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## BlackChakram

> Thankyou , how many different abitable worlds you can Imagine? or Astronomy can define as abitable? from positions in the system to size, features, etc etc ... I really can think only of a few but I bet there are dozens of other combinations I Can't think of .


I mentioned in this thread, that only real requirement for life as we know it is liquid water as a solvent and transport mechanism for complex organic molecules. Note that I said "as we know it". I suppose it's possible for some kind of life to exist without water, but it would be so foreign that we may not even be able to recognize or understand that it's alive.

So where do we get liquid water? The simplest model is based on distance from the star. This wikipedia article discusses that distance based on star size. We can modify this, though, from several things. If your planet is too far away, you need to warm it up. This could be through tidal forces like has been discussed here with Jupiter's moons. It could be a thick atmosphere that traps heat. It could be water deep underground that is warmed to liquid state by heat from volcanoes. Anything you can reasonably come up with here is good.

Same with being too close. You need to cool down to get liquid water. This could be in a band near the tops of mountains where things are cooler. It could be under an atmosphere that reflects the star's heat. It could be on the terminator of a tidally locked planet. Once again, any reasonable way you could conceive to drop the temperature.

As for size, I think someone already mentioned that here, but I could be wrong. Heavier worlds would create shorter, stockier creatures. I'm not a biologist, but I would assume you can reach a point with gravity where creatures can't hold themselves up, no matter how stocky or solid they are. Low gravity will breed more flying-type creatures. No need to waste energy if you can get around better by gliding than walking.

As for other features, I guess you'd have to look at that on a case-by-case basis. Mass Effect had the perfect example. On Palaven, the Turian homeworld, there was no planetary magnetic field. This allowed more dangerous radiation to reach the surface where it could affect the creatures. So life on that world evolved to have metallic components in their skin that could deflect / deal with this radiation. (They really nailed their science in that series.) 

There probably are endless combos, but I can think of at least one. A planet in the path of a pulsar would get bathed with deadly radiation thousands of times per second. But you could have a planet and a pulsar arranged in such a way that only one latitude gets exposed, so you have a dead strip on the planet where nothing can survive.

Hope this answers your question!

----------


## Chick

Just an off-topic comment .... when I saw this thread, I thought here is something I can really contribute to, having taught Planetary Geology at two different universities.  Instead, I am really impressed with the number of people who have posted really excellent answers.  Not sure why, but I pictured cartography as a technical art form, and instead I discover a lot of seriously good scientific knowledge here as well.  Very cool!

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## Naima

Please post the most of the information you can , I sugest even to make possible scenarios , those could be usefull as well , let your ideas and fantasy go wild but tied to physics and universe laws .

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## Naima

Ok here is another question ... 

I have a planet like earth , and I woudl like to have two moons , one double of moon the other sized as the moon , and by size I mean when seen by the ground , so they can be either smaller and more close or larger and more distant whatever fits best ... 

then I was wondering if I can also add two suns , one as normal sun and the other as a much minor sun visible in distance almoust looking like a very bright star ....

what woudl be the problems and advantages of such a configuration? how would environments react and also how can I manage to enstablish the cycles of all the year and moon phases? 

Also how those woudl have any effect on north pole , magnetism , tectonics, seasons , floods , weather etc?

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## BlackChakram

The setup of the moons would mostly affect the tides. Instead of a nice pattern of tides twice a day, you'd get a much more irregular and complex pattern of tides spaced oddly and of different sizes. Maybe you'd see some increased tectonics as well, but not likely to be much.

With the two suns, you wouldn't see much effect from the farther one. It may increase the average temperature of your planet by 5-15 C, but that's about it. If it's far enough away to mostly be just a bright star, then nearly all its heating and lighting effects will be too small to notice.

That's all that should be affected. Magnetism, north pole, seasons, and weather should all be pretty much unaffected.

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## Naima

how would rotate the suns around each other and how the moons? I can use any tool to calculate those?

