A class F star (1.4 times the mass of the sun — it has to go supernova) is expanding into the red giant phrase. Its Earth-like planet, with life on it, has slowly been pushed out of orbit as it expands, therefore buying its civilisation extra time. Eventually, Earth's orbit swings quite close to the orbit of a Saturn-like planet, whose orbit has also been expanding, but at a different rate. At this time, there are no other planets between 'Earth' and 'Saturn' (no idea why that would be, or that it would matter).

Earth is at the inner limit of the CHZ, Saturn and its moons are on the outer-limit. Earth is becoming quite inhospitable due to its proximity to the sun and some greenhouse effects, while some of Saturn's moons, which have been colonised for a while now, are becoming quite pleasant.

Tech Level

This civilisation has been moving people from Earth to Saturn's Moons for a few centuries. It's supposed to be our present era/ tech state + 1000 years, although due to resource scarcity and other catastrophes, the level has not increased linearly.


  1. Is this within the realms of plausibility, and is there anything that would make it more so?
  2. Given that this civilisation is trying to conserve resources, and assuming that Earth-Saturn travel is limited to a window period when both planets are fairly close to each other, what is the wait time between these periods when travel is possible? (In real Earth years...)
  3. What kind of flight technology would they be using (it has to take at least a month, if not more, for them to get between Saturn and Earth, when the planets are closest)
  • $\begingroup$ "No idea why that would be" idea: Gas giant for some reason pulls all material into it--or at least stops it from forming sizable clumps and planets. I believe this is why the asteroid belt exists in place of a small planet. $\endgroup$
    – Jax
    Commented May 19, 2015 at 13:38
  • 1
    $\begingroup$ present era/ tech state + 1000 years, although due to resource scarcity and other catastrophes, the level has not increased linearly. Technology is not expected to increase linearly but exponentially (for example, better computers mean that advances in other fields are easier, better physics gives better materials for building computers, biotechnology gives biocomputers/brain-computer interface). There has elapsed almost the same time since the first vapor machines to the first airplanes, than from those airplanes to current airplanes. 1000+ year technology probably will look like magic. $\endgroup$
    – SJuan76
    Commented May 19, 2015 at 16:39
  • $\begingroup$ @SJuan76 That fact is definitely a flaw in my story.... $\endgroup$ Commented May 19, 2015 at 17:45
  • $\begingroup$ I hate to be a party pooper, but F-type (at least of this mass) stars won't go supernova; they're definitely not massive enough. I think you're confusing the mass of the remaining core/degenerate object needed to form a neutron star with the mass needed to go supernova. $\endgroup$
    – HDE 226868
    Commented May 19, 2015 at 22:28
  • $\begingroup$ @HDE226868 Oh dear. So my star has to be 8 times the mass of our sun? I could use some microbial contamination theory to explain the population of intelligent life in the system.... $\endgroup$ Commented May 20, 2015 at 8:14

2 Answers 2


I was going to comment asking why the ♄-like planet would recede “at a different rate”, but I see question 1 at the end asks us, and you specified . So I'll tell you that no, it's not sensible.

The red giant loses mass because of the high solar wind and even outer layers that can puff off. The planets keep the energy they always had, but now the gravity is less. In a low-eccentricity orbit, the orbit will simply move outward to the new point where the (same) kinetic energy balances the (lower) potential energy by lifting it higher to increase the potential energy again.

All the planets will have a new "groove" for the lower mass star. The change in potential energy with increased distance gets smaller the farther out you go, so the outer planets will move outward more to gain X amount of potential energy.

Ah, here's an idea: the changing situation messes up the mutual stability of the orbits, so the gas giants or anything falling into resonance will throw them around and anything might happen.

The original planet in the habitable zone will be at a larger distance than ours, and the envelope-hydrogen-burning phase will puff out more than ours, too, since you specified a more massive star. You should check the respective sizes to see if the ⊕-like planet will simply be consumed or stay farther in (relative to our case) or what. Likewise, what is the needed distance of the habitable zone during the red giant phase?

