Help me with a short science fiction story, could such a planet exist?

[Kuansenhama]

I wanted to create a fictional planet, but I wanted it to be very different from earth while still being plausible within the realms of science as a possibly habitable world that one might actually find traversing the galaxy. So I ask to all you hardcore science people: is this possible?

Orbits a binary star system. One star is a main sequence, like our sun, but a little smaller. The other is a red dwarf. I wanted to put it around a binary of two red dwarfs since it seemed more plausible than giant/supergiant stars, but I've heard some people say that you can't have a planet around a mere red dwarf because it would be too close as to become tidally locked, as well as having problems with radiation and such.

Is the outermost in a system of moons orbiting a gas planet. (Probably with a resonance like Jupiter) The gas planet would probably be similar to Jupiter as well.

The potentially habitable moon itself would be smaller than earth. 60-90 percent radius, but be roughly as massive. Simply having more iron and metals rather than rocky compounds within it. A cross section probably resembling that of the planet Mercury. The planet would have a thicker atmosphere than earth, probably 2-5atm at sea level, and the planet would have a lower abundance of water, possibly only enough to allow for a few small seas on the surface.

The problems I could see with my model, but I don't know if they would be problems:

Protection from solar storms/cosmic rays.If I am within the magnetic field of the parent gas giant, but am on the outermost moon, am I safe. If I were on an inner moon, I might find myself in a radiation belt. If I were just on my own, I might potentially have my own magnetic field. (My planet is basically a flying iron core as it is, though.) But being in the magnetic field to a gas giant, but far enough from the giant to avoid radiation belts?

Would the eclipse of the gas planet be a problem causing the planet to cool too much?

Would the planet wind up tidally locked?

Would the tides in what seas there were be disastrous because the planet orbited a gas giant?

Orbital instability? Orbiting a gas planet orbiting a star that has another less massive star going around it?

Some other thing I'm not thinking of?

[peadar1987]

Pretty much every large moon of a gas giant is tidally locked to it. I can't think of any exceptions offhand. On the plus side, an earthlike atmosphere would protect you from pretty much all of the radiation, even in the worst of the belts, so long as you didn't climb too far above the surface. You wouldn't be able to fly without getting a pretty hefty dose.

I'd say an earth-sized moon would be completely possible though. The earth has a mass of about 100 moons. Jupiter has a mass of over 300 earths. If the moon can orbit the earth, I reckon the earth could orbit Jupiter.

[0111narwhalz]

Io isn't locked, is it? I thought it's vulcanism was due to tidal heating.

[magnemoe]

3 minutes ago, 0111narwhalz said:

Io isn't locked, is it? I thought it's vulcanism was due to tidal heating.

Its tidal locked but the moons get tides from each other. 

[kunok]

You don't really need a magnetosphere to be protected from the solar wind, look to Venus, it has a ionosphere protecting his atmosphere, with is made by the solar light (UV and more) ionizing the atmosphere. Is a lot harder to understand than the normal magnetosphere thing, though, and you will be losing more atmosphere than if you had a magnetosphere, but then again look to Venus. The radiation protection is given by the thickness of the atmosphere, not the magnetosphere.

In short if you have a thick enough atmosphere to have a significant ionosphere you will have solar wind protection, and that atmosphere will protect you from the radiation itself.

Edited November 24, 2016 by kunok

[TheCardinal]

7 hours ago, Kuansenhama said:

.....

Some other thing I'm not thinking of?

Some questions which arose while reading your post:

- What do you mean by "habitable"? For humans?

- What is the temperature range on the moon/planet?

 

[RCgothic]

Tidally locked to a gas giant is not necessarily a problem, it just means very long days.

[TheCardinal]

1 hour ago, RCgothic said:

Tidally locked to a gas giant is not necessarily a problem, it just means very long days.

There is more to it than just long days. Don't forget that one side of the moon receives light/energy and warms up for a long period while the other side is in darkness and radiates warmth into space. The temperature difference will cause winds. Most probably very strong winds. The strength of those winds depends on the amount of influx of energy on the sunside and the outflux on the darkside. which each in turn are dependant on several other factors like for instance the composition of the atmosphere, the distance to the star(s), the spectrum the stars emit, the revolutiontime of the moon around the gasgiant etc..

 

[peadar1987]

10 hours ago, TheCardinal said:

There is more to it than just long days. Don't forget that one side of the moon receives light/energy and warms up for a long period while the other side is in darkness and radiates warmth into space. The temperature difference will cause winds. Most probably very strong winds. The strength of those winds depends on the amount of influx of energy on the sunside and the outflux on the darkside. which each in turn are dependant on several other factors like for instance the composition of the atmosphere, the distance to the star(s), the spectrum the stars emit, the revolutiontime of the moon around the gasgiant etc..

