Working out a planet's attributes

[James Coote]

Interestingly, there was an article on the BBC website recently talking about Titan, and how it rained, but only every 5 or 10 years or so

The heating of a planet (or moon) core can be caused by radioactive decay of elements from when the planet is formed, it can be caused by tidal forces (i.e. the gravity of other moons and the planet it is orbiting, like on Io) stretching and compressing it, and I think it can also be caused by heat pressure of gravity, though I think in such cases, like Jupiter, it just results in a solid core rather than a liquid rock mantle.

It may also be at some point, there was a planetary collision, and the planet reformed from debris of the collision, meaning it could still be cooling down from that long after it would have otherwise become solid

Also the volcanism doesn't have to be liquid rock / mantle. Where you get 'ice' planets such as Europa, you get cryo-volcanoes (i.e. big ice geysers) and massive chasms in the ice covering of the planet

I read somewhere that Earth is quite unusual in having a solid metal core that revolves counter to the planet's direction of spin, helping to create a strong magnetic field.

Also, again just a guess, but if a planet had an orbit of 500 days and a mass twice the Earth's and an eccentricity of around 0 (i.e. roughtly circular orbit, rather than an elipse) then it would probably be a different distance from its star than Earth is. You'd have to run the orbital simulator as suggested, but you can cheat by making the star it orbits different (younger, hotter, more massive, less massive, older etc) so that your planet is still in the goldilocks temperature zone

Edit: Here is the wikipedia article on calculating orbits: http://en.wikipedia.org/wiki/Kepler_orbit

 

[undormant]

Interestingly, there was an article on the BBC website recently talking about Titan, and how it rained, but only every 5 or 10 years or so

The heating of a planet (or moon) core can be caused by radioactive decay of elements from when the planet is formed, it can be caused by tidal forces (i.e. the gravity of other moons and the planet it is orbiting, like on Io) stretching and compressing it, and I think it can also be caused by heat pressure of gravity, though I think in such cases, like Jupiter, it just results in a solid core rather than a liquid rock mantle.

It may also be at some point, there was a planetary collision, and the planet reformed from debris of the collision, meaning it could still be cooling down from that long after it would have otherwise become solid

Also the volcanism doesn't have to be liquid rock / mantle. Where you get 'ice' planets such as Europa, you get cryo-volcanoes (i.e. big ice geysers) and massive chasms in the ice covering of the planet

I read somewhere that Earth is quite unusual in having a solid metal core that revolves counter to the planet's direction of spin, helping to create a strong magnetic field.

Also, again just a guess, but if a planet had an orbit of 500 days and a mass twice the Earth's and an eccentricity of around 1 (i.e. roughtly circular orbit, rather than an elipse) then it would probably be a different distance from its star than Earth is. You'd have to run the orbital simulator as suggested, but you can cheat by making the star it orbits different (younger, hotter, more massive, less massive, older etc) so that your planet is still in the goldilocks temperature zone

Yeah I think I'm going to reduce the number of Earth days it takes to go around its sun to 400, which is only slightly longer than ours, putting it right in the correct place.

I have heard the argument that a magnetic field is needed isn't necessarily so, a decent atmosphere can protect life from radiation in the same way a magnetic field can. Apparently.

I've been looking through the internet all day for decent planet simulator only to find really simple ones that don't help me. They don't account for moons either. Can someone suggest a really good simulator?

R

 

[allmywires]

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.)

I'm a wee bit confused now myself but I'll try to collect my thoughts...

Earth's mass = 1, then Purple = 4. This would give our regular acceleration due to gravity, 9.81 m/s.

RE: the moons, what would probably happen is that the moon closest (assuming all moons have equal mass) would move the fastest, as the force on it due to gravity is strongest, giving it a faster angular velocity. I suppose these might lead to alignment, especially if you consider that orbit are more elliptical than circular. They might, for instance, all be aligned at the periapsis of orbit (the point where the elliptical orbit is closest to the earth, so is accelerating the fastest).

Again, sorry if too much jargon!

 

[Galacticdefender]

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".

Or you could keep it simple by simply saying 2 Earth radiuses?

 

[Galacticdefender]

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.

Hah, yes, I thought of your first point, but then I was like "it would be cool to have an orbital elevator on top of Olympus mons. And I don't think they have to be on the equator, but fairly close, and Olympus mons isn't too far off. And really, in the year 4100, they could probably find a different way to keep things in place around a planet.

 

[RJM Corbet]

... 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

'To him who hath, shall more be given. To him who hath little, even that little he hath shall be taken away.'

The law of gravity.

In effect the earth's gravity is trying to devour the moon's, but the distance is right, so it orbits?