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## BlackChakram

If you wanted to get some exact numbers, you'd probably need to use some software like Sandbox Universe. But essentially, here's a rough idea:



The star on the left would be your main one. The band around it represents the area where planets can orbit. Your planet with its two moons would be in this area. You na use any remotely realistic numbers here for orbit distance and moon distance. Your dimmer, other star is the one far on the right. Numbers on this scale are about as precise as you can get. You can make up anything you want as long as its within these ranges.

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## Azélor

> If you wanted to get some exact numbers, you'd probably need to use some software like Sandbox Universe. But essentially, here's a rough idea:
> 
> 
> 
> The star on the left would be your main one. The band around it represents the area where planets can orbit. Your planet with its two moons would be in this area. You na use any remotely realistic numbers here for orbit distance and moon distance. Your dimmer, other star is the one far on the right. Numbers on this scale are about as precise as you can get. You can make up anything you want as long as its within these ranges.


i tried the software but he has a serious problem with binary systems. You need to folow the tutorials to get it to work and still. My planet was at a good 50 AU from the center and was still unstable. 

I calculated the brightness of a star at around 75 AU. A big white dwarf... Nearly 3 time brighter than Venus and 600 time fainter than the full moon. By comparison, Sirius A, the brightest extra solar star as seen from Earth is about 50-60 times dimer than our old star. i used this formula to calculate the brightness : Inverse-square law - Wikipedia, the free encyclopedia
I suppose this formula can also be used to determine the energy dissipation ?  the impact on temperature)

infos from that topic: http://www.cartographersguild.com/co...rophysics.html

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## BlackChakram

Alright. I've whipped up some equations with some sample math. It was easier to do this as a word document. Post questions if you have 'em!

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## Azélor

so if the brightness of a star is around -16 in scale of magnitude, it's more than 100 times dimer than the Sun ?  

the apparent magnitude is -6 from the planet. I got that number using the inverse square law. I tried you formula and I got the same numbers  :Smile: .

so if I understand correctly, the second star increases the temperature by 10.38. That is a huge impact considering the star is at 216 AU. The difference between a temperate world and a tropical one. 
In my case I get 9.37 degrees. I was going to wonder If something was wrong with the formula, but no. I tried with Neptune and got similar numbers to the wiki.

0.008 times is the brightness of the star compared to the Sun and it is located at 75 AU.

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## BlackChakram

all sounds about right to me!

And yeah. 10 k does make a difference. But that can easily be undone by reflective cloud cover, planetary tectonic activity, or any other number of factors.

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## s0meguy

> There probably are endless combos, but I can think of at least one. A planet in the path of a pulsar would get bathed with deadly radiation thousands of times per second. But you could have a planet and a pulsar arranged in such a way that only one latitude gets exposed, so you have a dead strip on the planet where nothing can survive.
> 
> Hope this answers your question!


Wouldn't the gamma radiation just go through the planet and irradiate the other side too? As well as irradiate the atmosphere, that circulates on the planet between the part that is facing the pulsar and the part that isn't.

But perhaps complex life forms could develop that utilize the radiation could be possible under such circumstances too? 

This article is about how at least a fungi can use radiation for sustenance.

Chernobyl Fungus Feeds On Radiation

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## BlackChakram

> Wouldn't the gamma radiation just go through the planet and irradiate the other side too? As well as irradiate the atmosphere, that circulates on the planet between the part that is facing the pulsar and the part that isn't.
> 
> But perhaps complex life forms could develop that utilize the radiation could be possible under such circumstances too?


Gamma radiation can be stopped with 15 inches of lead or 3 feet of dirt.  :Smile: 

The only kind of radiation or particle that would be able to go through a whole planet would be neutrinos, but the good thing is that they'd harmlessly pass right through you as well.

The fungus thing is cool, though. Could definitely present some interesting opportunities.

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## MarilynA

> On the whole, the balance is probably tilted toward cooling.  The year after Penetubo (sp?) erupted was a cold and wet one for much of the northern hemisphere.  The year after Krakatoa erupted was known as the year with not summer, with snowfall recorded in the summer in many paces int he United States.  A prolonged period of eruptions by multiple volcanoes may allow greenhouse gasses to build up to a greater extent than these isolated eruptions did, but the amount of particulates would be correspondingly greater as well.



