Working out a planet's attributes

[undormant]

Hi there,

I am new to this forum, there used to be a really good one called Newmars and we used to talk about terraforming. It was a bit technical for me but I used to like it because I could ask for technical details about certain things.

I am trying to write a novel and am having trouble working out the planets attributes. If anyone can tell me if I have any details wrong on a technical level can they please help me out and let me know?

We shall call this planet 'purple' for sake of argument. It is twice the size of Earth with roughly the same mass, therefore roughly the same gravity as Earth (I hope my logic is correct there).

It has a 30 hour day (an hour being the same length as one of our hours by the way) and takes 500 Earth days to go around it's sun (or 400 native days if I have worked it out correctly).

It has very similar amount of ocean percentage-wise and a very similar axial tilt.

It either has a slower rotation around it's star or it's Sun is warmer than ours, either way temperatures are comparable to Earth and 'Purple' is in the 'Goldie locks zone.'

All of that kind of makes sense to me. It's when adding moons it gets a bit more complicated. Purple has 4 moons, all twice the size of our moon, they have gravity of about the same as Earth, therefore they have an Earth similar mass. As a sidenote they are all like Earth in terms of oceans and atmosphere etc. I guess the main question is: Can Purple hold 4 moons of this description in orbit without much trouble? Gravitational bodies pull on each other and if Purple has a mass of 1 and these moons have a pull of 1 each... do we stack the pull of the moons to 4?

I need the gravity on all bodies to be Earth-like, but I need the physical size of these bodies to be as stated, is it mathematically possible?

R

 

[Gumboot]

Put simply, no a planet cannot have four moons the same mass as itself, or even one for that matter. A moon is a satellite, which means it is in secondary orbit around the parent body, and is of lesser mass.

If the bodies are of equal mass they might in theory form a binary planetary system where they both rotate around a point exactly between the two bodies, but neither would be a moon; both would be planets. So in theory this might work with five bodies, but would be incredibly unstable I would imagine. Bear in mind in such a scenario it would play havoc with the planet's rotation, and those bodies would not appear in the sky like moons. My guess is you'd end up with orbital lock whereby the same side of of the planets would always face each other (like with the moon and earth) so the other planets would always be in the same place.

There are some really good orbital simulators where you can create your own planetary systems with their own statistics, and the simulator will run their orbits so you can check if they're stable (that's how I determined the configuration for my world which has three moons).

Bear in mind that a planet twice the size of earth with the same mass would have half the average density which would substantially effect a whole host of things from plate tectonics to building construction, to the magnetic field, to volcanism.

To be honest, the scientific reality is that there's a quite narrow band of characteristics within which a planet can support life. If you go much outside earth's parameters the environment becomes hostile to life.

I would be exceptionally wary of creating a life-supporting planet (at least supporting life in a form we understand) with characteristics that varied much from earth.

To give you an example, Venus is 85% of earth's size, but is too small to allow for convection in the mantle, thus there is no plate tectonics, no volcanism, etc, which are vital for life.

A planet 200% of earth's size would have substantially greater tectonic activity which would make the surface environment too unstable to support life.

Likewise, a planet 200% of earth's size, but at the same mass, would have a substantially smaller iron core, and substantially slower rotation rate (with the five-body system it might rotate slower still) which would result in a substantially weaker magnetic field, making the surface and atmosphere vulnerable to solar winds and radiation which is harmful to life.

 

[Gumboot]

Also, I just had a thought, the force of gravity relates not only to mass but distance from the centre of gravity, therefore a planet twice earth's size but with the same mass would have weaker gravity as you would be further from the centre of the mass when standing on the surface.

Also, when you say "double the size" do you mean double the volume or double the circumference? Because this would make an enormous difference. A planet double the circumference of earth would be 8 times the volume which means for the same mass it would be 1/8 as dense.

 

[Galacticdefender]

There are some really good orbital simulators where you can create your own planetary systems with their own statistics, and the simulator will run their orbits so you can check if they're stable (that's how I determined the configuration for my world which has three moons).

Anyone know where I can find one of these simulators? That sounds quite useful.

 

[Galacticdefender]

To give you an example, Venus is 85% of earth's size, but is too small to allow for convection in the mantle, thus there is no plate tectonics, no volcanism, etc, which are vital for life.

Well, actually: http://en.wikipedia.org/wiki/Volcanism_on_Venus

There is (or was) quite a bit of volcanism on Venus, so size seems not to be a factor. Especially when looking at jupiter's moon Io. (Though that volcanism is caused by jupiter pulling on it)

Mars has several extinct volcanoes as well, and it is significantly smaller than Venus.


Also why are volume and circumference different? My tired brain is having a bit of trouble comprehending this at the moment.

 

[Warren_Paul]

Mars has several extinct volcanoes as well, and it is significantly smaller than Venus.

Not sure if it matters (I don't have a clue about these things), but Mars actually has the largest known volcano in the whole system. Olympus Mons. So at least at one point it had some very serious volcanism going on.

