How short can Milankovitch Cycles be on a world with a stable orbit?

Okay so there's a project I've been thinking about for a long time wherein humans colonise a world where the climate appears warm and benign only to discover that the local climate oscillates from something like the height of the last Ice Age to something like the Medieval Warm Period on a decadal timescale (using Earth years for reference dating), that's at least 10 and not more than 99 years not swings every 10 years.

There are two ways I can see to do this, (A) use a bright star and a large, eccentric orbit, this is however very regular and predictable so option (B) is orbital forcing which I'd like to explore.

My question therefore is can I just drop three or four zeros from the period of the Milankovitch Cycle components and call it even or would that mean that the planet was too unstable in its orbit? If such a drastic increase in the "standard" variables isn't viable is there another orbital variation I could use to get the effect I'm looking for?

• I doubt such a world would be idyllic for life as we know it. Evolution doesn't happen on decades (unless the reproductive cycle is shorter than months, which rules out anything bigger than a fly), and glaciations were big hitters on flora and fauna population. – L.Dutch - Reinstate Monica Oct 24 '17 at 13:22
• @L.Dutch Fair point, have edited the question to focus on the climatic implications that are most important. – Ash Oct 24 '17 at 13:25
• A planet like this was actually a key plot point in the Generation Warriors series by Anne McCaffrey and Elizabeth Moon, so you might take inspiration from there – Benubird Oct 24 '17 at 15:55
• @Benubird Cheers I'll have to have a look at that. – Ash Oct 24 '17 at 15:58
• You could get a fairly drastic swing on those scales with some kind of orbital resonance. – AJMansfield Oct 24 '17 at 22:00

It's the Sun that fluctuates, not the planet

Our sun has natural cycles that fluctuate its brightness and energy. The most obvious cycle is the 11-year sunspot cycle, but there are other cycles that can now be observed thanks to constant satellite monitoring. One cycle is less than 5 hours, while another cycle appears to be much longer, at least several decades and possibly longer than we've been able to observe. Our sun's variations are very slight, the 11-year cycle fluctuates by only about 0.1%. I found many articles online:

A handful of astronomers have suggested that some of the fluctuations observed by the Kepler Space Telescope that were identified as exoplanets orbiting extremely close to their suns, are not exoplanets at all but a natural cycle of the star itself. One such extreme case is Kepler-78b which was observed with an 8.5 hour dimming cycle. The official theory is it has an Earth-sized planet with an 8.5 hour year. This same alternate theory suggests that pulsars which rapidly fluctuate in milliseconds are not spinning hundreds of times per second, but simply have a cycling magnetic field. In reality, there are a few pulsars that apparently break the speed limit spinning over 1000 times per second, supposedly rotating faster than physically possible.

Your situation might involve a perfect storm where several solar cycles peak together. I don't know how your astronomers will miss it, but one possibility is the peak energy occurs only in certain spectra which has been masked by an accretion cloud. Another possibility is that a very long "century" cycle was at a low during the survey, but is now approaching its peak. Still another possibility is that humans are fallible and often choose to disregard warning signs, sometimes deliberately when profit is involved.

To directly "touch" the core question, no. I honestly don't see how the planet shifting orbit or wobbling around can be explained with a 10yr cycle that would be unobserved but stable longterm, or how a colony could be discovered, surveyed, and settled in less than 10 years. That time frame does not seem realistic. I would find it an objectionable plot detail. A century-long solar cycle amplifying a decade cycle can explain how your colony got established but later experiences drastic weather changes each decade.

• I'm going to assume the fact that you've gone for the "something else" option means you don't want to touch the core question? – Ash Oct 24 '17 at 15:09
• @Ash, I have edited to "touch" what I find unbelievable in your core question. – wetcircuit Oct 24 '17 at 15:47
• Sorry I just realised I made a bad assumption "decadal" isn't every 10 years rather more than 10 and less than 100 years. – Ash Oct 24 '17 at 15:51
• Ahh, I see "decades" not "a decade", that is probably better. My initial thought is it still seems too short to be caused by another gravitational object in the system. I will reconsider, but my gut tells me that object is very unsubtle, as someone else said it's like a Jupiter next door. It's influence is very strong if it's enough to cause weather changes, and then there's the issue of the planet "popping back into its socket" once the monster has passed. Saturn has moons that trade orbits but this would also be obvious: apod.nasa.gov/apod/ap051102.html – wetcircuit Oct 24 '17 at 16:02

I have no idea how short such a cycle can be, but I can tell straight away that the idea of people capable of interstellar travel and colonization of new worlds not noticing it in advance is not credible. I am pretty sure that every starship has navigation systems that can rapidly and accurately calculate all possibly relevant orbital mechanics of any object the ship might want to reach. Most cyclical patterns would be noticed straight away and automatically. An unusually rapid cycle such as you want would be obvious.

