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So I decided to revive an old project which I was seeking assistance a year ago on the realism of the worldbuilding. WB links from the past: Reality of a tidally locked planet and Slowly Sculpting the Planet - Sky within a Red Dwarf System

In short, my world is a planet orbiting around a red dwarf. Since it is generally habitable (I will make it habitable if it isn't, and it's not decided yet the habitants are descendants of earthlings or just humanoid that evolved on this planet) both the day an night side of the planet are developed and populated. However, due to political influence/cultural difference/technological constraints, "day" hemisphere and "night" hemisphere became two big "nations" being diplomatically hostile or at least opposite to each other. After generations, habitants from either side of the planet have never seen people from the other side.

For an important plot purpose I would like to have a celestial object that is only detectable from the night side of the planet but hidden/invisible to most of the day hemisphere.

At first I was thinking of this: Red Dwarf System 1

Basically the planet is having a natural satellite with a geostationary behaviour, thus it's hidden from most of the day hemisphere. However after a bit of research, I figured out that a tidally locked planet is unlikely having a natural satellite. Even if it exists the system will be unstable.

Then I came of this: Red Dwarf System 2

Two planets are synchronized in orbital period, they have the same length of a year and the smaller planet will always be in the shadow. Will it be too much of a coincidence or is it scientifically possible?

If both are not possible, is there a third proposal that can be compromised? I don't mind artificial satellites, but the problem is that it's not large enough for the part of the story to take place. I would want enough room for this celestial object to have equipment and underground systems implemented which is completely unknown from the day side.

Any out of the box suggestions will also be appreciated.

Thank you!

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    $\begingroup$ You say that part of your story takes place on the hidden planetish thing. Does this mean at least one or both of your societies have space flight technology? This would imply satellite technology. Which would imply that no matter what you come up with, it won't be hidden. $\endgroup$ – JBH Oct 18 '17 at 0:21
  • $\begingroup$ Simply in my story, this planet has close to earth or bit more advanced technology, but good amount of the planet's population is busy maintaining other stuff like the balance of the climate system. Anyway the main point is the people are already busy with their lives and are only given what they need for their living. Technology like space travel is only in the hands of government officials. It's not the right or responsibility of the people to take care of the space. A limited number of people may know the truth, but they won't let the public know. $\endgroup$ – Yinnes Oct 18 '17 at 8:54
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The good, the bad, and the ugly

The Ugly

All it takes to prove the existence of a planet, even one that is visibily hidden, are observation, time, and mathematics. We've been proving the existence of "bodies of mass" for a long time because orbits don't make sense unless everything is taken into account. Therefore, while your planet may be visibily hidden, it cannot be mathematically hidden. If your light side peoples have calculus, they can prove the existence of the planet. Which might not be that ugly, as it could be an interesting plot point. Things you can't see are easily forgotten/ignored, even when a small group of scientists keep reminding people, "there's gotta be something there, and here's how massive it is...."

The limitation is that the mass of the two objects will need to be nearly identical (I think) to minimize instabilities. The greater the difference, either the greater the distance between the two worlds (making travel very difficult) or the greater the possibility one will want to orbit the other rather than both orbiting the star.

The Bad

while your proposed orbit is theoretically possible, the reality is that it is unstable. Any change in mass, distance from the star, even the rotation of either or the worlds (or a passing comet, for that matter), and the orbital sync would fall apart. If either planet has a moon, it's probably impossible (but only an astrophysicist could confirm that statement).

However, that might not be a show stopper becasue things can take a honking long time to change when it comes to stellar phenomena, so it might be that the two orbits are not actually sync'd, that one is only just faster than the other, and we happen to be in a... say... 1,000 year period when your hidden planet is actually hidden. That might actually be a useful plot point for you as the existence of the planet would be common enough knowledge for people to not actually think about it (how often do you think about the back of your knees?) and, better still, it's existence would have basically become myth. Of course, those pesky scientists are still reminding people it's there, but your average response might be, "yeah, and according to popular fairy tales, dragons live there... don't we have something more important to talk about?"

The Good (or, at least, the really cool)

It might be a bit more complicated, but with a bit of handwavium, you could set up an argument for a Lissajous orbit around Lagrange Point L2. Lagrange points are (simplistically) gravitational eddies that can be orbited like an actual body of mass. L2 is directly in line "behind" a planet such that the planet always shields the point from the sun. A Lissajous orbit is one that requires no artificial propulsion to maintain. Granted, it ususally requires a loop around earth to maintain momentum, but I mentioned handwavium, right?