 

Io's orbital period is only 1.7 days though. And it orbits at about 6 Jupiter radii out, which means it won't spend too much of its time eclipsed. (Assuming a circular orbit, the orbital path is 2*PI*r long, of which r/3 will be behind Jupiter. Roughly 1/18th of the time, if my maths is correct)

[TheCardinal]

11 hours ago, peadar1987 said:

Io's orbital period is only 1.7 days though. And it orbits at about 6 Jupiter radii out, which means it won't spend too much of its time eclipsed. (Assuming a circular orbit, the orbital path is 2*PI*r long, of which r/3 will be behind Jupiter. Roughly 1/18th of the time, if my maths is correct)

I wasn't referring to being eclipsed by the gasgiant. (In the case of IO it's actually about 1/11th of the time).

Tidally locked means that the same face is always turned towards the planet, In other words, the moon rotates once around it's axis in the same time it takes the moon to rotate once around the planet. Half of that time any spot on the surface of the moon is bathed in light, the other half in darkness (ignoring eclipses).  This causes a temperature between the dark and the light side. As there is an atmosphere, winds will develop. The magnitude will depend on those factors i already mentioned (and i probably forgot a lot of others). It would need someone current in gas/fluid dynamics (which i'm not) to make a good estimated guess.

As a thought-experiment, what do you think would happen if earth would have a day 1.7 times the current one?

[Tex_NL]

Io might have a 1.7 day period, @Kuansenhama's moon will not. He clearly stated he wants it to be the outermost moon. A high orbit will mean a long orbital period but that also reduces the chances of a tidal lock. If it would be locked the day/night cycle would be very long. If not the cycle can be pretty much whatever you like.

One way to limit the temperature differences between night and day would be to place the parent gas giant beyond, or on the far edge of the parent stars habitable zone therefore limiting the amount of light the moon receives. With less light difference between day and night the temperature differences will also be smaller. And a dense atmosphere would generate a greenhouse effect retaining a significant amount of heat.
The lack of solar energy can be compensated by geothermal energy (induced by tidal forces) heating the surface both day and night reducing the difference even more. Any indigenous life could, for example be chemosynthetic instead of photosynthetic.

Geothermal tidal forces could also explain why the moon still has a molten metal core. With a molten metal core it could have its own magnetic field. Together with a thick heavy atmosphere it could shield the surface from the worst of the gas giants radiation. As an added bonus it could also induce some incredibly spectacular auroras.

'60-90 percent radius, but be roughly as massive', '2-5 atm at sea level', ' a lower abundance of water, possibly only enough to allow for a few small seas on the surface '.
To me this says gravity not too far below 1G. 0.7-0.8 perhaps. 2-5 atm is dense but definitely something the human body can handle given enough time to adjust. 'A few small seas' would mean the water would be liquid, between 0 and 100° C. Especially the lower end is 'survivable'.
To recap: gravitational forces, atmospheric pressure and temperature all within reasonable comfort levels.

Edited November 25, 2016 by Tex_NL

[KerikBalm]

On 11/24/2016 at 0:54 AM, Kuansenhama said:

I wanted to create a fictional planet, but I wanted it to be very different from earth while still being plausible within the realms of science as a possibly habitable world that one might actually find traversing the galaxy. So I ask to all you hardcore science people: is this possible?

Are we to take the plausibility of its formation into account, or just ask that if it could exist as described (ie, we allow for alien supertechnology to make/move something, and then have it continue to be habitable "hands off"?

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Orbits a binary star system. One star is a main sequence, like our sun, but a little smaller. The other is a red dwarf. I wanted to put it around a binary of two red dwarfs since it seemed more plausible than giant/supergiant stars, but I've heard some people say that you can't have a planet around a mere red dwarf because it would be too close as to become tidally locked, as well as having problems with radiation and such.

You can have a planet around a red dwarf, but that places severe restrictions that prevent it from being Earth like.

Does this red dwarf orbit inside the orbit of this habitable world? or is it more distant?. I doubt Earth would be stable if there was a red dwarf where mercury is. If we moved earth outward, it would go too far out... the reddwarf's light would be insiginificant compared to a G type star (the classification of our Sun), or even a K type (on size down). Note that red dwarf's can be main sequence stars... in fact, most main sequence stars are red dwarves, and that will become more true with time. Due to the low rate of fusion, a red dwarf can remain in the main sequence for well over a hundred billion years (possibly past 10 trillion, depending if it was on the small side or large side of the mass range of a red dwarf),

If it was out where sedna is... fine.  Consider Alpha Centauri vs proxima centauri. Even Aplha Cen A vs B are 15,000 AU apart. The second star should not be orbiting inside the orbit of the habitable world... It can be a "nearby" destination for interstellar travel without all the problems of normal interstellar travel.