Wee small correction here.  Krakatoa was in 1883. The "Year Without a Summer" was courtesy of Mount Tambora. The volcanic erruption was in April, 1815,and the YWAS was 1816.

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## BlackChakram

The "Little Ice Age" from about the 1300-1700s was something like this too. Although more nebulous in cause.

----------


## MarilynA

> The "Little Ice Age" from about the 1300-1700s was something like this too. Although more nebulous in cause.


Mixed causes. There was more vulcanism, possibility of reduction in solar activity, possibility of reduction in oceanic circulation, and more.

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## Azélor

I have some questions about astrophysics. They are mainly related to the CWBP2. 

One of our planet is tide locked to the star. The star is a G star just like the Sun. I was wondering how close the planet need to be in order to become tide locked. It's about the size of Mercury.

I would also greatly appreciate if you could give me your opinion on this topic : http://www.cartographersguild.com/co...tml#post252715

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## BlackChakram

Hm... the equation for tidal locking is really complicated. But even Mercury isn't tidally locked, so you'd have to do one of two things: 1) Wait longer, as all things tidal lock with time, or 2) get closer.

I'd go with option 2 and do something like 0.2 AU. But this is just a rough estimate. Like I said, the equation is complicated.  :Smile:

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## xpian

@BlackChakram - For tidal locking, you could also use some special circumstances, could you not? I mean, sure, it would take billions of years for a planet like Earth to tidally lock to a star like the sun, but what if that mars-sized object that struck the Earth long, long ago--the one that caused the formation of the moon--had hit us at *just the right angle*? What if it had hit Earth in such a way that it slowed the planet's rotation dramatically, allowing it to settle into a tidally-locked situation.

These things happen. Just look at Uranus: the planet orbits ON ITS SIDE. That's one huge-ass planet, and it didn't start off pointing its pole down near the ecliptic. No, something extremely violent happened to Uranus in the very distant past, and it completely and permanently put the planet in a goofy orientation.

Even if you're not going to invoke gods or aliens as the reason why your planet is tidally locked, you could invoke bizarre circumstance.

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## BlackChakram

> @BlackChakram - For tidal locking, you could also use some special circumstances, could you not? I mean, sure, it would take billions of years for a planet like Earth to tidally lock to a star like the sun, but what if that mars-sized object that struck the Earth long, long ago--the one that caused the formation of the moon--had hit us at *just the right angle*? What if it had hit Earth in such a way that it slowed the planet's rotation dramatically, allowing it to settle into a tidally-locked situation.
> 
> These things happen. Just look at Uranus: the planet orbits ON ITS SIDE. That's one huge-ass planet, and it didn't start off pointing its pole down near the ecliptic. No, something extremely violent happened to Uranus in the very distant past, and it completely and permanently put the planet in a goofy orientation.
> 
> Even if you're not going to invoke gods or aliens as the reason why your planet is tidally locked, you could invoke bizarre circumstance.


This is quite valid. You're right, bizarre situations like this could very easily allow tidal locking for almost anything you want. Hence the joy in creating worlds, eh?  :Smile:

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## johnvanvliet

as above "tidal locking" is complicated 
a example might be kepler 10b 


a good analogy is all the moons of Saturn 
while not "locked"  they are in sync 
a harmonic sync of 2 to 3 or 3 to 5 

the orbits somewhat stabilize out at distances ( relative to mass) into 1:1 or 2:3 or 4:5 periods

----------


## Soloeus

If a gas planet (like Saturn) migrated close (but not too close) to a star (maybe the size of Alpha Centuari), would it be viable for the moons of this gas planet to absorb gases from the planet similar to the companion effect that impacts stars? If the Gas Planet had plenty of Oxygen and Nitrogen, could it be possible for life to be sustained on these moons?  Could the atmosphere of a gas planet comprised of oxygen (primarily) extend beyond the moons, which if solid and contained enough water, produce or sustain life?

I am looking at the creation of a starfield and am looking at details where possible.  I don't mind stretching a bit, but I want to be believable, and design something not "too" far fetched from what "could" happen.