 

[Galacticdefender]

I have an orbital elevator set atop Olympus Mons (the volcano Paul just mentioned) in my Space Opera universe lol.

 

[Karn Maeshalanadae]

Or skip terraforming altogether. It's not inconceivable that in the entire universe there aren't other planets like Earth, so go with that.


Or how about you go down a different evolutionary road? Scientists on Earth say water is vital for life. Yes it is-ON EARTH.


That's not to say that there could be a world where water would be a deadly toxin to its races, much like chlorine would be to us. It could be there are races in the universe that would have their eyes burned out by our degree of sunlight, or races that would die of hyperthermia on Antarctica, or hypothermia on our equator. If you set it up well enough, almost anything could be believed.

 

[allmywires]

Not sure if it matters (I don't have a clue about these things), but Mars actually has the largest known volcano in the whole system. Olympus Mons. So at least at one point it had some very serious volcanism going on.

Venus and Mars have abundant volcanism because they have a mantle, like out planet, which needs to release heat somehow - that's what volcanoes are essentially, only vents. The reason Olympus Mons is so big is not indicative of the amount of volcanism on Mars, it's just because it's so old and so much lava has erupted. Think of Earth: our volcanoes occur at subduction zones and hotspots. Moving tectonic plates causes our volcanoes to go extinct, or become completely destroyed. On Mars it is completely static, same with Venus.

It's not as simple as making the planet twice as big as Earth. You said that giving it twice the mass would make the gravity about the same - not quite right, as the force of gravity is an inverse square rule, so is proportional to 1 over the radius squared. This would mean the planet would have to be either smaller with less mass (less dense) or larger with more mass (denser). This again leads to problems with geochemistry in the mantle and the size of the core but I won't go into that here. There is a reason Earth has life and no other planets around us do: it's a very specific set of events that have led to it. Venus once had plate tectonics and even oceans but it was unsustainable and led to a 'runaway greenhouse event'. These days the atmosphere is fill with sulphur dioxide and rains of sulphuric acid.

As for the moons theory, well, if you want the maths I'd go here: http://en.wikipedia.org/wiki/N-body_problem However after doing an exam on differential equations in January I'm afraid even looking at those sums is giving me PTSD-type flashbacks....

(Hope I could help, just trying to get the science a bit clearer! )

 

[Gumboot]

Anyone know where I can find one of these simulators? That sounds quite useful.

The simulator I used is called "AstroGrav".

 

[Gumboot]

Blegh, I had a brain fart. Smaller planets can still have volcanism, but they don't plate tectonics, that's the difference. Plate tectonics are vital for forming geography, in particular divisions of land and water, and mountain ranges (both vital for life).

 

[Gumboot]

Also why are volume and circumference different? My tired brain is having a bit of trouble comprehending this at the moment.

Man, I am having a bad day. Not circumference, diameter, that's what I meant. Although the same applies for circumference.

If you double the diameter both the surface area and the volume increase substantially more than double (but not the same amount).

Imagine a cube that's 1x1x1. The diameter is 1 (obviously a cube doesn't actually have a "diameter" but the distance through a cube is the same as the length of one side), the area is 6^2, and the volume is 1^3.

Now double the diameter so it's 2x2x2. The area is now 24^2 (four the original area) and the volume is 8^3 (eight times the original volume).

As you can see, when you talk of something being "twice the size" it makes an enormous difference whether you mean "twice the volume", "twice the area" or "twice the dimensions".

 

[allmywires]

Plate tectonics are vital for forming geography, in particular divisions of land and water, and mountain ranges (both vital for life).

Absolutely right. Volcanoes and spreading ridges provide nutrients from the exuded mantle, and shallow seas caused by continental rifting provided the vital step-up from life in the seas to life on land. Not to say you can't be creative with it, but the Earth model is pretty much the only way to go.

 

[Gumboot]

Absolutely right. Volcanoes and spreading ridges provide nutrients from the exuded mantle, and shallow seas caused by continental rifting provided the vital step-up from life in the seas to life on land. Not to say you can't be creative with it, but the Earth model is pretty much the only way to go.

Another thing I thought of is that mountain ranges compress weather systems and lift clouds, causing the vapour to condense back into water which then rains on the earth and forms rivers, establishing the water cycle.

Without mountain ranges you wouldn't get localised concentrations of water vapour dense enough to turn into precipitation, so while the planet might have plenty of water, it would mostly be in vapour in the atmosphere, and not concentrated on land where life can access it.

Exactly as you say, the Earth model is basically the only way to go - you can deviate slightly in any direction but not by much. A planet that is too small is too stable to support life. Too big and it's too unstable to support life.

 

[allmywires]

Another thing I thought of is that mountain ranges compress weather systems and lift clouds, causing the vapour to condense back into water which then rains on the earth and forms rivers, establishing the water cycle.

A great point that I hadn't thought of, actually. Also global temperature, which controls the amount of water locked up in ice sheets, glaciers, et cetera. Though not too much, or else you get the rather comically named 'Snowball Earth' scenario.