 

[Vertigo]

I'm not that hot on my orbital physics but I think if you try to get five moons to orbit together (ie same period of orbit) but at different orbital radii then I suspect it just won't work. And if those moons all had the same mass as the moon (never mind the Earth) and were closely following each other then the tidal forces on Purple would be truly horrendous. I could imagine tides in the order of many tens of metres. Coastal life would be difficult to say the least and trying to set up a harbour would be a nightmare.

RJM I wouldn't actually say that. They are really unequal partners, both orbiting about their comon centre of mass. In the same way we do not actually orbit about the sun but around the common centre of mass. In this case the difference in mass means that that common centre is well inside the sun's radius. This of course makes the sun wobble which is one of the tell tales of for finding exoplanets in other systems. I believe when we get an alignment of all the major planets the common centre of gravity is actually just about outside the sun itself.

 

[RJM Corbet]

Vertigo: sincere question. Eliptical orbits? It confuses my mind. I mean, I suppose a comet is first dragged outward by the greater gravity of some other galactic object, then back into the sun's gravity, etc?

 

[chrispenycate]

Do there have to be five moons? I can do you a Klemperer rosette, but stability is improved with six bodies of equal {no, that's not quite right, balanced. You can have three large and three small} mass. Tides would depend on distance between them but if they're close enough to be seen as moons, rather than a point like Venus, they are going to be interesting, assuming the bodies have maintained sufficient rotation for a reasonable day/night cycle, but not impossible, as relative positions are maintained. Mind you, I can see no possible way in which this situation could come about naturally; perhaps you'd start with a torus of matter rotating retrograde in the preplanetary disk… It's far more likely to be, like Niven's "Fleet of worlds", an artificial construct by an exceedingly technologically sophisticated race.

Now, who are all these people saying "make life impossible"? We have exactly one example, for the time being, of a planet that supports life, and does so in any environment where there is energy and raw material. This would lead us to believe (without any evidence apart from this one case) that once life has got started it will adapt to a very wide range of conditions. Until we have a representative sample of other planets, we can't say more. And if life is dissipated as spores or on cometback, rather than having to spontaneously, all bets are off as regards where it can turn up, and flourish.
And those mountains, with their particular weather patterns? We've seen you can get vulcanism without tectonics; we have no way of guessing whether weather can be generated by land/sea junctions (ever mobile because of those tides) could give convection enough to get clouds and precipitation; weather modelling doesn't even work yet on this planet, where we've been looking into the problem for several tens of thousands of years; how can we make judgement about so far unencountered conditions?

Vertigo: sincere question. Eliptical orbits? It confuses my mind. I mean, I suppose a comet is first dragged outward by the greater gravity of some other galactic object, then back into the sun's gravity, etc]

Actually practically all orbits are ellipses; it's just a question of how far they are off circular. Most of the planets are pretty close, possibly because both they and the sun condensed out of a disk of matter. Pluto, of course, isn't, suggesting that either it was not part of the original construction, and was pottering about in interstellar space before getting captured by the sun, or its giant moon Charon was in that situation, and was captured by the little planet, severely modifying its original orbit.

Comets, similarly. There might be millions of them in roughly circular orbits way outside the warm region the planets inhabit, with a tiny minority getting flung into closer flybyes, or they may be captured wanderers from completely outside the system. Until our telescopes get a lot better we're not going to find out. But their orbits are more or less completely predictable, unless they pass too close to something of planetary mass. Newtonian mechanics handles it nicely; no need for phantom black holes dragging them back out into the cold dark.

 

[Gumboot]

Or you could keep it simple by simply saying 2 Earth radiuses?

When dealing with a sphere diameter and radius change at the same rate.

 

[Gumboot]

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.

This is incorrect. The planet would be more massive, but less dense. A planet with double the radius has eight times the volume, so if you increase the mass by a factor of four it will still be half as dense.

 

[Piousflea]

In response to the OP: A planet twice Earth's radius would need to have four times Earth's mass to have equal gravity. This means it would have half Earth's density. (4x mass / 8x volume = 0.5) This would require an exceptionally metal-poor planet. (with problems such as relative scarcity of iron and lack of magnetic field)

The Newtonian "n-body problem" says that any system with more than 3 bodies orbiting in space is inherently unstable. The instability becomes minimal if one of the bodies is much more massive than the others, such as 8 planets orbiting the Sun. (still, over time instability happens - thus we observe comets getting flung into Jupiter or the Sun) Any system with more than 3 bodies of similar mass is extremely unstable. Even in a Klemperer rosette (multiple evenly spaced planets orbiting at the same speed) each planet requires station-keeping thrusters, otherwise its orbit would eventually destabilize and it would collide with its neighbor. (see Larry Niven's Fleet of Worlds for a really good description of this phenomenon)

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.

 

[RJM Corbet]

... Newtonian mechanics handles it nicely; no need for phantom black holes dragging them back out into the cold dark.

I still don't get it, really. How does the gravity of the sun work on the highly eliptical orbit of a comet? I mean it's obviously more complicated than swinging a bucket of sand round your head on a rope? What are the forces, apart from gravity, creating eliptical orbits, Crispen?