Note that you can of course make it part of the story that the system was never surveyed properly and the colonization force had no capability to fix the oversight. You could even fairly easily make a story where they were aware of it, but had to colonize anyway. Explaining why a datum that comes relevant few decades or centuries later gets forgotten is fairly trivial. And depending on your story forgotten detail might be exactly what you want. It is a fairly common solution in fiction, actually.

Alternately, you can easily explain why the orbital parameters change after colonization. A close encounter with a fairly large object that was far in interstellar space when the system was surveyed and colonized can do all kinds of things you want with the planets orbit. It can do them fast too. And there would not really be anything people could do about it unless they happen to have a Death Star™ or equivalent available.

• You can't see precession etc... without prolonged close observation, and that's if you're looking for it which they wouldn't necessarily on a preliminary survey, assuming any survey was carried out at all. – Ash Oct 24 '17 at 13:32
• This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post. - From Review – L.Dutch - Reinstate Monica Oct 24 '17 at 14:32
• I agree with @VilleNemi; to shorten Milankovitch cycle that much nothing less than a Jupiter-size planet on Mars orbit is needed. That is bound to be noticed. Plot can fairly easily explain why they may want to colonize despite problem, but overseeing it would be unfit for any starfaring race (unless they grew a lot dumber in last travel). – ZioByte Oct 24 '17 at 14:32
• @Ash You mean: "We didn't see it", don't you? You are probably right about the close observation part even with future technology since you need the resolution, but you are hugely underestimating the benefits practical starships give to astronomy. Think space telescopes but dirt cheap and without mass limit (not really, but compared to us, and the actual needs to spot this) and backed with ship computers programmed to do routine observations and astronomy automatically. They would have the instruments and observations and the calculations by default because it would be essentially free. – Ville Niemi Oct 24 '17 at 20:23
• @ash If in a different stellar system, then they have the ability to have observed the system over a period of many years, with effectively arbitrary resolution. If they have FTL, that gives them, effectively, the ability to observe the system over an arbitrary period of time. Want to see what it was 100 years ago? Jump 100 light-years away and press the button to deploy your standard-package Jupiter-sized array of telescopes. Obviously, it depends on tech level, but observing at high resolution over large periods of effective time would be standard practice. If STL, travel gives the time. – Makyen Oct 24 '17 at 21:00

Between 10 and 99 Earth years is the length of 1 year on many planets. Jupiter takes 11.8 earth years. One Saturn year takes 29 earth years. Instead of the Milankovitch cycle, your planet circles its star more slowly. A winter occurs every few earth decades, followed by a summer.

It sounds like you also want a shorter warm/cold cycle that resembles Earth winters/summers. Your planet also revolves around a gas giant, completing a revolution every few months. The planet's path around the gas giant is at a 50 degree angle from the gas giant's revolution path around the star, so some star light hits the planet even when on the far side of the gas giant, but a reduced amount. During this colder period the planet is closer to the gas giant's northern hemisphere.

Then in the warm period, the planet is closer to the southern hemisphere of the gas giant, and has an unobstructed line of sight the the star.

• Nice idea. Simple and effective, no hand-waving or special conditions required. Plus one. – a4android Oct 25 '17 at 3:42
• Good answer. To address the observation concern from Ville Niemi's answer, I'd suggest that the colonists knew this would occur, but didn't realize the degree to which it would affect things. – Bobson Oct 25 '17 at 17:14
• My only complaint, if that's even the right word is that this cycle, in terms of it's timing etc... is still relatively predictable given a short observation of the system. The reason I opted for Milankovitch Cycles in the first place is that they are not immediately obvious, without long term observations, on the order of a few years with such a rapidly changing schedule, it took centuries worth of data to pin down Earth's changing orbital and precessional parameters and we're still not sure it's right. – Ash Oct 29 '17 at 11:34

I think the sort of change you need might be better sourced from within the planet itself. Two large scale forces that can be arranged to flip flop the climate relatively quickly would be ideal.

If the topography of the continents was such that snow and ice could easily build up over vast areas relatively quickly it could act as a big radiator cooling the planet down.

If there were to be regular eruptions on the time frame you had in mind, in and around this vast snow covered area driven by some inner movement of the planet this could rapidly reverse the situation.

Locally volcanic heating and lava would melt some ice, but more importantly and over a much wider area, volcanic dust from these volcanos could easily cover vast areas relatively quickly. If the volcanic dust was very dark (not implausible) then the ice would melt and the albedo of the planet could be upset relatively quickly. Even if the ice did not melt a sufficient layer of dust might still change the albedo for some time. Some areas might become covered by very thin unstable ice sheets which could easily be destabilized.

In a relatively short period the radiator has gone and is replaced by an heat absorbing dark surface warming the planet up.

The warming is significant but in very high latitudes snow still starts to cover the dark ground surface. As the volcanism dies down the snow line creeps south (or north in the southern hemisphere) until we return to snow cover and another set of eruptions.