The movement of the planets or close approach will not be a huge factor in making the trip feasible. You'll get a launch window repeating just over one year apart.

The deadline might be made by other means: will their planet be engulfed? Maybe the destabilized planet orbits will drive it into a dive or escape. But we're talking millions of years here, so not a rush on the human scale. Perhaps an impending collision will do the trick.

Yea, say they have an asteroid belt too, and not only are the asteroids going to be scattered, but the increasing eccentricity of the planet (due to driving by the giant) will push the perihelion into the asteroid belt. Every year is a gamble against possible major impact events, and the chaos means they can't be accurately forecast. You might need a little hand waving as the reshaping of the orbits are slow processes to get the dire situation set up. Maybe the asteroid belt can be squished to lower its aphelion, too. Hand waving with chaos theory and some buzzwords like strange attractor and phase change in phase space can gloss over that. In any case, invoking a chaotic process makes dramatic outcomes more plausible and any suggested outcome harder to refute.

Point 2: covered above. Not a factor of closeness, but some other deadlines can be brought to bear.

Point 3: more like years, not a month. Transfer orbits have bodies moving the same way as any other orbit: once around takes a year at the inner point, and the other planet's year at the outer point. Roughly figure the time based on the highest point, and you traverse somewhat less than half of it.

For a mass migration, look into a permanent Aldrin Cycler (or several with staggered schedules) transit liner that only needs to be put into its orbit once, and it keeps going around. At each rendezvous you shuttle between it and the planet with light craft.

  • $\begingroup$ Basically the danger comes from part of Earth's orbit overlapping into the asteroid belt? As more time passes, it becomes more dangerous to be on Earth and more dangerous to organise pick-ups? Maybe they have an Aldrin Cycler (didn't know of its existence before, thanks), some asteroids are entering its trajectory, its predicted it won't come around another cycle? $\endgroup$ Commented May 19, 2015 at 12:38
  • $\begingroup$ Asteroids might collide with the cycler or the planet. $\endgroup$
    – JDługosz
    Commented May 19, 2015 at 15:39

The issue of the planetary movement has been covered quite well by JDługosz, so I will answer what sorts of methods of travel could be used.

Since we are talking 1000 years in the future, most technology would appear magical to us, but ideas that are fairly difficult or just out of reach will be mature technologies by then. The simplest means of travel once you escape into Earth orbit would actually be solar sails. As far back as the mid 1970's K Eric Drexler proposed an ultra thin foil sail, made by evaporating a molecule thick layer of metal on a "wax" substrate, then allowing the "wax" to evaporate. The entire process was designed to be done in orbit, and calculations showed that sails like this would have incredible acceleration (for solar sails). Since sails can continue to gain momentum until someone actively brakes the sail (changing the orientation to the sun, for example), these sails can accumulate huge velocities, and one estimate was such a sail could carry a payload from Earth to Pluto in a flypast trajectory in as few as 3 years.

Slowing and stopping such sails would be more of an issue than starting out, but clever "sailing" would make this relatively easy, and after a 1000 years of practice, the best sorts of orbital trajectories would be well known.

Similar "sails" using the plasma wind (Magsails) or electrostatic interaction with the solar wind (electric sails) are also possible, and they have similar parameters as far as performance is concerned. Changing the direction of flight and slowing is a bit simpler, since the sail can be controlled in this case by varying the magnetic field or power sent to the elements of the sail.

These sorts of ships would be dominated by the vast "sail" area, and passage through the chaotic regions of the former asteroid belt could be fraught with some difficulty if a piece of debris were to strike the sail or spacecraft. On the other hand, there would be essentially no limitation on when you needed to launch, although orbital mechanics means that you will still have an easier time when to orbital alignments are correct for minimum trajectory transfers.


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