----------


## Azélor

No it's not possible. Put it simply, you need to take in consideration the Roche limit. It's like an invisible soft barrier the moons hit when it get too close to the planet. At that point, the moon start to disintegrate due to the gravitational pull of the planet. I don't have the numbers right now but the Roche limit is located at least at 2 or 3 times the maximum altitude of the atmosphere.

----------


## Chick

> If a gas planet (like Saturn) migrated close (but not too close) to a star (maybe the size of Alpha Centuari), would it be viable for the moons of this gas planet to absorb gases from the planet similar to the companion effect that impacts stars? If the Gas Planet had plenty of Oxygen and Nitrogen, could it be possible for life to be sustained on these moons?


Not impossible if there were any reason for the gasses to migrate from the planet to the moon, but I can't think of why/how that would happen.




> Could the atmosphere of a gas planet comprised of oxygen (primarily) extend beyond the moons, which if solid and contained enough water, produce or sustain life?


Impossible, due to Roche Limit, as Azelor said.  http://en.wikipedia.org/wiki/Roche_limit

All of that said, there is no inherent reason that the moon of a gas giant couldn't contain an atmosphere, and if within the habitable zone of a star, liquid water and potential life.

Consider Titan, satellite of Saturn, which contains an atmosphere denser than Earth's, and Europa, satellite of Jupiter, which is suspected to contain liquid water beneath the ice surface (melted by gravitational friction due to Jupiter's proximity).  If either of these were in the habitable zone of Sol, they might well be habitable.

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## Soloeus

I see.

One of my "Colony Worlds" is a gas planet approx the size of Saturn, which contains 5 moons.  1 moon is aquatic, 2 moons are capable of hosting life (but did not evolve anything beyond vegetation).  Since those living there are colonists, I can assume that would be okay. I am glad I don't have to go out of my way to handwave why the moons of a Gas Giant could contain life.

----------


## Chick

Just put the primary (planet) in the habitable zone of the star and you should not get any complaints  :Smile: 

It's true, as JohnVanVliet pointed out, the moons would be tidally locked to the primary (meaning they always keep the same side toward the planet), their revolution around the planet would expose the other sides to the star (sun).  So you would expect to have a portion of the planet-facing moon surface that was not exposed to sunlight, and the rest of the moon having unusual cycles of sunlight, but your story can probably deal with that.

----------


## Soloeus

> Just put the primary (planet) in the habitable zone of the star and you should not get any complaints 
> 
> It's true, as JohnVanVliet pointed out, the moons would be tidally locked to the primary (meaning they always keep the same side toward the planet), their revolution around the planet would expose the other sides to the star (sun).  So you would expect to have a portion of the planet-facing moon surface that was not exposed to sunlight, and the rest of the moon having unusual cycles of sunlight, but your story can probably deal with that.


Indeed, a difficulty for the colonists who live there.  I am creating a local sector map (part of alpha quadrant of this galaxy) which will feature different stars that have qualified planets for life. What surprised me from a little more searching is how rare such a Star is. Too big and the lifebelt is bombarded with lethal radiation. Too small and the life belt would be too close to the star and the planet would not be very stable.  I want the things I use to be believable in the sense that it could happen without having to stretch too much. 

Thank you all, by the way, for using your knowledge to help those of us who lack it. I did take an Astronomy class but it was an online class and the real thing I learned is that I don't learn in that kind of environment. Not my cup of tea, which is a shame because the course is only offered online in my region.

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## johnvanvliet

> t's true, as JohnVanVliet pointed out, the moons would be tidally locked to the primary (meaning they always keep the same side toward the planet)


not one sided

the moons are locked ( AND ROTATING) in a orbital resonance 

for example pluto's ? moons? are within about 5 to 15% of 
Stix  has a 1:3 ratio  
Nix has a 3:4 ratio 
Kerberos has a 4:5 ratio 
Hydra has a 5:6 ratio 

moons and planets tend to  fall into the 1:2:3:4:5:6:7:8..... orbital ratios 

earth and  mars are close to a 1:2 

getting a planet or moon to be " one sided"  like our moon is rather complicated  orbital mechanics 

