You canna change the laws of physics. . .or can you?

[Strebor Nosnibor]

I came across this one system today that was decidedly strange. (I think I took a screenshot of the system map, but I'm not on that PC at the moment). It was a single star system, with several planets, but the orbital periods made no sense whatsoever. The planet closest to the star had an orbital period of about 30 days, the next one out was 300plus days, then the next one was sixty. In the middle was a planet with 3500 days, and the next one further out was four hundred and something.

Have I just discovered a major space-time anomaly for which I can expect to be royally compensated by Universal Cartographics?

 

[iain666]

Probably not. Orbital period depends on the mass of the orbiting body as well as the radius of the orbit. Unless all your planets had similar masses - then that's a bit odd.

 

[Arry]

Not all the bodies have to be from the same source, to become orbital, correct?

 

[iain666]

Not all the bodies have to be from the same source, to become orbital, correct?

Planets can be flung out of systems and captured by others, if that's what you mean.

 

[Ziljan]

In general, the period always goes up with orbital distance regardless of the mass of the planet. This is because the mass of the orbiting body is usually insignificant compared to the mass of the star. If this was a brown dwarf system with with a tiny main star and several extremely massive gas giants, then the periods going up and down might make sense. However...

The formula for calculating the period is:



Where T is the Period, a is the radius, G is a constant, M1 is the mass of the Star and M2 is the mass of the planet. Normally M1 >>>>>> M2. So M2 might as well be ZERO for the purposes of this calculation. Hence the period only varies with the radius a^(3/2), otherwise known as Keplers 3rd Law.

EXAMPLE:

the mass of the Sun is 1.98855 x 10^30 kg. If you add the mass of jupiter the mass of the sun (M1 + M2) you get 1.9904 x 10^30 kg. A difference of less than 0.1%

Or to put it another way, if you put Earth at Jupiter's orbit, your year would be 4335.34 days. Compare this to Jupiter's actual year which is 4332.59 days. That's right, a mere 2.75 day difference for a planet that weighs 317.8 times as much as the Earth.

 

[Fen Harel]

I have wondered about the stability of some complex systems we all run into now and then. Say, ones with a (relatively) close binary pair, with each star having planets. Off the top of my head it doesn't seem like these would be stable over the course a several billion years.

I'm willing to believe they have done all the math right, at least for Newtonian physics, but I have wondered about the viability and long term stability of some complex systems.

 

[Asteroidnerd]

I have wondered about the stability of some complex systems we all run into now and then. Say, ones with a (relatively) close binary pair, with each star having planets. Off the top of my head it doesn't seem like these would be stable over the course a several billion years.

I'm willing to believe they have done all the math right, at least for Newtonian physics, but I have wondered about the viability and long term stability of some complex systems.

The mathematics is definitely not right in this system. For a single star system, the planet orbital period will be proportional to the orbital radius to the power 3/2. Put another way - the orbital period will always increase as distance to the star increases.

For close binary systems IRL, planets can orbit around the centre of gravity of the binary system, and the same laws apply. Planets closely orbiting individual stars will be the same as single systems. If the planet's orbital radius is a significant fraction (system dependent) of distance between the stars, then the orbit may well be chaotic or unstable.


Just a note from a friendly astrophysicist.

 

[Jackie Silver]

Can you post pictures of the star and planet details from the system map, please? I'm very curious to see.

 

[domaq]

Who's to say our model of physics is complete or that the mathematical simulation is correct?

 

[Ziljan]

Who's to say our model of physics is complete or that the mathematical simulation is correct?

I am almost positive that the game uses Keplers law, or a simplified approximation of it. I would like to see a data page for each planet to see if it is indeed an error or simply an accidental reading of the planet's day length rather than it's year length.

 

[Janichsan]

I came across this one system today that was decidedly strange. (I think I took a screenshot of the system map, but I'm not on that PC at the moment). It was a single star system, with several planets, but the orbital periods made no sense whatsoever. The planet closest to the star had an orbital period of about 30 days, the next one out was 300plus days, then the next one was sixty. In the middle was a planet with 3500 days, and the next one further out was four hundred and something.

Have I just discovered a major space-time anomaly for which I can expect to be royally compensated by Universal Cartographics?

It could be that some of these planets orbit around a barycenter. In that case, the game shows the orbital period to travel around this center, not the central star. Highly excentric orbits could also be an explanation.

A screenshot of the system map is really required to decide what's the matter.

 

[Kancro Vantas]

It could be that some of these planets orbit around a barycenter. In that case, the game shows the orbital period to travel around this center, not the central star. Highly excentric orbits could also be an explanation.

A screenshot of the system map is really required to decide what's the matter.

Exactly my thoughts. I think OP was probably reading binary planets which data is relative to their barycenter instead of main star.

 

[domaq]

I am almost positive that the game uses Keplers law, or a simplified approximation of it. I would like to see a data page for each planet to see if it is indeed an error or simply an accidental reading of the planet's day length rather than it's year length.

Exactly. Would be interesting to know what they based the Star Forge on if for nothing more than giggles.

 

[Strebor Nosnibor]

Thanks for all the thoughtful responses; I knew Ziljan would have the relevant equation to hand! As he says, the relationship is the basic Keplerian one.

However, mystery solved (sort of) and as you might expect, it is user error. I went back to the system map last night and realized that what I hadn't noticed (I know, never explore when you are tired, blah blah blah) was that some of those planets were also orbiting one another. Looking at the singletons in the system, everything looked more logical. So what tripped me up here is that when you click on the info for a planet after having scanned it, what you get is information about orbital period, etc., that is related to the immediate center of gravity around which it is orbiting. Currently, there doesn't seem to be any way for you to look at, say, a pair of planets and see their orbital data relative to their parent star. Or is that something else I am missing?