• I'm going to assume the fact that you've gone for the "something else" option means you don't want to touch the core question? – Ash Oct 24 '17 at 15:10
• @ash Yes. It’s not my area but from what I do know it seems that such changes tend to be subtle and slow in part because there’s so much inertia involved. If they happened too quickly and weren’t strong enough there would be a danger that other environmental feedback mechanisms might act as a brake and average the changes out. That’s why I went for the geological option. You need something to make really drastic changes really quickly to overwhelm any other feedback mechanisms. – Slarty Oct 24 '17 at 16:44

While there are plenty of objections, time to look at the science for this science-based tagged question. The difficulty is coming up with a significant orbital that is stable from a distance yet dramatically unstable from afar.

A lone moisture farmer on Tatoo a planet orbiting two suns

Sure, there may be an iconic image we all dream up when world building about a double sunset and a young man destined for great things in a sci-fi fantasy novel, but how stable are planets orbiting binary stars? You'd be surprised.

A binary star frequently has solar eclipses - of the kind where a sun would eclipse the other sun. This is because the suns are moving, and to an extent, have a large amount of momentum. Any moving object can cause gravity assists, and a binary sun would be no exception. This can wreck havoc for anything that gets too close. However, from far away, the combined binary star is quite stable, and our young farmer can gaze longingly into the double sunset without fear of Disne an evil empire:

A massive red planet in a highly elliptical earth crossing orbit, in the counter direction can disrupt this:

This is because the red planet is slowly robing momentum from the binary stars and stealing it from our inhabitable planet, thus slowing it down and causing it to drift closer in on a notable time scale. Such a feed would not be noted using current technology. If the blue and red planets rotated in the same direction, the blue planet would be pulled out of the habitable zone. Again, none of this would be detectable with current technology. We don't know which direction a binary star orbits unless they have different spectrum, which a near-twin binary star wouldn't have.

(I obtained this diagram using this gravity simulator and the program below:)

//Gravity fun at TestTubeGames
_settings(gravity: r^-2, n: Binary Sun);
_type0(m: 750, col: 2, pic: 0);
_type1(m: 0.01, col: 4, pic: 1);
_type2(m: 3.5, col: 5, pic: 1);
_add(type: 0, x: 0, y: 10, vx: -4.5, t: 0);
_add(type: 0, x: 0, y: -10, vx: 4.5, t: 0);
_add(type: 1, x: 125, y: 0, vy: 3, t: 0);
_add(type: 2, x: -70, y: 90, vx: 3.31, t: 0);

• Okay so this would pull a colony out of a warm climate situation would that world drop back into the goldilocks zone or is this a one way trip? – Ash Oct 25 '17 at 10:05
• This may be a one way trip, but it's surprisingly easy to add heat to a planet. We're doing it without intending to. Plunging back into a warm area can be as simple as being an explorer accidentally opening a massive methane vent. Or it can be as complicated as the large planet getting shockingly close and having one of it's moons collide, adding massive amounts of energy. – Mark Oct 25 '17 at 15:04

This might work. Your Earth-mass planet orbits a bright hot star. This means it will have a long orbit. Its orbit is flanked on either side by two gas giant mass planets and they are in similarly long orbits.

Whenever the earthlike planet is passed by either gas giant it is "pulled" into either a higher or lower orbit. This it alternates between two climactic states. Deep glaciation and a medieval hot time.

The long orbital timescales will explain its decadal rate of variation. This is virtually a short-term Milankovitch cycle.

A planet like this will qualify as habitable. Refer to Stephen H Dole's Habitable Planets for Man (1964; 2nd edition 1970) for the range of proposed habitability criteria. Also available here and here.

Starship navigation systems and planetary colonisation surveyors will readily identify the climactic characteristics of this planet. Provided it is within habitability criteria, then colonisation will be allowed. Other colony planets in your fictional universe may be similarly habitability compromised; not perfect but viable.

• "Starship navigation systems and planetary colonisation surveyors will readily identify the climactic characteristics of this planet." Why does everyone assume someone actually looked that carefully before dropping on this particular rock? Never yet have humans colonised anywhere they really knew the ins and outs of. – Ash Oct 25 '17 at 9:55
• @Ash That was then, things will be different in future. It will be relatively easy to determine these factors. The vast majority of planets will be uninhabitable. It's reasonable to assume potential colony planets will be examined for any hazards or adverse environmental conditions. Even with ridiculous cheap FTL travel, it's improbable colonists will flood planets without doing their research first. We're not smarter, just cautious. – a4android Oct 25 '17 at 11:52

You could create such a situation on a temporary basis by having a really large comet breaking up on a pass near the star. Thereafter there is a cloud of gravel that some years hits the planet and some years not. It will depend on the mutual interaction of the period of the comet and the planet. If the comet and the planet's orbits are co-planar then periodically the planet gets plastered with a few hundred megatons of small rocks moving at high speed. This has the initial effect of heating the upper atmosphere a lot, then leaving enough dust for a nuclear winter for a few years.

The comet fracture could happen either after the colony was created, or soon enough before that the preliminary reports didn't see it.

Eventually the gravel spreads through the orbit and you get hit to a smaller degree every year.