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I have two planets and humans live on both of them. Humans need not have developed on these planets, but the planets should support human life with the allowance that the humans have technology comparable to modern day Earth.

Planet A is roughly the size and climate of Earth, with a circular or near-circular orbit.

Planet B can be a different size (probably smaller), and has the tricky orbit I can't quite picture.

Is it possible that the two planets can orbit the same star such that:

  • Planet A and Planet B occasionally come very near to one another, but are usually distant

  • Planet B's climate is at least reasonably hospitable

What would PlanetB's orbit have to look like for this to happen?

Edit: This "nearness" should mean that casual (or crude) space travel becomes possible between them, like a trip. The planets would remain so close to one another for a short time (days or a couple weeks at most) and then not line up with each other again for 5-10 years.

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    $\begingroup$ Define "near." Do you mean "the two planets revolve around the star at nearly the same rate so they rarely see each other" or "it's now just a day trip to the other planet, but only for these few days and not again for a millennium"? $\endgroup$ – Frostfyre Dec 12 '17 at 18:00
  • $\begingroup$ The latter. Although you also raise the good point about the periodicity of this day trip-level nearness. I'll update the question to be more specific, thanks. $\endgroup$ – KoaxialKable Dec 12 '17 at 18:02
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    $\begingroup$ If two planets should come periodically as close as Earth and Moon, I would say stable orbits are inconceivable. But maybe I'm wrong and such stable orbits can be found. $\endgroup$ – Alexander Dec 12 '17 at 18:08
  • $\begingroup$ That's what I'm wondering about. The relative masses of the two planets is negotiable, so maybe it's possible? I just wonder if the smaller planet would eventually become a moon or just be thrown out of orbit or something. $\endgroup$ – KoaxialKable Dec 12 '17 at 18:12
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    $\begingroup$ @RDFozz Journey to the Far Side of the Sun. en.wikipedia.org/wiki/Doppelg%C3%A4nger_(1969_film) $\endgroup$ – Jiminion Dec 12 '17 at 19:56
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You could put both planets in the habitable zone on horseshoe orbits. Janus and Epimetheus orbit Saturn on this type of orbit. From the point of view of one moon, the other follows a horseshoe shape around Saturn (or the star in your case). Most of the time they are relatively far away, but once every cycle the two planets come pretty close to each other and have a gravitational encounter -- close enough to have a pretty giant object in the sky for a short time. One planet's orbit gets a little closer to the star and the other's gets a little farther.

Here is what it looks like for Janus and Epimetheus, from a frame of reference orbiting Saturn. (Keep in mind that they are both orbiting Saturn far faster than they make horseshoes)

enter image description here

This is just one sort of peculiar form of the 1:1 orbital resonance. (For more see here, here or or here).

From the comments I'm seeing a lot of confusion about what this type of orbit actually looks like. Here is a nice animation comparing different reference frames: https://youtu.be/gsHBE3DWCP4

And here are a couple more animations I found. This one shows that Janus and Epimetheus don't really change orbital distance all that much: https://youtu.be/jIlTyFU4kUw. The actual change is less than 1 part in 1000, so it would not have much of an effect on the climate. I suppose if you had a more extreme mass ratio between the two planets, then the smaller one could have larger excursion in orbital radius.

And here is a really nice article on Janus and Epimetheus' orbits: http://www.planetary.org/blogs/emily-lakdawalla/2006/janus-epimetheus-swap.html

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Well if you put planet B in planet A's L3 Lagrange point it would be on the opposite side of the star and pretty hidden and inaccessible.

https://en.wikipedia.org/wiki/Lagrangian_point

enter image description here

This orbit is not stable over a long term, the planet would most likely drift in a so called horseshoe orbit, which would periodically bring the planets closer to one another. Several of Saturn's moons are in these types of complicated orbits of one another, the moons Epimetheus's and Janus are co-orbital with close approaches every four years (compared to their orbital period around Saturn of less than a day).

https://en.wikipedia.org/wiki/Horseshoe_orbit

enter image description here

This is very unlikely to occur naturally, but is theoretically possible.

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    $\begingroup$ Horseshoe-type orbits are not that crazy. They arise once in a while from simulations of planet formation. They're much much less common than Trojan setup (2 planets on mutual L4/L5 orbits) but they can happen (planetplanet.net/2014/05/22/…). There are even other classes of 1:1 resonant orbits in which the two orbits trade off eccentricity: oklo.org/2008/03/30/11-eccentric $\endgroup$ – Sean Raymond Dec 12 '17 at 18:39
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I'm actually surprised noone came up with this yet.