Maybe discussing all of this is taking the fun out of world building...accuracy is good, but I guess some of our very salient points may be ignored

 

[Gumboot]

A great point that I hadn't thought of, actually. Also global temperature, which controls the amount of water locked up in ice sheets, glaciers, et cetera. Though not too much, or else you get the rather comically named 'Snowball Earth' scenario.

Maybe discussing all of this is taking the fun out of world building...accuracy is good, but I guess some of our very salient points may be ignored

Absolutely. Ultimately, what we're writing is fiction, and it's up to every author to decide for themselves how far down the realism rabbit hole they want to go in their own project.

 

[Vertigo]

I have an orbital elevator set atop Olympus Mons (the volcano Paul just mentioned) in my Space Opera universe lol.

Minor point but don't space elevators have to be on the planet's equator as that is the only place a stable geostationary orbit can be set up. the top end of a space elevator must be in geostationary orbit or powered.

Another point is that multiple massive moons, let's say similar to our own moon's mass (rather than the planet's mass as already discussed) would also probably not be conducive to life. Assuming they are all in different orbits the varying tidal forces would be complex to say the least and in the case of alignment of the moons probably catastrophic.

Don't forget that yet another feature of our planet that makes it conducive to life is the size and orbit of our moon. That stabilises Earth and in particular stabilises axial wobble.

 

[undormant]

First of all, thank you for all your replies, I wasn't entirely clear what I meant by twice the size I think, I meant if Earth has a Circumference (equatorial) of 40,000 km then Purple would have 80,000 km.

It also seems a got my mass thinking wrong, but I was assuming Purple is made up of much lighter materials, which also has an effect on mass right? I mean a soccer ball and a bowling ball are the same size, but have different mass because they are made up of different materials right?

So by that thinking could Purple has the same mass as Earth if made up of lighter materials? If that were the case in then would it not have the same gravity?

Thank you for your patience.

R

 

[allmywires]

I think it's quite difficult to think in terms of circumference. As a starting point, Earth's radius is about 6000km, using 2*pi*r that gives a circumference of 37000km, approximately 40000km, as you said. So using that in reverse you get the radius of Purple to be about 12000km, therefore having the same effect (ie double the radius, and 4 times the actual size of Earth.)

May be getting too technical here but to compensate for the larger radius (so following 1/r^2, a weaker gravitational pull) the planet would need to be denser, not lighter, and by a factor of 4, not 2.. This would probably mean a larger core (which is assimilated from dense elements like iron and magnesium) which would also affect tectonism.

Really, it's only consequential if you're planning on going into great detail with this in your work - otherwise I'd say just go with it and forget the technical details, though the 4 moons might be the biggest thing to try and rationalise. The planet and what it's made of - well, probably most people would just accept it would work. And as Vertigo says, having moons have life on is another whole can of worms.

So to conclude, essentially if Purple were twice the size yet four times as dense then yes, in theory, it would have the same gravity.

 

[undormant]

I think it's quite difficult to think in terms of circumference. As a starting point, Earth's radius is about 6000km, using 2*pi*r that gives a circumference of 37000km, approximately 40000km, as you said. So using that in reverse you get the radius of Purple to be about 12000km, therefore having the same effect (ie double the radius, and 4 times the actual size of Earth.)

May be getting too technical here but to compensate for the larger radius (so following 1/r^2, a weaker gravitational pull) the planet would need to be denser, not lighter, and by a factor of 4, not 2.. This would probably mean a larger core (which is assimilated from dense elements like iron and magnesium) which would also affect tectonism.

Really, it's only consequential if you're planning on going into great detail with this in your work - otherwise I'd say just go with it and forget the technical details, though the 4 moons might be the biggest thing to try and rationalise. The planet and what it's made of - well, probably most people would just accept it would work. And as Vertigo says, having moons have life on is another whole can of worms.

So to conclude, essentially if Purple were twice the size yet four times as dense then yes, in theory, it would have the same gravity.

You're going to have to excuse my ignorance here, imagine Data in Star Trek has just said something complicated and then Jordi La Forge or Troy says "oh, its like when..." and breaks it down into easy to understand sound bites lol.

So, what you are saying is, in terms of density, that Purple holds more heavy materials. It is twice as big as Earth but holds four times as much heavy materials? This would give an Earth-like gravity on the surface right? Out of interest, in this instance if Earth is mass = 1 what does Purple = ? Not sure why I'm so hung up on mass.

Ok so I had the moons as being Circumference (equatorial) 20,000 km and again with Earth gravity on the surface. Our Moon is about 10,000 km so willing to go down to that if it makes it work. Willing to go to 0.8 Earth gravity maybe.

The moons would be tidal locked to Purple like our system, I would want the moons to line up so they appear slightly behind each other at all times, but that would require moon 2 to be moving faster than moon 1, and moon 3 to be moving faster than moon 2 etc not sure that's natural.

R