This concept could allow you to create a moon that is never seen by the light side. Note that those pesky scientists are still yammering about some mass that keeps affecting the tides (and it would be a wild and wooly affectation, too... did I mention plot point?), but there are ways to discredit/silence/ignore scientists.

No matter what solution you come up with, it will always be mathematically visible. Tides for moons. Orbital perturbations for planets. Observation, time, and mathematics will detect them all. For the sake of realism, you'll need to deal with this nasty problem in your story. But it's a cool problem, wouldn't you say?

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    $\begingroup$ People from the day side might have a problem with any celestial observations. $\endgroup$ – Alexander Oct 18 '17 at 0:15
  • $\begingroup$ Not particularly. The OP said it was a red dwarf. Red dwarfs have much lower luminosity than our own sun. Possibly, low enough to allow the same planetary observations we make ourselves in the morning and evening (then again, maybe not, as there might not be enough light to reflect off the other planets). It's an interesting conundrum. However, you can still make measurements of your own planet. Not as simple, but still possible. $\endgroup$ – JBH Oct 18 '17 at 0:19
  • $\begingroup$ I presume that lack of luminosity is compensated by smaller orbital distance, so that the planet in question and Earth would end up receiving similar amounts of energy. $\endgroup$ – Alexander Oct 18 '17 at 0:35
  • $\begingroup$ I think I'm okay with a small population being able to indirectly observe such hidden celestial body. I just want the general public not knowing it as common sense. The government on the day side can be enforcing indoctrination and arrest people for "spreading conspiracy theories" and "cause public panic". Noise made by a group of scientists and such will always spice the story up. That actually gave me more ideas on how it can go on. The only problem is with the probability of this celestial body's existence. I will read all answers and see what fits best. Thank you! $\endgroup$ – Yinnes Oct 18 '17 at 9:21
  • $\begingroup$ Good anser, but: The Good: How would a locked planet with an L2 moon have any tides? As long as the day nation does not visit low earth orbit with satellites or ICBMs I don't see how they can calculate such a thing and not just think that the planet is just bigger or heavier instead or has some strange heavy metal deposit where the sea level is lower than it should be (which at least we on earth measure with a satellite). $\endgroup$ – Henning M. Oct 18 '17 at 9:28
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A synchronized orbit like you described is, simply put, not possible.

For every orbit, there is a set speed for that orbit. No faster and no slower. Gaining or losing speed changes the orbital properties. So, something in a further orbit would have a different orbital period. Additionally, eccentricity would come into play, as orbits aren't circular. Things would get out of whack pretty quickly.

I don't think it's possible to have a natural satellite in the manner you want. That said, an artificial satellite could work.

If the inhabitants are descendants of colonists, the colonists could have established a station in orbit. Using the L2 Lagrangian point and some stationkeeping, it would stay precisely where you want it to. There are numerous reasons for staying in this position, but one significant one is that it blocks out the sun's light, allowing for better celestial observation. What the station was originally intended to observer, or why it is observing, is a different matter entirely.

The problem is, in such a position a station or other object wouldn't be easily visible. It wouldn't be reflecting sunlight for obvious reasons, so the only way to "See" it would be to see it blocking stars or other objects.

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    $\begingroup$ Asteroids do tend to collect in Lagrangian points. But it's worth mentioning that the only objects that can remain in a Lagrangian point are those which have a negligible gravitational impact on the system. So no Moon-sized objects, artificial or otherwise. $\endgroup$ – IndigoFenix Oct 18 '17 at 11:11
  • $\begingroup$ If I go for artificial satellites instead, is there a limit on the mass or size of it? $\endgroup$ – Yinnes Oct 19 '17 at 0:52
  • $\begingroup$ @Yinnes Yes and no. Technically, something exceedingly large would still have issues. Realistically, though, even a gigantic satellite would probably still be small enough to not have major issues. Plus, being artificial, it can have stationkeeping devices. $\endgroup$ – Andon Oct 19 '17 at 1:26
  • $\begingroup$ Does that mean unless the artificial satellite is as large as a natural moon, it can stay in position for a very long time? $\endgroup$ – Yinnes Oct 19 '17 at 8:40
  • $\begingroup$ @Yinnes I don't know the math behind it, and natural moons vary a lot in size. But, for story purposes, it would probably work just fine. $\endgroup$ – Andon Oct 19 '17 at 18:05
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Your first scenario could exist for a while, but it is unstable and extremely unlikely to occur by chance. Your second scenario flat-out impossible. I can think of a few more options, but they may not be any better than the first two.