Make the two planet's orbital planes about adjacent, and add a bit of ellipticity to them:

enter image description here

Pros:

  • The planets barely meet each other - their orbital period can be made about the same, without the risk of collision.
  • Even when they're close, their relative speed is extremely high: they are moving adjacent.
  • Traveling between the two planet always needs very high delta-V, so it's technically very challenging, and has a really high energy cost. A slingshot maneuver is probably required, which takes a lot of time – years.
  • Story-telling-wise, it's really easy to explain: it's a much simpler concept that the Lagrangian orbits.

Cons:

  • The system is somewhat unstable - except some special cases - as comments pointed out. While you didn't specify if you want one, this may be a problem if you do want a hard-science answer - otherwise you could just hand-wave it, possibly blaming some other planets' (unexplained) stabilizing effect.
  • Astronomically, it's really unlikely a locally born planet would earn such an off-plane orbit without a serious event.

So, how on Earth (khm) could a planet end up with such an orbit? I see three options:

  1. It's an alien planet: it came from the outer space billions of years ago, and got caught. It's more ancient than any other planet there - sounds like a cool plot point, with huge possibilities. ;)
    Note: This needs some other, possibly gas-giant-sized planets in your system to interact with.
  2. The solar system experienced a cataclysmic event in the distant past: experienced an orbital system breakdown, got "hit" by a fast-moving black hole, encountered another solar system, etc., that highly disrupted the planetar orbits. This must've caused collisions and an extreme bombardment as well – so fill your system with asteroids and craters ;)
  3. Even better - the planet got caught from the other star system causing the cataclysm above! This has all the advantage of (1) and sounds better and more feasible than (2), actually. (I'd imagine it's a bit hard to earn a 90° orbital plane shift from zero... but if the planet always had it... ) Maybe it's life was already developed at that time? An advanced civilization was destroyed in the cataclysm?... story points, story points everywhere...
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    $\begingroup$ The only way this would be stable is if it is in what's called an "eccentric 1:1 resonance" (see here: oklo.org/2008/03/30/11-eccentric). That type of resonance can indeed be stable for billions of years. If it's not in resonance it will be unstable fast -- it's much much less stable than the Solar System. I have generated this type of resonance in computer simulations of orbital instabilities in systems of gas giant planets (thought to be very common -- see here: youtu.be/dCRdEFU_lIo). It's a rare outcome but it does happen. $\endgroup$ – Sean Raymond Dec 13 '17 at 8:40
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    $\begingroup$ I'm not sure if a stable configuration of orbits ever include moments where the two are really close to each other. But if this is the case, no matter the delta-v, you can still transfer without a huge rocket or long flight times. As both planets are assumed to be habitable, you'll just need a good enough aim to hit the atmosphere at the right angle and the mother of all ablative heat shields. $\endgroup$ – mlk Dec 13 '17 at 20:21
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    $\begingroup$ The moment when you read about the mother of all ablative heat shields and start giggling. BTW, +1 for going the Ulysses route. (en.wikipedia.org/wiki/Ulysses_(spacecraft)) $\endgroup$ – Mark Dec 14 '17 at 2:23
  • $\begingroup$ Significant eccentricity can start to mess with the habitability of the planet as well. $\endgroup$ – NeutronStar Dec 14 '17 at 4:04
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    $\begingroup$ Neptune and Pluto seem pretty stable, and they're in a 3:2 resonance rather than 1:1. $\endgroup$ – supercat Dec 14 '17 at 23:03
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Of all the solutions yet posted, I do not see any mention of expanding the habitable zone such that two planets with long, slightly-different orbital periods would both fit. This would require a very luminous star, I believe doubling the luminosity (average over wavelengths) would triple the distance to the inner and outer edges of the habitable zone.

Our solar system has two planets (Uranus and Neptune) at 20 and 30 AU, which are in conjunction about once every 170 years. To expand the habitable zone from it's current 0.7 AU to 1.5 AU boundaries, we need a star about 8-10 times as luminous as the sun. Assuming that the star is the same colour as the Sun that would be a class IV subgiant, which would be too short-lived for life to evolve on, but could theoretically support human life that arrives there for millions of years. These are not uncommon stars, so it is completely plausible that humans find and colonize one if they are out colonizing anyway.

If you need to allow for longer or shorter periods between when transfer between the planets will be possible, then you can adjust for star brightness and change the size of the habitable zone accordingly. In the interest of realism, make sure that your chosen brightness falls in an area on the Hertzsprung–Russell diagram with the same colour as the Sun (above or below it on the diagram) in a relatively dense part (i.e. no Sun-coloured 100 luminosity stars).

