There are several sci-fi movies where planetary bodies are shown in the sky in order to indicate that the characters are on an alien planet. A good example is in Predators (2010):

Alien Planet

Is it actually possible for such planetary bodies to exist so close to each other as to be visible in such detail to the naked eye? Wouldn't gravitational forces cause disruptions on them that inhibit the development of life (whether plant or animal)? In our Solar system there's a good example in Io which has severe volcanic activity resulting from tidal heating due to its proximity to Jupiter.

  • $\begingroup$ Also, for better clarity, isn't a sci-fi related question more relevant to this community? $\endgroup$
    – KalenGi
    Commented Jul 9, 2014 at 14:12
  • $\begingroup$ My qualification for being on this site is that I watch a lot of scifi. In real life I do programming. Nothing of that qualifies me to answer your question although it is scifi-related. My impression is, that this is about the profile of most of scifi.se users. The guys at physics.se or space.se have a different qualification profile. They can answer this question without even knowing who Schwarzenegger is. $\endgroup$
    – Einer
    Commented Jul 9, 2014 at 14:19
  • $\begingroup$ @Einer I do agree, since I fit the very same profile that you describe, but I've come to learn that sci-fi seems to attract a lot of science-oriented minds. For example, this question scifi.stackexchange.com/q/33131/13185 has a lot to do with an understanding of chemistry and biology, but to get a really good answer for it it helps to have an interest in sci-fi $\endgroup$
    – KalenGi
    Commented Jul 9, 2014 at 14:24
  • $\begingroup$ While the question was sparked by something sci-fi, the heart of the question is about (astro/)physics, so it would probably do better on space or physics SE. Moving closer towards an answer, while Io is full of volcanic activity, Europa is not, and may even have a liquid ocean beneath its surface. Titan (one of Saturn's moons) has also been hypothesized as a birthplace for life. $\endgroup$
    – Brian S
    Commented Jul 9, 2014 at 14:25
  • $\begingroup$ @BrianS in that case, it would be great to have it move to space SE rather than closed since an answer is still required. $\endgroup$
    – KalenGi
    Commented Jul 9, 2014 at 14:33

2 Answers 2


Quick calculation of Roche limit for two identical earthlike (fluid) bodies. Roche distance is about equal to 2.44 X radius X(density1/density2)^1/3. For identical bodies the density conveniently cancels, so we get Rd = 2.44 X radius. Earth radius is 6378 km. So Rd = 2.44 X 6378 = 15500 km (center to center). The distance surface to surface (15500- 6378) is 9184km.

At that distance, by trigonometry, another earth would subtend about 44° of sky, pretty big!, and just at the bleeding edge of stability.

The nearest planet in the provided picture looks like it might be at about that distance. Evacuation might be in order as the other three planets or moons will degrade the close binary's orbital stability.


Have a look at this question (Help in determining the features of an unusual, fictional star system) for one possible hypothetical scenario in which such a planet (or a satellite) could exist.

For a satellite of a giant planet, there will be tidal forces, sure, but after a few hundred million years of formation of the system, it is likely that there will be tidal locking due to the dissipative forces within the satellite, which will ensure that the effects of tides are minimal after that (or, in other words, the effects are 'static'). On a related note, I do find it interesting that Io isn't tidally locked to Jupiter yet, but this is a different question altogether.

On the other hand, I believe it actually helps to have a Jupiter-sized body around as it drives meteoroids away from the moon you live on, making life a little safer (this might be wrong, but I think this is how it will work out; also, this does not help if 'panspermia hypothesis' turns out to be correct).

Having multiple planets nearby, as shown in the picture, is a little more difficult due to many body systems not being very stable. You could possibly have a binary planetary systems and life in the moons at Lagrangian points, but it is very likely that such systems will lose some of their bodies due to perturbations from other planets or similar second order effects due to them being unstable. You can have a moon revolving around the binary planetary system some distance away as an alternative, which would be fairly stable, I guess. However, with the current planet formation hypotheses, it seems very unlikely that you will have large planets near each other, since they won't be formed in the protoplanetary discs in a way that they don't coalesce to form a single larger planet.

Also try this: http://www.stefanom.org/spc/ . You'll soon realize that having many planets close by, in a stable system is difficult.

  • $\begingroup$ I think that game has some precision problems. I dropped 4 ice giants near the star. I'm getting stable orbits for decades at a time, then a sudden shift into new stable orbits. I know when I tried to write something like that (without the graphics) errors during close flybys were a real bugaboo. $\endgroup$ Commented Jul 9, 2014 at 23:41
  • $\begingroup$ It's likely. I haven't looked into the code of that game. Also at 2 AU scale, it can be difficult to have the close flyby distances (or impact parameters, which would be more accurate) very precise based on visual input. And often it is those distances that can change the dynamics of the whole system. Also the 'decades at a time' part can happen because of resonance-like features where two nearby planets' synodic periods (relative periods wrt each other) are much larger - of the order of decades - and they only come close once in a while. $\endgroup$
    – Takku
    Commented Jul 10, 2014 at 10:53
  • $\begingroup$ While you can have long period resonances it's pretty hard to have it happen when everything is within 2AU, generally within 2/3 of an AU. What happened to me when I tried to code such a program was for any reasonable stepping interval a step moved across too much of the gravity well during a flyby. (A simple test showed it was caused by the stepping: The shorter the stepping interval the better it behaved.) $\endgroup$ Commented Jul 10, 2014 at 22:59
  • $\begingroup$ Io isn't tide-locked? My impression is that the ‘tidal’ stress comes from the resonance of its orbit with those of other moons. $\endgroup$ Commented Feb 28, 2018 at 7:07

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