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Is the outermost in a system of moons orbiting a gas planet. (Probably with a resonance like Jupiter) The gas planet would probably be similar to Jupiter as well.The potentially habitable moon itself would be smaller than earth. 60-90 percent radius, but be roughly as massive. Simply having more iron and metals rather than rocky compounds within it. A cross section probably resembling that of the planet Mercury.

I think you'll need to increase the radius/take the high end of that range. 60% the radius of earth is 21.6% the volume... you would need to be nearly 5x as dense to be of the same mass. Mercury vs Earth composition wouldn't help much. The light volatiles that Earth retains but Mercury has lost (hydrogen, Nitrogen) don't account for much of Earth's bulk mass. Nearly 65% of Earth is Iron and Oxygen. With Iron being 1/3rd the total mass. Most of that Oxygen is not in the form of our atmosphere, or even water, but with metals as oxides. Mercury also has a lot of oxygen in the form of metal oxides. Oh, lets not forget silicon (15%) which is also similar in abundance on Mercury, magnesium (14%). Sulfur/Nickel/calcium/aluminum each make up 2.9/1.8/1.5/1.4 %. All other elements including hydrogen and nitrogen, make up less than 1.2% of Earth's mass. We may think that Earth has a lot of lighter volative elements compared to mercury... after all its covered in oceans of H2O! our atmosphere is mostly just nitrogen! That's only the surface.... and hydrogen is a small % of the mass of water. Earth is in fact the densest planet known.

I would say that its bad sci-fi to postulate a planet 5x denser.... sure its not impossible... you could make a planet of mostly U-238, but it seems rather unlikely at this point in the universe's life. Maybe if it formed very close to where a star previously went supernova (where the heavier elements are made)

 

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The planet would have a thicker atmosphere than earth, probably 2-5atm at sea level, and the planet would have a lower abundance of water, possibly only enough to allow for a few small seas on the surface.

Well, that can work if its farther out and thus the temperature at the top of the atmosphere is colder. Given the lower mass, if you want to hold on to lighter gasses (note: CO2 as on Venus is quite a bit heavier than water vapor and neon), it should be farther out. More volatiles in the atmosphere seems inconsistent with a denser composition. Why does it have all these volatiles around it, yet its surface and interior must be depleted in them to get the required densities. - not naturally formed? Why does it have all these volatiles, but very little water? even outgassing which will outgass water vapor... unless that vapor is being lost while CO2 is being retained. Its a mostly CO2 atmosphere? Its possibly habitable for some form of life but not humans?

To me, having few seas and a dense interior should mean something happened that depleted the volatiles... I'd go for a thin atmosphere, but thick enough for water.

 

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Protection from solar storms/cosmic rays.If I am within the magnetic field of the parent gas giant, but am on the outermost moon, am I safe. If I were on an inner moon, I might find myself in a radiation belt. If I were just on my own, I might potentially have my own magnetic field. (My planet is basically a flying iron core as it is, though.) But being in the magnetic field to a gas giant, but far enough from the giant to avoid radiation belts?

If it orbits close enough to be protected by the magnetosphere of the gas giant, its definitely locked. If it orbits at a certain distance... its in the radiation belts (like Europa/Io/Ganymede of Jupiter). Jupiter's 4th large moon, Callisto, orbits mostly outside Jupiter's radiation belts. Radiation there isn't much worse than interplanetary space. A dense world like you suggest should generate its own magnetic field to protect itself anyway. (note, an Earth mass moon would be 50x more massive than Callisto/40x more massive than Ganymede)

 

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Would the eclipse of the gas planet be a problem causing the planet to cool too much?

Would the planet wind up tidally locked?

Would the tides in what seas there were be disastrous because the planet orbited a gas giant?

Orbital instability? Orbiting a gas planet orbiting a star that has another less massive star going around it?

Some other thing I'm not thinking of?

Well, lets assume the gas giant is similar to Jupiter, and your world (its a moon, not a planet!!! ) has an orbital period similar to Callisto (I question if a moon 50x bigger than callisto would form around a jupiter mass planet... but whatever).

Callisto has an orbital period of 16.7 days. Jupiter is 1,900,000 km away, and 140,000 km in diameter. 140/1900 = 0.0737 ... so it would take about 4.2 degrees of arc in the sky.

4.2/360 = 0.0117 of the orbital period. The eclipse (assuming no inclination) would last 0.196 earth days... or 4.7 hours. Not a huge problem. Sure it would get colder, but especially if your planet has a thicker atmosphere, it should be fine.

[Hast5250]

The moon would be exposed to the deadly ionization belts around the gas giant, the ion flux tube as its called would constantly beam charged particles at the planet.

 

The planet would also have a very long night, if it was tidally locked, then its days would last as long as its months.

Edited November 25, 2016 by Hast5250
added more stuff