In your first scenario, you've got the satellite perched at the planet's L2 Lagrange point. This is one of five points in a two-body system where the gravitational and centrifugal forces on a third body all cancel out and it can sit in place relative to the planet and star. The L2 point, unfortunately, is unstable. The potential there is sort of like a saddle. If the satellite gets nudged ahead or behind the planet (or out of the plane of the solar system), the combined gravity of the star and planet will pull it back. However, the slightest nudge toward or away from the planet will send it out of equilibrium and, most likely, flying out into its own orbit around the star.

Such a natural satellite would not remain in place for long. Manmade satellites can exist at the Earth-Moon and Earth-Sun L2 points for quite a while, but that's because they were given a precisely-calculated push and have thrusters to keep them from drifting away.

It is worth noting that the L4 and L5 points, however, are stable. A nudge in any direction to an object at either of those points will simply put it in orbit around that Lagrange point. However, as these points are about 60° ahead of and behind the planet in its orbit, you won't be able to hide a satellite from both the light and dark hemispheres there.

Your second scenario, as I mentioned earlier, is flat-out impossible. The formula for the orbital period of a planet orbiting a star is given here. Note that the only factors are the radius of the orbit, the mass of the star, and a few constants. Planets farther away from their stars orbit more slowly, period, end of story. Your "satellite" will be about as visible from the larger planet as Jupiter and Mars are from Earth.

This, however, raises the possibility of a third option: If there is a thick asteroid belt or Saturn-like ring between two planets, that'll increase the effective mass of the star for the second planet, causing it to orbit faster. For this to work, however, the mass of the material in the ring must be an appreciable fraction of the mass of the star. There are a lot of reasons to suspect that this shouldn't work, and frankly, I'm not well-versed enough in the mechanics of this sort of thing to offer a clear opinion. On the one hand, all the material seems like it ought to have coalesced into a planet at some point. On the other hand, Fomalhaut has a ring substantial enough to be visible from telescopes on Earth... but then again, the Fomalhaut system is quite young and Fomalhaut itself is almost twice as massive as our Sun. If I were to take a wild guess, I'd say that this scenario is no more implausible than a planet managing to maintain a satellite at its L2 point for long enough for intelligent life to evolve... if the second planet is even visible through the ring at all.

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  • $\begingroup$ you may hide a L4/L5 moon with a very dark, unreflecting surface from the day side. $\endgroup$ – Henning M. Oct 18 '17 at 9:31
  • $\begingroup$ @HenningM. That'd make it hard to see from the night side as well... but given that it's impossible to see stars and planets during the day on Earth due to sunlight scattering off the atmosphere, you may have a point. However, such a small or dark-colored L4/L5 moon will only be visible from less than half of the night side of the night side, which isn't exactly what the OP is going for either. $\endgroup$ – Someone Else 37 Oct 18 '17 at 20:29
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An interesting configuration that unfortunately still suffers from similar instability issues to those mentioned before is to have the tidally locked world situated at the l1 lagrange point of a large gas giant. Your "hidden object" could then be one of the normal moons of the gas giant. The reason this is an interesting alternative is that the people living on the dark side would not have to live in complete darkness, since there would be light reflected from the gas giant. This might also help stabilize the extreme weather patterns that are assumed to occur of tidally locked worlds due to the temperature gradient.

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    $\begingroup$ It should then be tidally locked to the gas giant and not the sun I think. $\endgroup$ – Henning M. Oct 18 '17 at 9:34
  • $\begingroup$ If it is at the sun/gas giant L1 then being tidally locked with one is the same as being tidally locked with both. $\endgroup$ – Lex Oct 18 '17 at 16:00
  • $\begingroup$ which is instable as you said, i see, sorry, you are right. $\endgroup$ – Henning M. Oct 18 '17 at 16:19
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One possibility is an object that has an extremely eccentric orbit with a perihelion (closest approach to the star) not too far outside the tidally locked planet's orbit. And a resonant orbital period. Thus the object might complete exactly ten or a hundred or some other number orbits for every one the tidally locked planet does.

The outer object with the eccentric orbit would travel faster the closer it was to the star as it got closer to the star. So when it was at perihelion it would be moving very fast as it swung around the star with enough excess speed to travel far back out again. And the duration of it's closest flyby of the star might thus be about the same as the period of the tidally locked planet's year/day.

