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I want the surface of my fantasy planet to be made hostile every few thousand years for long enough to make every species go extinct that breeds on land. The easiest explanation seems to be other planets' gravitation, causing every volcano to erupt at the same time when everything is in a certain constellation. Ash will cover the sun and everything freezes for some time.

What size, distance, and constellation would the planets have to be to support such a system?

Your assumption is reasonable, as Tidal Heating is the reason that Io is the most geologically active body in our Solar System. Although, a problem with using Tidal heating is that the energy is drawn from the gravitational energy, so - without additional planets involved - your system will eventually decay into circular orbits and no longer produce the heating. But, that can be somewhat mitigated with other massive objects in the system to 'kick' your planets back into an elliptical orbit

Since you want this to be a long process, I would suggest looking at Neptune and Pluto, which periodically swap order - for example, from 1979 to 1999, Neptune was actually farther from the sun than Pluto. ("My very efficient memory just stores up planets nine") Pluto takes 248 years to complete an orbit, and Neptune takes 165 years, so they reach the closest point in a cycle of about 495 years

Because these are sufficiently separated vertically, then can never get closer to each other than 8AU (1AU is the distance from Earth to the Sun) - however, your planets can get closer than that. Also, when 1 planet is at one of the intersection points, the other planet may be elsewhere in its orbit.

All you need is that every 20,000 years the planets both reach the intersection point at about the same time, and pass close enough to stress each other. To magnify the effects, your inhabited planet is probably the smaller of the pair. When stretched and squished, then this means:

• The interior of the planet is heated up, providing plenty of magma for volcanos


• The crust of the planet is put under strain, providing plenty of opportunities for earthquakes, and resulting tsunamis

In terms of stability? Both the Jupiter-Io and the Neptune-Pluto systems are stable. You will need other planets in the system to help, but with only 1 interaction every 20,000 years this does not need a massive amount of help.

Another planet seems unlikely, given the time frame. Even Pluto's orbit is 'only' about 500 years, a far cry from 20.000. Also, it would not explain why the hostile period is only a thousand years.


Ice ages might be a better solution, though ice ages on Earth have lasted from ca. 50,000 years up to a hundred million years. Nor are they very regular.



Poul Anderson's 1974 novel Fire Time has a planet that experiences a period of extreme heat every thousand years due to the planet's irregular orbit around three suns. This might be tweaked to suit your needs.



Perhaps the best fit might be if is your planet and its sun is in a very eccentric orbit around a far larger sun, or possibly a black hole with a accretion disk emitting hard radiation. A characteristic of eccentric orbits is that most of the time, the orbiting object is far away from the larger object it orbits and spends a relatively short time whizzing close past that object before returning to a farther distance. An example is the comet Ikeya-Seki, which has an eccentricity very close to 1 (which is the limit for orbiting objects). Its aphelion (furthest distance from the sun) is roughly 200 times that of the Earth, but when it last came closest, it came within 450,000 km of the sun's surface - not much more than the distance between the Earth and the Moon. Its orbital period is roughly a million years. This is far more extreme than what you need, but shows that it is possible.



Finally, since your planet is a fantasy planet, there might be a fantasy explanation. It may be that the dragons breed every 20,000 years and get a bit rowdy during that time. Or there may be a bit more exotic explanation like in the Earthdawn role-playing game, which has a recurring Time of Horrors.

I am slightly skeptical that you could have a stable system involving a doomsday planet that comes close enough to cause tidal heating without seriously disrupting the orbit of both planets after the first pass. Its the sort of thing that might cause any moons of your inhabited world to leave for somewhere more gravitationally friendly.

Now, I am no orbital mechanic, but I think that a more plausible means of periodic catastrophes are meteorite showers. One source might be gas giant planets in the outer solar system having an orbital resonance that periodically disrupts the local equivalent of the asteroid or Kuiper belt, flinging a bunch of comets and asteroids into the inner solar system some of which end up falling onto your unfortunate world.

Big asteroid impacts are obviously Quite Bad in themselves, but they are also linked to major volcanic events too, if that's important to you.

Biological Infestation

Have you read the Pern books by Anne McCaffrey? The series is science fiction, though much of it is indistinguishable from fantasy, as it's a colonial world that has regressed technologically and depends on fire breathing 'dragons'.

One of the major environmental issues in the Pern books is an organism called Thread, which floats onto the planet every time a rogue planet, with a highly elliptical orbit, passes near Pern. The Thread organisms consume pretty much anything organic they touch.

Obviously you wouldn't want to directly copy this, but it could be a good jump start on thinking about other ideas.

While I'm inclined to favor the orbit-involves-something-nasty answers (note that the nasty object could also be a fast-spinning neutron star, the jets can be powerful) you've got a far bigger problem here:

After you have wiped out the land breeders the first time there will be no more land breeders. Your period between encounters is nowhere near enough for new land breeders to evolve.