This resonance is also why large gas giants near the parent star LIKE ( really LOVE) to fling smaller planets out of that solar system 
some random googling 
--- this is a good game 
http://www.stefanom.org/spc/
or
http://astro.unl.edu/naap/pos/animations/kepler.swf
or
http://phet.colorado.edu/sims/my-sol...system_en.html

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## Chick

Depending on the relative sizes and distance, the tidally locked moon might or might not receive much sunlight on the planet-facing side.  As we have all seen the full Moon of Earth, the planet-facing side receives plenty of sun, but the Moon is large relative to Earth and fairly far.  A closer, smaller moon might be well into the shadow of the planet when that side faced the star.

Here is a nice explanation of some of the factors influencing habitability of a tidally locked planet, and some of it applies to moons.
http://www.astrobio.net/news-exclusi...-inhospitable/

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## Trollface13

well if earth had no moon the orbit would destabilize and all tides would sieze but worst of it is the weather stops then tectonics stop and that means te carbon cycles stop and eventually life on earth as we know it would stop and our blue ball will turn red like mars.... lack of a moon also gives lack of a magnetic field irradiating the planet killing almost all life on the surface. 

if earth had 2 moons however its a different story. the extra moon would cause some extreme conditions such as larger hurricanes and vast extremes in wind speed and dust storms it does strengthen the magnetic field but it would also mean more earthquakes and volcanic activity. but really it depends on the moons mass. if its larger than our current one it could rip our planet apart but if it is smaller it would not be so bad.

now earth has had ice ages before and the correct term for tropical age is "Hot House Age" and we had alot of those during the cambrian, carboniferous, and the cretacious/eocene times the Eocene being completely iceless the Eocene epoch was so hot in fact that germany and antarctica were covered in jungles and canada was covered in swamps.

on alien worlds however in terms of climate can vary on atmospheric density and gas composition. take pandora from avatar for instance its atmosphere contains high carbon dioxide levels nearly 20 percent to be exact that is why humans wore masks on the planet as carbon dioxide is toxic to us.... however alternate biologies of the lifeforms on pandora allow the existance of complex life just as long as the 4 biochemicals exist on the planet(oxygen, nitrogen, carbon, hydrogen). however he same climate conditions can be created with an atmosphere like earth just by increasing atmospheric density and/or distance from its parent star. thing is if you want a jungle world in the solar system. reduce the density of venus's atmosphere and filter the carbon and sulfur out and maybe add a few large asteroids as moons you would get the same results as cretacious earth. the reason why the pressure is so high on venus is because of the heat it generates from the surface and sunlight assisted with the carbon compounds in the atmosphere as well as the higher density. but take away one or two of those and you can make many different conditions but maybe not all earth like unless you add a few things too.

as for gravity..... well watch james camerons avatar and look at the trees and the na'vi, they are frakking huge. you know why? because pandora has similar mass to venus.  meaning less gravity. if a person is born on the moon and is raised there they could reach about 6 feet at age 10. if a person is born on mars however they would only be around 4-5 feet at age 10.

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## Trollface13

i can list several possible planets that can ocurr naturally in our universe
-Jungle
-Desert
-Ice
-Ocean
-Volcano
-Terran
-Water Vapor Gas Giant
-Diamond

For Jungle & Terran is our own earth as i stated before in the eocene the earth was covered in vast jungles but now its a multiclimate aka Terran.
For diamond if a carbon planet is in eccentric orbit of a sun it can crystalize into a diamond planet after millions of years due to the process of heating and cooling and geological & gravitational friction. 
For ice..... well you know what europa is its an ice ball orbiting jupiter plus earth was in a global ice age billions of years ago called "snowball earth"
For volcano we have jupiters Io it is the most volcanically active planet in the solar system.
As for desert.... Mars takes the cake.
Ocean planets are mainly extrasolar superearths. though someday we may find a weird oceanic island planet.
Water Vapor Gas giants now this one fascinated me because these two extrasolar planets are the only evidence that these exist and could harbor floating giant aliens. HAT-P-11b & HD 209458 b both are gas giants containing the 4 key elements for life.