 

[allmywires]

This is incorrect. The planet would be more massive, but less dense. A planet with double the radius has eight times the volume, so if you increase the mass by a factor of four it will still be half as dense.

Ah yes. I was confusing myself. This thread makes my brain hurt...unless any of us are experts on orbiting bodies I don't think there is going to be a clear answer here.

 

[RJM Corbet]

Ah yes. I was confusing myself. This thread makes my brain hurt...unless any of us are experts on orbiting bodies I don't think there is going to be a clear answer here.

Let's wait for Crispen to come back. He's better than wiki ...

 

[undormant]

In response to the OP: A planet twice Earth's radius would need to have four times Earth's mass to have equal gravity. This means it would have half Earth's density. (4x mass / 8x volume = 0.5) This would require an exceptionally metal-poor planet. (with problems such as relative scarcity of iron and lack of magnetic field)

The Newtonian "n-body problem" says that any system with more than 3 bodies orbiting in space is inherently unstable. The instability becomes minimal if one of the bodies is much more massive than the others, such as 8 planets orbiting the Sun. (still, over time instability happens - thus we observe comets getting flung into Jupiter or the Sun) Any system with more than 3 bodies of similar mass is extremely unstable. Even in a Klemperer rosette (multiple evenly spaced planets orbiting at the same speed) each planet requires station-keeping thrusters, otherwise its orbit would eventually destabilize and it would collide with its neighbor. (see Larry Niven's Fleet of Worlds for a really good description of this phenomenon)

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.

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.

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.

Years ago someone told me a formulae for working out whether a planet could hold a moon or not, I wish I could remember it now!

R

 

[Piousflea]

I still don't get it, really. How does the gravity of the sun work on the highly eliptical orbit of a comet? I mean it's obviously more complicated than swinging a bucket of sand round your head on a rope? What are the forces, apart from gravity, creating eliptical orbits, Crispen?

All orbits are elliptical, there's no such thing as a perfectly circular orbit. Gravity is not a rope, it does not have a length, and there is nothing stopping celestial bodies from changing distance with respect to each other.

A comet, asteroid or planet orbiting a star will have a perigee (point of closest approach) and an apogee (point of furthest distance). A planet like Earth happens to have a very nearly circular orbit. Other celestial bodies have a highly elliptical (eccentric) orbit.

The relationship between orbital period, stellar mass and average distance from star to planet is complicated to calculate. For a planet to have reasonably similar climate to Earth, if its star is Sun-like it will be at a similar distance and similar orbital period. If the star is much smaller than the sun then the planet will have to be closer, with a shorter year. If the star is much larger than the sun then the planet will be further away with a longer year. However, very large and hot stars will have problems with radiation exposure and stellar instability.

You can also increase distance / length of year by having a greenhouse-effect planet. This implies a very high carbon dixoide content, which implies toxicity to humans and native lifeform gigantism. ("Avatar" and the Na'vi actually did a good job with this one)

 

[Gumboot]

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.

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.

Years ago someone told me a formulae for working out whether a planet could hold a moon or not, I wish I could remember it now!

R

I think the issues relate more to "not enough to allow life to survive".

A larger planet spins more slowly, and a planet with less metal (relative to volume) has a smaller metal core.

Both of these directly dictate the strength of the planet's magnetic field, which is essential for two purposes:

1) Protecting the surface from harmful radiation
2) Protecting the atmosphere from severe disruption due to solar winds etc.

Without a sufficiently strong magnetic field, the planet can't support life.

 

[RJM Corbet]

All orbits are elliptical, there's no such thing as a perfectly circular orbit. Gravity is not a rope, it does not have a length, and there is nothing stopping celestial bodies from changing distance with respect to each other.

A comet, asteroid or planet orbiting a star will have a perigee (point of closest approach) and an apogee (point of furthest distance). A planet like Earth happens to have a very nearly circular orbit. Other celestial bodies have a highly elliptical (eccentric) orbit.

The relationship between orbital period, stellar mass and average distance from star to planet is complicated to calculate. For a planet to have

All this I know. But if a lesser body orbits (falls) around the the gravity of a greater body how do you explain highly eliptical orbits? Is it the original escape velocity, constantly dragged back by gravity? There must be other forces than the gravity of the central body acting upon an eliptically orbiting body? Well, there are, it happens, it's a fact, but I don't get the expanation?

 

[undormant]

I think the issues relate more to "not enough to allow life to survive".

A larger planet spins more slowly, and a planet with less metal (relative to volume) has a smaller metal core.

Both of these directly dictate the strength of the planet's magnetic field, which is essential for two purposes:

1) Protecting the surface from harmful radiation
2) Protecting the atmosphere from severe disruption due to solar winds etc.

Without a sufficiently strong magnetic field, the planet can't support life.

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