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I have a galaxy with many civilizations each with several planets. Many of the civilizations are at war with their neighbors, and use technologically advanced weapons in battle.

My question is about the colonies. If one of these civilizations want to colonize a new planet, and want to choose it based only on its natural defenses against future attacks, what kind of planet would they look for?

In order to make it simple, consider only weapons operating from orbit, outside the (potential) atmosphere, with no planet-side assault.

Weapons targeting the planet from orbit may include:

  • Huge inert projectiles (like deflected asteroids)
  • Long range lasers
  • Bombs (Dropped from high altitude, and exploding on impact)
  • Missiles (Small or big, sometimes heat-seeker to explode on a structure or a someone)
  • Weapons of Mass Destruction (Rarer, missiles or bombs that would raze a city if they explode on the ground)

UPDATE:

I'm don't want to consider planet natural resources in this question. Colons would bring with them enough life support (food, water, oxygen etc) and construction materials with them if it is not present on the planet.

Underground structures would be, unfortunately, not a considerable solution in my precise case: I'm asking this in order to make a game, and I don't plan to manage this kind of structure.
Underwater (or any other liquid), however, are an interesting point.

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  • $\begingroup$ What about an atmosphere which contains elements which obscure guidance and detection? $\endgroup$ – Francine DeGrood Taylor Sep 29 '15 at 17:56
  • $\begingroup$ Are you looking for planet to hide your base using natural color(camouflage) and mask your activities using magnetic field or thick atmosphere or perhaps giant gas planet to distort or eliminate any intelligible signal? Or do you mean the planet itself makes landing or orbiting difficult and the atmosphere can even block GRB effectively? $\endgroup$ – user6760 Sep 29 '15 at 23:24
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Getting the atmospheric composition right will go a long way towards natural, passive planetary defense.

Projectile Weapons

Shooting small unguided munitions at surface targets through the atmosphere just doesn't work because ...reasons. Highly turbulent atmospheres may interfere with guidance systems on a guided missile. With radio countermeasures in place, any orbital guidance may not get through to the missile. This only works for missiles or rocks below a certain size though.

Really giant rocks (>100km diameter) don't really care about atmosphere and will just keep going as if the atmosphere isn't even there. (Note that "space" on earth is technically defined at 50km so a 100km rock is going to touch the surface while it's top is still in the vacuum of space.) If an atmosphere is the planet's armor, space can always find a bigger rock to throw to punch through that armor. Thicker and thicker atmospheres require bigger and bigger planets to hold onto that atmosphere and the colonists eventually stop looking at rocky super Earths and start looking at gas giants.

Beam Weapons

Earth's atmosphere absorbs radiation across most bands except IR, visible, near-UV and radio. These colonists will want an atmosphere at least as good as Earth's because of the good natural protection from the higher energy radiation found in space (and giant orbital super weapons). A planet with a high particulate content (dust or smoke) will scatter visible light lasers, making them less effective.

Can you even make orbital weapons based on radio waves? Perhaps not but radio does make for a great sensor suite so you'll still want to take some precautions against them.

Earth's Absorption Specturm

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  • $\begingroup$ Small unguided munitions can be incredibly precise through atmosphere if they're going fast enough. However by that point they're either going to burn up (more accurately: ablate down), explode partway, or hit the ground with enough mv^2 to make precision a somewhat moot point... $\endgroup$ – Joe Bloggs Oct 1 '15 at 9:14
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Your best bet is probably not a planet at all but a large, molten-cored moon of a super-jovian, like a bigger version of Io. The gravity of the jovian would make it incredibly difficult to hit you with a large asteroid, and you'll be wrapped in flux and plasma tubes which will be hostile to any high technology in orbit (the plasma would also make laser and particle beams less effective).

The downside to this is that the surface is likely to be inimical to life, so you'd probably have to build the bulk of your colony underground. That would add greatly to your defence as well, but on the other hand you could do that anywhere.