The light of the planet's star scattered around the daytime sky will drown out the light of most celestial bodies except the star. Especially if the atmosphere is more dusty or foggy or opaque on the sunward side of perpetual daytime.

On Earth, objects visible in the daytime sky include:

1) the Sun, apparent magnitude -26.74 (the higher the magnitude the lower the brightness)

2) the Moon, apparent magnitude -12.90 at full moon, down to -2.5 at new moon.

3) comet Ikeya-Seki (1965) maximum apparent magnitude -10.00. The brightest comet to be visible during the daytime.

4) Iridium satellites. Maximum apparent magnitude of flares -9.5.

5) Supernova seen in 1006. Maximum apparent magnitude -7.5.

6) Crab supernova seen in 1054. Maximum apparent magnitude -6.00.

7) International Space Station. Maximum apparent magnitude -5.90.

8) planet Venus Maximum apparent magnitude -4.90, minimum -3.2.

Objects with apparent magnitude less than -4.0 are not visible when sun high in sky.

9) planet Jupiter. Maximum apparent magnitude -2.94, minimum -1.61.

10) planet Mars. Maximum apparent magnitude -2.91, minimum +1.84.

Objects with apparent magnitude less than -2.5 are not visible in daytime sky even when the sun is less than 10 degrees above the horizon.

11) planet Mercury. Maximum apparent magnitude -2.45, minimum +5.7.

12) star Sirius. Apparent magnitude -1.47. All other stars and planets are not as bright as Sirius.

There are about 9,096 stars visible in dark skies to average human eyes from Sirius at -1.47 down to stars at apparent magnitude +6.5.

Note that it is possible to see a few of the brighter stars and planets at twilight. Perhaps the nightsiders rule the twilight zone and keep the daysiders out of it.

The tidally locked planet would not have a moon. The author can decide if the daysiders have any artificial satellites bright enough to be seen in the day. Comets bright enough to be seen in the day are rare in our solar system, and supernovas visible in the daytime are much rarer in our region of the galaxy (which is one of the necessary conditions for life on Earth). The author can decide if those conditions are different in his solar system.

The author can decide if there will be any stars bright enough and close enough to be seen in the daytime of his planet. The author can decide if there are any planets in the solar system that sometimes get bright enough to be seen in the day. In short the author can ensure that the only celestial object the daysiders ever see in the sky (except for rare and unpredictable events like supernovae and daylight comets) is their sun.

The nightsiders will see the stars all the time and count time by the time it takes for a particular star to rise above the horizon, reach it's highest point in the sky, and set below the horizon, and repeat. They will know that the outer celestial body will take a specific number of such periods to return to sight.

So the only problem is to make sure that the outer celestial body is not bright enough to be seen in the daylight sky until it passes below the horizon as seen from the dayside.

You may need to have someone run some computer simulations to find a set of orbits that makes the outer celestial body only visible when passing by the night side.

It is possible that as the outer object passes close to the night side of the planet it suddenly flares up briefly in brightness due to developing a coma like a comet or having a frozen atmosphere that becomes vapor and makes the body much more reflective.

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Aren't we all overthinking things a little bit?

Most planets and the stars are known to be mostly visible at night. Sometimes you can see Jupiter, Mars or Venus in daylight, but they are relatively faint - even Jupiter can only be seen during daylight with binoculars or more powerful equipment.

So all you need is a regular planet on a regular orbit. It must be farther or smaller than Mars, enough that it should not be visible under daylight, but feasible enough still, and quite large enough for whatever you need to have on it.

In our own solar system, if Earth had a day ength equal to its year, Ceres would be a good candidate.

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  • $\begingroup$ It sounds like OP is looking for a Earth like moon. Ours is visible at night so the only way to make it work would be to reduce it's reflectivity. However, people would still ask "Where do our tides come from?" $\endgroup$ – Muuski Feb 23 '18 at 15:23
  • $\begingroup$ @Muuski OP is talking about a tidally locked planet. Such a planet would have no tides. Also OP said they're willing to go for asteroids, space stations or whatever as long as it is only visible by night. $\endgroup$ – Renan Feb 23 '18 at 15:29
  • $\begingroup$ I meant tides from the natural satellite that you suggested orbit the planet. I must have missed where OP mentioned accepting objects outside the planet's sphere of influence. $\endgroup$ – Muuski Feb 23 '18 at 15:33
  • $\begingroup$ @Muuski I never mentioned a sattelite orbiting the planet. $\endgroup$ – Renan Feb 23 '18 at 17:18

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