Working out a planet's attributes

[Gumboot]

Back to the low-density world problem, I really don't see anything wrong with using a planet of unusually low density. It's plausible that you could have a planet with enough silicon and calcium to stay solid (ie, not a gas giant) but very low amounts of iron and heavier metals (thus low density). As of 2012, our knowledge of planetary formation and planetary cores is incomplete. So who's to say that all solid planets have a density similar to Mercury/Venus/Earth/Mars? That's a sample size of 4.

I don't think the issue is whether a solid planet can exist at that density (pluto is substantially less than 1/2 earth's density, but still solid), but whether a planet can exist at that density which would support life.

One thing to bear in mind is the lower density and greater size would collectively make plate tectonics and volcanism far more active on the planet which would create major issues for anything attempting to live on it.

 

[undormant]

I don't think the issue is whether a solid planet can exist at that density (pluto is substantially less than 1/2 earth's density, but still solid), but whether a planet can exist at that density which would support life.

One thing to bear in mind is the lower density and greater size would collectively make plate tectonics and volcanism far more active on the planet which would create major issues for anything attempting to live on it.

How much more active to be an issue, there would still be large, massive land masses that were not even on a fault line or even near a volcano. Much of Earth is uninhabitable for the most part for example and we have settled not only on the parts that are inhabitable but on fault lines and next to volcanoes

R

 

[Gumboot]

I did hear a suggestion that a decent atmosphere can protect almost as well against solar wind/radiation as a magnetic field can.

However, can we assume the magnetic field from Purple will be half of Earth's magnetic field? Still possibly enough to protect the population?

R

Well it's fiction, we can assume anything we want, but it's not scientifically likely.

Atmosphere doesn't protect against solar wind, rather the magnetic field protects the atmosphere. There are elements of the atmosphere that can protect against radiation (such as the ozone) but these need to in turn be protected by the magnetic field.

A prime example is Mars, which has almost totally lost its atmosphere since its magnetic field turned off.

 

[Piousflea]

Ok this is very useful. So how about Moon1 is 10,000 km, Moon2 is 12,000 km, Moon3 is 14,000 km, and Moon4 is 16,000km? That way looking at them in the sky they will appear to be the same size as they get larger further away from the planet.

The problem with the arrangement described in your OP is that it doesn't make any physical sense. Planet Purple is 2x Earth radius with 1x Earth gravity and 0.5x Earth density. Each of its moons is presumably Luna sized (0.25x Earth radius) but with near-Earth gravity. They would need to have 4x Earth density, and each moon would have ~1.6% of Purple's mass. Earth's moon has 1.2% Earth masses and we don't have any other moons. A moon of 1.6% the planet's mass is probably large enough to destabilize the orbits of the other moons.

The 8-fold difference in density between planet and moons is silly enough. The excessively large mass of the moons is an even bigger problem. You would basically have to invoke an alien race to create and maintain Planet Purple and its four moons. It would not be naturally possible.

Also, in terms of lack of iron etc, it depends what you mean by a lack of these materials, from a story point of view it could be a good lack of resource type story wars etc. As long as there was a large mineral deposit near a population then they could consider themselves to be no worse off than we are.

Depending on just how rare iron was, things could be similar to Earth but with iron as an expensive "precious metal", or iron could be so rare that it dramatically changes the planet's biology. Earth-bound bacteria already attempt to steal iron from each other with chelators. In an ultra-iron-poor world, native lifeforms could evolve entire biochemical systems to hoard iron and prevent other organisms from acquiring it. Their biology would be radically different from Earth biology - imagine animals with non-iron based blood. The levels of iron present in Earth plants and animals might be so high that many native lifeforms find it toxic.

The lack of magnetic field in an iron-poor planet would also be a big deal. It would require a relatively small and cold star (low radiation output), as solar radiation would otherwise destroy the atmosphere of a non-magnetic planet. Even with a red dwarf, the radiation exposure on the surface might still be significant. Native lifeforms would have to be highly radiation-tolerant.

About Plate tectonics - I honestly don't know how much of a differnece there would be on an iron-poor planet. Plate tectonics are caused by the interaction between the crust and the mantle. Both of them have much lower heavy metal content than the planetary core (on Earth they are mostly silicon, calcium and oxygen while Earth's core is iron), and plate tectonics are way superficial to the core anyways.

 

[Ursa major]

If one had a colonised planet twice the surface area of the Earth, but with approximately the same gravity as here, would such a world still be capable of producing the necessary magnetic field (all other things being equal) to allow the planet to support life in the way the Earth does?

 

[Piousflea]

If one had a colonised planet twice the surface area of the Earth, but with approximately the same gravity as here

Twice the habitable surface area of Earth is pretty easy honestly... it could be virtually the same size as Earth, but with more land and less water, or 22% larger and 18% less dense with a similar proportion of water. The composition of the planet wouldn't be that different from Earth.

 

[RJM Corbet]

Twice the habitable surface area of Earth is pretty easy honestly... it could be virtually the same size as Earth, but with more land and less water, or 22% larger and 18% less dense with a similar proportion of water. The composition of the planet wouldn't be that different from Earth.

Oh? The crust alone? What about the mantle and the nickel/iron core?

 

[Ursa major]

It's twice the surface area of the planet as a whole, Piousflea, not necessarily twice the land area. The difference being what the place looks like to someone on its surface, e.g. they can see farther and can take in more in a glance (give or take the presence of view blocks, like buildings and hills).

