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I've asked another question about trinary planet systems which turned out to not be possible.

Is there a way for three worlds to co exist within sight of each other? Sight being defined as making out continents with the naked eye.

The planets need to be large enough to have gravity of 0.8g, 1g and 2g (ish), humans can have minor modifications (thicker skin, stronger bones) to survive but are essentially the same as they are now.

I'm now thinking of moons orbiting a gas giant, but I don't know if this would work with them having to be close enough to the star for heat & light and whether the gravitational stresses of the planet would make life too unpleasant.

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    $\begingroup$ Do you want them visible at all times, or would occasional proximity (once a year for a few days, say) work? Without a gas giant you're looking at a binary planet, but getting the third close enough may be impossible. $\endgroup$ – rek Aug 14 '16 at 7:38
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    $\begingroup$ @rek occasional proximity is good enough, I'm trying to create a world where you can be inspired and know there's other people out there $\endgroup$ – Chris J Aug 14 '16 at 10:35
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    $\begingroup$ Do they have to be planets? I think it might be possible to have a 2g planet with a 1g and 0.8g moon orbiting it. Don't forget the gravity at the surface isn't only dependent upon mass, but on distance. The radius of these other planets could be smaller. Just a thought. $\endgroup$ – ozone Aug 14 '16 at 11:49
  • $\begingroup$ Note it is possible for two moons to almost share the same orbit and periodically swap orbit en.wikipedia.org/wiki/Epimetheus_(moon) $\endgroup$ – DarcyThomas Aug 14 '16 at 20:53
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Orbiting a gas giant is a good idea. Make it a hot jupiter, because that will put the gas giant planet right in the habitable zone. The three planets will need strong magnetic fields to protect from the possible radiation belts around the hot jupiter.

Drawbacks for the three planets will include increased seismic and volcanic activity and the planets being tidally locked. The fact that the planets are geologically active increases the probability of there being life on the planets. This won't make life too unpleasant. Humans can live with volcanoes and very active earthquake zones. Presumably life on the three planets will have their evolutionary histories to help them adapt to such inconveniences. Tidally locked will means that the other planets can only be seen from specific regions on each planet. Not the worse problem a planet can have.

Nevertheless, there would be one serious drawback of tidal lock. Either those moons are close and face serious volcanism, OR they are more distant ones and instead face awfully long day-night cycle (like weeks) which causes serious temperature fluctuations and turns interiors of continents into areas with really extreme temperatures.

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  • $\begingroup$ They'll be tidally locked to the Jupiter, not the star. It shouldn't pose habitability problems. $\endgroup$ – Loren Pechtel Aug 14 '16 at 17:58
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    $\begingroup$ @LorenPechtel Indeed. The planets would be moons in effect. $\endgroup$ – Mast Aug 14 '16 at 18:05
  • $\begingroup$ If they are tidally locked to the Jupiter and have orbital periods of weeks, then they will have solar days that are weeks, which would be bad (though not unsurvivable) for life. $\endgroup$ – Jarred Allen Aug 14 '16 at 23:40
  • $\begingroup$ If it was orbiting a hot jupiter, the planet would not be habitable. en.wikipedia.org/wiki/Hot_Jupiter $\endgroup$ – alessandro Aug 15 '16 at 1:06
  • $\begingroup$ @zeeman "Stellar tides brake a planet's rotation and, together with tidal migration, act to remove satellites; this process limits the lifetimes of larger moons in extrasolar planetary systems. Because more massive satellites are removed more quickly than less massive ones, we are able to derive an upper mass limit for those satellites that might have survived to the present day. For example, we estimate that no primordial satellites with masses greater than 7×10-7M⊕ (~70 km radius for ρ=3gcm-3) could have survived around the transiting planet HD 209458b for the age of the system." $\endgroup$ – a4android Aug 15 '16 at 4:59
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Provided we keep some moons reasonably close to a large planet, but far enough from each other, stable orbits are possible.

A Jupiter sized planet orbiting a sun-like star, at 1AU would have a Hill sphere of about 10 million km. And an Earth Sized moon would have its own Hill sphere of about 1/10 of its orbital distance.

So placing the three moons at distances of (say) 200,000km, 300,000km and 500,000 km puts them well inside the planet's gravity well, far enough that they should be able to orbit without disrupting each other's orbit and close enough at opposition from Jupiter for continents to be clearly seen.

The usual caveats apply. These moons will be tidally locked, heated internally by the gravity from the planet (lots of volcanoes?) and orbiting in powerful radiation belts.

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  • $\begingroup$ Could there be something like a system of moons keeping them locked in place? $\endgroup$ – Puppy Aug 14 '16 at 12:37
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    $\begingroup$ No. The moons can be in resonant orbits (eg for every 1 orbit the outer moon makes, the inter moons make 2 and 4 orbits respectively (this is the case for the moons of Jupiter.) But the moons can't have the same orbital period and be stable orbits (not even using Lagrange points) $\endgroup$ – James K Aug 14 '16 at 13:00
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For them to be close enough to see each others' continents with the naked eye, they'd have to be about as close as the moon to earth. Earth sized planets at 400,000 km would exert around 2.5 x 10^-4 m/s^2 acceleration on each other. That's around 1.5 x 10^26 N of attractive force.
These are back of the envelope calculations, so I assume I'm off by a factor of 100. Despite that 2 such planets would not remain in stable orbit. And three is right out.

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  • $\begingroup$ "they'd have to be about as close as the moon to earth". Well, assuming their eye sight is similar to ours. What if they have much better eye sight? changing 400k km to 600-800k km should change the force quite a bit. $\endgroup$ – Bakuriu Aug 14 '16 at 20:04
  • $\begingroup$ Not really, no. I've already pointed out that a factor of 100 would make no difference to the outcome; a factor of 2^2 is negligible. Secondly, our eyes are fine. The reasons we can barely make out the shapes on the moon are: the movement of air and dust in the atmosphere blurring the details, and the moon being too small. Making the moon bigger helps with the second, not the first; making it bigger, but moving it further back, largely cancels out. The apparent size of an object is the angle it subtends on our retina, so, 2*arctan(r/x), where r = moon radius, x is earth to moon distance. $\endgroup$ – nzaman Aug 15 '16 at 6:06
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Give the inhabitants Eagle-like vision.

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  • $\begingroup$ «there a way for three worlds to co exist within sight of each other? Sight being defined as making out continents with the naked eye.» It is a direct answer: instead of contriving the planets to be closer together, improve the character’s natural ability to see them. $\endgroup$ – JDługosz Aug 17 '16 at 22:19
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    $\begingroup$ Answers should not require comments that are longer than the answer to explain them. $\endgroup$ – Brythan Aug 18 '16 at 0:38

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