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Planetary Composition

Just as Frostfyre says, one of the key natural defenses of the planet would have to be a colossal underground cave/tunnel system. It must also have to be really deep underground so that little pebbles measuring ~10 - 20 km don't bother your much (other than completely blocking out your exits and choking you inside, that is).

Another defense I could assume would be a somewhat dense atmosphere which also happens to be slightly caustic. A casual mixture of sulphur dioxide (20%), nitrogen dioxide (10%), hydrogen chloride (20%), phosphorus V oxide (10%) and oxygen (40%) would be handy (I hope Venus cringes in jealousy!). If the atmospheric pressure is ~5 times the atmospheric pressure on earth, it would be further helpful.

The crust should be largely composed of titanium. Large mountains made of diamond would also be a big help.

How Does It Help Naturally?

You are living miles deep under the crust so asteroid impacts don't bother you much.

Due to slightly corrosive and mildly dense atmosphere, most of the asteroids (anything lesser than half a mile) would be obliterated to ash within the atmosphere, before reaching the ground.

Due to the same aspects of the atmosphere, any bomb or missile would glow red in the atmosphere and burst tens of miles up in mid-air, far before reaching the surface.

Even high energy lasers would not be able to reach or penetrate the surface, thanks to the extremely dense atmosphere and the incredibly hard crust.

A Slight Problem

Forget any damage from anything dropped, shot or launched outside the planet's atmosphere. It will not damage you at all (unless they all gang up on you and throw a moon-sized body on you).

However, you might have a little trouble landing on this planet and safely making to the underground tunnel system. Good luck.

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If we are only considering defenses native to the planet rather than technological ones introduced by the colonizing species or environmental ones influenced by the same, then the only really viable option is to build underground.

Orbital bombardment (drop a chunk of rock/metal) is pretty much the ultimate weapon, short of the Apex Absolute Annihilator of Handwavium (AAAH) Bomb (copyright pending). Getting hit by a large asteroid is basically the same thing, just an undirected version. We can see the consequences of this ourselves, as it happened on Earth some 65 million years ago (Chixculub).

The best protection against orbital bombardment is to build deep underground. You could build underwater, but water (any liquid, really) is much less dense than rock and less of a defense against most meaningful weapons.

You want your target world to be comprised of, or have large quantities of, a structurally sound material, something that won't shatter/buckle/break/bend under repeated impact, is resistant to shock waves, and is ecologically feasible (e.g., high heat tolerance to insulate the colony).

The best choice would probably be some form of carbon, like carbon fiber, but that kind of world would almost certainly be the result of planetary engineering.

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A very dense atmosphere

The Earth's atmosphere will burn up small meteors. A denser one will burn up even larger ones. The faster they hit atmo, the more they will burn up, so an attacker would have to drop either a very large rock (making their attacks less feasible) or do it slower than they'd like (giving more time to do something about).

At a high enough density, any manufactured weapons will simply be crushed.

This will also decrease the terminal velocity of the object reducing it's impact energy. Since impact energy is the square of the velocity, this can be a big change.

A very dense atmosphere also makes floating easier. Instead of living on the surface, live in floating cities! Then it doesn't matter how many bombs they drop on the surface.

A very turbulent atmosphere

This increases the complexity of flying small craft, such as missiles, in atmo; they'll get blown around.

A very opaque atmosphere

The water and carbon dioxide in the Earth's atmosphere absorb many wavelengths, throw more gasses and dust into the mix and you'll absorb even more. This makes observation difficult and will reduce the power of energy weapons.

A very strong magnetic field and a radiation belt

The Earth has a belt of energetic charged particles about 1,000 km to 60,000 km above the surface called the Van Allen belt. It's caused by Earth's strong magnetic field trapping things like cosmic rays and the solar wind. The Van Allen belt is strong enough to mess with sensors and damage unshielded electronics. Our hypothetical planet would have a much stronger belt able to fry even shielded electronics.