 

[Piousflea]

It's twice the surface area of the planet as a whole, Piousflea, not necessarily twice the land area. The difference being what the place looks like to someone on its surface, e.g. they can see farther and can take in more in a glance (give or take the presence of view blocks, like buildings and hills).

On Earth, you never really percieve the curvature of the planet unless you are either in the ocean or flying at a very high altitude. In any other situation the non-flatness of the ground vastly outweighs the "true" curvature of the horizon. A few tens of feet of elevation over several miles is all it takes.

There are much better ways to make a place look different "to someone on its surface". Just being in a large valley produces the optical / psychological effect of "being able to see further" more effectively. Ask anyone who has ever visited Grand Teton National Park.

 

[Gumboot]

On Earth, you never really percieve the curvature of the planet unless you are either in the ocean or flying at a very high altitude. In any other situation the non-flatness of the ground vastly outweighs the "true" curvature of the horizon. A few tens of feet of elevation over several miles is all it takes.

There are much better ways to make a place look different "to someone on its surface". Just being in a large valley produces the optical / psychological effect of "being able to see further" more effectively. Ask anyone who has ever visited Grand Teton National Park.

Atmosphere makes a big difference too. On a really settled dry day without much dust, water and material being blown about in the sky, you can see much further (or at least, you can see far off things much more clearly which gives the impression that you're seeing further).

 

[Ursa major]

The effect on the viewer is as much psychological as actual, in that they know the planet is bigger and react to what they see in that knowledge.


However, my original question remains: would a planet with twice the surface area, but much the same gravity, as the Earth have a much altered magnetic field? (For reasons I won't go into, I don't really care whether the magnetic field becomes ineffective or not, but I do need to know which it is.)

 

[Gumboot]

The effect on the viewer is as much psychological as actual, in that they know the planet is bigger and react to what they see in that knowledge.


However, my original question remains: would a planet with twice the surface area, but much the same gravity, as the Earth have a much altered magnetic field? (For reasons I won't go into, I don't really care whether the magnetic field becomes ineffective or not, but I do need to know which it is.)

I don't think it would change enormously. Twice the surface area would mean 41% greater radius and 280% greater volume.

 

[Piousflea]

The effect on the viewer is as much psychological as actual, in that they know the planet is bigger and react to what they see in that knowledge.


However, my original question remains: would a planet with twice the surface area, but much the same gravity, as the Earth have a much altered magnetic field? (For reasons I won't go into, I don't really care whether the magnetic field becomes ineffective or not, but I do need to know which it is.)

Impossible to say, but it is more plausible to have a decent magnetic field on a planet with 82% the density of Earth.

 

[Ursa major]

Seeming not to be flying in the face of physics in too obvious a way is all that is required, really.

 

[Karn Maeshalanadae]

Seeming not to be flying in the face of physics in too obvious a way is all that is required, really.

Exactly. You simply don't want to have a planet twice the size of Jupiter with a gravitational pull weaker than our Moon, pretty much, as an example.

 

[RJM Corbet]

... However, my original question remains: would a planet with twice the surface area, but much the same gravity, as the Earth have a much altered magnetic field? (For reasons I won't go into, I don't really care whether the magnetic field becomes ineffective or not, but I do need to know which it is.)

The magnetism is generated by the spin of iron core against the mantle, so I don't know how the maths would work proportionately, but bigger core = greater magnetism?

Conversely, if the core stays the same mass, but the increase is in the mantle and the crust (ie: they're less dense) then the magnetism should stay the same.

Gravity will be a different equation, relating to mass alone (= weight for the purpose) regardless of surface area?

 

[Ursa major]

The Earth's magnetic field is generated by the liquid (outer) part of the core (which is also known as the geodynamo, I think).


As for the quantitative stuff, Wiki sums up the problem thus:

The equations for the geodynamo are enormously difficult to solve, and the realism of the solutions is limited mainly by computer power.

 

[Vertigo]

Gravity will be a different equation, relating to mass alone (= weight for the purpose) regardless of surface area?

No not related to mass alone. Mass and radius from the centre.

 

[allmywires]

How much more active to be an issue, there would still be large, massive land masses that were not even on a fault line or even near a volcano. Much of Earth is uninhabitable for the most part for example and we have settled not only on the parts that are inhabitable but on fault lines and next to volcanoes

Well, plate tectonics is really a product of heat flow, and in a planet that large there would be a huge mantle (assuming crustal thickness remains the same, which I doubt, but that's another point). So there would be a much higher build-up of heat inside the planet, which would lead to more hotspots (volcanoes) and spreading ridges, which make oceans. Due to the high heat flow, the processes would be much faster than they are on Earth (as a reference point, one supercontinent cycle is about 300 million years).

 

[RJM Corbet]

Thanks Vertigo yes that's true, of course. But I just meant that the planet's magnetic field would have no relation to its gravity?

Elipticals?

It appears that the eliptical is caused by changing velocity in the orbiting body, like a rubber band stretching to its limit, then snapping back again.

The velocity of a comet, say, increases as it rushes towards the sun then, luckily for the comet, just misses hitting the sun, swings around it, and starts its outward journey again, trying to break free but slowing down as the forces of gravity and velocity compensate, until gravity wins, the rubber band reaches the limit of its stretch, and the comet is pulled accellerating back again towards the sun.

The pull of gravity upon the comet, as you say, being more powerful in (inverse) proportion to the comet's closeness to the sun.

So really, the eliptical is decided by an orbiting body's original velocity, tangent etc, when it first becomes captured by the gravity of the larger body?