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    $\begingroup$ In only a couple billion years our sun will expand much further than 1.5AU. $\endgroup$ – cybernard Dec 13 '17 at 22:47
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    $\begingroup$ @cybernard: Yes, but at that time it won't have the correct colour spectrum. I wonder if Uranus and Neptune, after having much of their atmosphere boiled off, might leave behind habitable rocky cores though! $\endgroup$ – dotancohen Dec 13 '17 at 23:39
  • $\begingroup$ +1 for the creativity, though the horseshoes are a bit better from a story standpoint, as they have more frequent encounters (I'd imagine humans crossing worlds and having to cross back will be more useful in a sci-fi novel then the 170 year conjunction). $\endgroup$ – Mark Dec 14 '17 at 2:19
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    $\begingroup$ The idea that brighter stars can fit more planets in their habitable zones is not true. The width of the habitable zone is not actually a limiting factor. The habitable zone is indeed wider for brighter stars, but since planetary orbits are naturally spaced logarithmically, the number of planets that fit into the habitable zone is to first order independent of the stellar properties. See here for details: planetplanet.net/2014/05/19/… $\endgroup$ – Sean Raymond Dec 14 '17 at 12:22
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    $\begingroup$ The idea isn't to fit more planets into the habitable zone (our solar system already has three) but rather to move the habitable zone further out, so that conjunctions of the habitable planets happen less frequently. $\endgroup$ – dotancohen Dec 14 '17 at 23:19
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I think an inclined orbit with similar orbital period with an offset of a half year would do it.

You don't have much flexibility in orbital period and still be in the same habital zone of the star. That leaves inclination, eccentricity, and where in the orbit is the "new year point".

Eccentricity is going to affect whether you're on the cold or hot edge of your habital zone. But not do much in terms of planetary distance from eachother.

An orbital inclination will increase the distance between the objects by moving it out of plane, without changing the climate much. By having a high inclination on one planet (45 degrees) and zero on the other, and by offsetting their orbits a half year, the planets will vary between about a third orbit to a half orbit from each other.

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  • $\begingroup$ My first thought is a 90 degree 'tilted' orbit, and you'll only have relative proximity when both planets were near the intersection point. The only caveat is that whilst you could stagger them and 'time' the period so the 'dead time' is 5-10 years, they'll also probably be relatively close together for multiple cycles after that. $\endgroup$ – Sobrique Dec 13 '17 at 10:32
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Another approach: We need to make the system a binary star system, although the partner might not be hot enough to glow in the visible spectrum. The two planets are in resonance orbits with the binary partner which will go a long way towards making the system more stable.

Both orbits are a bit elliptical. On your close approaches the outer world (at the inner point of it's orbit) zooms past the inner world (at the outer point of it's orbit)--while they are pretty close to each other in space orbital speed difference will be enough to ensure there's a single transit window between them at each encounter.

While in general being close physically but far apart in speed is not considered close in space this is a special case as your target has an atmosphere--which means aerobraking. All you have to go is graze the world, you don't have to match velocities. The mission path is basically to get run over by the other world.

Note that the binary partner will not protect moons--these worlds will have to be moonless as the close encounters will wreck havoc with their orbits.

There's still no such thing as crude space travel, though. The energy requirements of a space mission are simply too high. It takes some pretty sophisticated engineering in order to get the energy density to go into space. Crude ships simply don't have the power density to make it.

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The planets couldn't hold their orbit that nearly.

Suppose you find an appropriate orbital arrangement.

Putting manned craft (not just machines) on Low Earth Orbit with today's technology is getting cheaper, but it's still very expensive, and no one would call it crude nor casual except in very relative terms.

For another planet to be more reachable, it would need to be nearer than the satellites in LEO and/or pass by much more slowly, with slowness being much more important. When planets are near each other for extended time frames, their gravity affects each other. So they wouldn't hold their orbits.

It's possible, pending calculation, that they couldn't even hold their structure, i.e. their roughly spherical form, with the other planet pulling the near side more strongly then the far side.

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Stable orbits are not possible with large earth-like planets approaching very closely. Each planet will exert a gravitational attraction on the other which will be particularly strong at closest approach which will in turn modify the orbits of both. That said it might happen for a few million years as an unstable arrangement depending on how close “close” is.

One configuration that would provide what you want would be for one planet to orbit close into the warm edge of the habitable zone and the other to have an elliptical orbit ranging from the inner edge of the habitable zone to the outer edge of the habitable zone. In this way occasionally the planets will be close together when they both happen to be at closest approach to the sun at the same time. However as mentioned above this orbit would be unstable in the long term.

One key element you should consider is velocity. It’s all very well being in close proximity to your destination but if the destination proceeds to fly past you at several km/s you’re not going to be able to land there.

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