While the belt has huge gaps at the poles, so it would only restrict an attacker's plausible approaches, or require them to add excessive weight for shielding. Sort of like an orbital minefield. And because it's a belt around the equator, any orbiting object will have to pass through it, or orbit at a height to avoid it.

Receive a lot of solar radiation

The dense, opaque atmosphere will protect or colonists, but the poor saps attacking from orbit will be receiving the full solar bombardment. Not just heat, but radiation as well. They will either have to deploy bulky and vulnerable sun shields, or always remain in shadow on the night side.

Orbit a very active star

A coronal mass ejection aimed at our planet is bad news for the attacking fleet in orbit. They'll either have to high tail it out of there, or hide in the planet's shadow. Meanwhile, the colonists are protected by the atmosphere and magnetic field.

In conclusion... visit beautiful Venus! (BYO magnetic field)

With it's ultra dense and opaque atmosphere full of corrosive gasses, Venus is extraordinarily hostile to everything. What about our poor colonists? They'll be safe in their floating cities above the more toxic gases and crushing depths. Bomb the surface all you want, it's hot enough to melt lead down there with enough pressure to crush a nuclear submarine!

Venus has a very weak magnetic field, so our colonists would have to find a Venus-like planet with a strong magnetic field.

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If underground planets are out of question, you're out of defence against anything really massive falling from the sky. Underwater habitats will be just smashed by the shockwave in the water. Suggesting a planet with an ocean under a thick ice core is just cheeky.

That aside, there's nothing else natural on a planet that will semi-reliably kill a hardened, shielded single-purpose device or munition, but will not kill a multi-purpose, meaning-of-life-seeking colonist. You can't beat earth in this regard, a shovel is a soldier's best friend.

Your best bet is security through obscurity.

An ideal planet would be the one with multiple obscuring layers:

  • a thick cloud layer at 10-12 km (assuming Earth-like atmosphere density) to obscure your planes from satellites if you need air transport
  • another thick cloud layer at 2-3 km to make aerial recons largely fruitless
  • high trees with lush canopies forming a third layer covering what you have on the ground

Also if the enemy is using lasers and you know their preferred wavelengths, choosing a planet with atmosphere with electromagnetic transparency windows in wrong places won't hurt. However, choosing an atmosphere with abundance of small metallic particles to fool radars will hurt your engines and hurt a lot.

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You could hide your colony within a star system that is hidden behind a black hole. (from the perspective of your enemies.) Your radio signals, etc. would not be detected because they would pass into the event horizon of the black hole. Nobody would even know that there was a star there unless they bothered to travel out to a new frame of reference to look.

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  • $\begingroup$ But what if the colony want to communicate or travel to systems on the other side of the background? The black hold would forbid them to do it directly, wouldn't it? $\endgroup$ – Aracthor Oct 1 '15 at 5:03
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    $\begingroup$ That's correct. I just figured the best defence would be avoid a fight at least until you have finished building up your arsenal. $\endgroup$ – Lorry Laurence mcLarry Oct 1 '15 at 5:08
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If one of these civilizations want to colonize a new planet, and want to choose it based only on its natural defenses against future attacks, what kind of planet would they look for?

A very inhospitable, resource-poor one.

This assumes that the civilization has the means of making it hospitable for itself and that such means are not, and will not for the foreseeable future, be available to the other civilizations (for, at that point, the planet becomes hospitable, therefore attractive, therefore likely to be attacked).

The civilization in question could have modified itself in order to better exploit inhospitable planets (e.g. they don't need a natural environment, open air, sunlight etc. and are happy living in artificial shells deep inside the molten mantle of a tidally locked low-density airless worldlet bombarded by the radiation of its sun).

They could be "uploaded" beings, and live in a virtual worlds which are physically located wherever convenient: this would explain why they choose planets based on their natural defenses instead of, say, habitability. They would then rely on the abundance of lifeless worlds to ensure they'd not be disturbed by encroaching neighbours.

A sandy world (think Arrakis) or a mostly water world would provide a ready natural defense against most of the weapons you quote.

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