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## xpian

"well if earth had no moon the orbit would destabilize and all tides would seize"
No, the sun also influences tides. Tides would be smaller. I have no idea what "the orbit would destabilize" means.

"the weather stops"
No, weather is generated by Earth's own processes, owing to things like land, sea, and air being warmed by the sun and the planet itself rotating.

"then tectonics stop"
No, plate tectonics on earth have very little to do with the moon. The heat within the Earth, either left over from the planet's formation or generated by decaying radioactive elements, keeps much of the material of the planet below the crust in either liquid or plastic form, allowing huge chunks of solid material called cratons (the building blocks making up the continents) to move about, crashing into each other.

"te carbon cycles stop"
The moon has nothing to do with the carbon cycles of life on Earth.

"lack of a moon also gives lack of a magnetic field irradiating the planet killing almost all life on the surface."
Earth's magnetic field has nothing to do with the moon. Most scientists agree that our magnetic field is generated by the great dynamo at the heart of our planet, the spinning metallic core. Mars, by contrast, seems to have lost most of its magnetic field and the conjecture is that the red planet has cooled off and slowed enough that its core is no longer able to generate one.

"if earth had 2 moons however..."
Some of your predictions are more likely than others, but at least you do point out that the mass of the additional moon is important. To that, however, I'll point out that even a quite massive moon wouldn't necessarily "rip the earth apart". A big moon could fall into a tidally-locked orbit with its planet, one not rotating with relation to the other, both just gently circling for a very long time.

"Hot House Age"
True. Earth has been completely ice-free several times that we know of, when global temperatures and sea levels were both much higher. Also, we've had a "snowball earth" at least twice that we know of, when a runaway albedo effect caused a global ice-age all the way to the equator. If an alien had visited earth in those days, the planet would have looked like Hoth.

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## xpian

"because pandora has similar mass to venus. meaning less gravity."
Venus is Earth's sister world, just a little smaller in size and mass. Surface gravity there is about 91% of what it is on Earth. And we don't really have any good data as to how living things that evolved in 1G would react and grow in 0.9G. Maybe they'd get really tall--maybe not. Maybe it would take countless generations of evolution for organisms to adapt to permanently lower gravity.

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## Chick

Everything xpian said, I second.   Trollface, you really need to do some reading on scientific websites about some of your ideas.  They really make little sense.

Xpian, the idea of the Earth's orbit destabilizing without the Moon is a misunderstanding of the correct idea that the *axial tilt* of the Earth would likely be more variable (NOT the orbit), without the stabilizing gravity of the Moon.  The Earth's orbit is not significantly affected by the presence of the Moon, since the Moon is in the same solar orbit as the Earth.

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## Trollface13

There is a possible way a planet can host earthly life in the cold zone of a star system..... A gas giants moon if close enough to its parent planet can give off geothermal heat in the atmosphere via volcanic activity but the thing is it has to contain ice in the first place for it to work. not all life depends on sunlight to live deep oceans geothermal vents proved that. just imagin humanoids that use nocturnal vision to see walking in a dark Chemosynthetic forest. the area filled with hydrothermal oceans and geothermal hot spots. the normally toxic chemicals wouldnt be a problem because of the chemosynthesis the strange plants use. it would be very very dark like an eternal night so humans would need nightvision goggles. But it is indeed possible that such a place exists.

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## xpian

"Xpian, the idea of the Earth's orbit destabilizing without the Moon is a misunderstanding of the correct idea that the axial tilt of the Earth would likely be more variable (NOT the orbit), without the stabilizing gravity of the Moon."

Ah, yes. Now I see what he was going for. I have heard that idea as well.

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## Trollface13

Thanks for the correction.... i need to update my knolege.... turns out my info was incorrect i thought the moon was vital to earths survival but i guess i was informed wrong.

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## Trollface13

however i do have a question..... can a city planet like coruscant even exist? or would you need to keep some natural environment in a few places so the place doesnt kill itself?

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## Azélor

If the technology is advanced enough, they might not need the natural ecosystem. Everything the nature does could be made by man or machine but at a great cost. 
That depends what you mean by killing itself.

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