Now I feel embarrassed at having encouraged anyone to expend brain power on this. . .

 

[TruffleSnout]

Currently, there doesn't seem to be any way for you to look at, say, a pair of planets and see their orbital data relative to their parent star. Or is that something else I am missing?

No, unless I'm missing the same thing. I've always wanted to know what the orbital period/semi-major axis for the whole planetary pair/tri (is that even a word?) is, but have found nothing. It's particularly bothersome when the outermost planets around a star are in a pair/tri and I'm in the system map trying to decide if they are close enough to warrant cancelling my FSD charge.

 

[Kancro Vantas]

It could be that some of these planets orbit around a barycenter. In that case, the game shows the orbital period to travel around this center, not the central star. Highly excentric orbits could also be an explanation.

A screenshot of the system map is really required to decide what's the matter.

Thanks for all the thoughtful responses; I knew Ziljan would have the relevant equation to hand! As he says, the relationship is the basic Keplerian one.

However, mystery solved (sort of) and as you might expect, it is user error. I went back to the system map last night and realized that what I hadn't noticed (I know, never explore when you are tired, blah blah blah) was that some of those planets were also orbiting one another. Looking at the singletons in the system, everything looked more logical. So what tripped me up here is that when you click on the info for a planet after having scanned it, what you get is information about orbital period, etc., that is related to the immediate center of gravity around which it is orbiting. Currently, there doesn't seem to be any way for you to look at, say, a pair of planets and see their orbital data relative to their parent star. Or is that something else I am missing?

Now I feel embarrassed at having encouraged anyone to expend brain power on this. . .

No, unless I'm missing the same thing. I've always wanted to know what the orbital period/semi-major axis for the whole planetary pair/tri (is that even a word?) is, but have found nothing. It's particularly bothersome when the outermost planets around a star are in a pair/tri and I'm in the system map trying to decide if they are close enough to warrant cancelling my FSD charge.

There is currently no way to know this in the game right now. Only thing is to travel to the barycenter of both objects, then check distance to the parent star, then use equations to find out.

 

[Ziljan]

There is currently no way to know this in the game right now. Only thing is to travel to the barycenter of both objects, then check distance to the parent star, then use equations to find out.

In general, the distances between planets orbiting each other is tiny compared to the distance to the star. So you could just target either planet immediately after jumping to the star and just use that distance and get an answer with an insignificant margin of error. If you use the equations enough, you can start to get a feel for what to expect based on the mass of the parent star and the AU/Ls to the planets, and won't need to crunch the actual numbers. In astronomy, error margins can be in powers of ten, so being able to intuit a rough answer rather than calc it is extremely useful.

 

[Jackie Silver]

You have a couple of options for finding the SMA of a binary pair from the system map information:

First) Quick, easy, absolutely unreliable option - if the binary pair is between other planets of the same type (e.g. a binary pair of terrestrials, with a terrestrial orbiting outside of them and a terrestrial orbiting inside of them) you can make a reasonable guess that the sma will be roughly a+ ((b - a) / 3) where a is the sma of the inner planet and b is the sma of the outer planet.

Second) Accurate, not always applicable, requires you to have already scanned them, time-consuming option - If one or either of the binary pair have no atmosphere, you can work out the blackbody effective temperature at that distance by correcting for the planet's albedo and then by using the details of the star you can work out at what distance that effective temperature applies.

NB

Being a numpty I not only didn't bother to check this, but gave a wrong formula above. I've corrected it now.

 

[Strebor Nosnibor]

That is a good point about the relative distances. I keep forgetting how Super Cruise distorts your sense of the scale of the systems, particularly relative measures.

And on another note, I should say that one reason I like hanging out in the Explorer's section rather than other places on the Elite forums is that no one immediately responded to my query with "RTFM Noob!" or some such. Maybe explorers are a little more inclined to accept that a procedurally generated universe, much like the real one, might be a little buggy, and that contributes to the interest of it all.

 

[Ziljan]

You have a couple of options for finding the SMA of a binary pair from the system map information:

First) Quick, easy, absolutely unreliable option - if the binary pair is between other planets of the same type (e.g. a binary pair of terrestrials, with a terrestrial orbiting outside of them and a terrestrial orbiting inside of them) you can make a reasonable guess that the sma will be roughly a+ ((a + b) / 3) where a is the sma of the inner planet and b is the sma of the outer planet.*

Second) Accurate, not always applicable, requires you to have already scanned them, time-consuming option - If one or either of the binary pair have no atmosphere, you can work out the blackbody effective temperature at that distance by correcting for the planet's albedo and then by using the details of the star you can work out at what distance that effective temperature applies.


*I should test that properly, but if the planets are at rough intervals 1,2,4,8 and so on that should work.

Orbital resonance theory? Interesting, but remember that there are different modes of resonances just like harmonics on a guitar string. So it's not just a doubling like octaves, but could also be in ratios of 5:3, 3:4, 5:9, etc... actually, it's pretty amazing seeing the similarities between orbital mechanics and musical instruments. The reason that harmonies sound pleasing to us is quite similar to the reason that stable orbits exist for moons.

This video says it all quite eloquently:
[video=youtube;i_0DXxNeaQ0]https://www.youtube.com/watch?v=i_0DXxNeaQ0[/video]

That is a good point about the relative distances. I keep forgetting how Super Cruise distorts your sense of the scale of the systems, particularly relative measures..

If you check the semi major axis on planets that are orbiting each other it's usually 0.00 AU. I suppose they could use a different unit to measure the distance between double planets/moons, but then people would need to convert the units to even understand what the scale really was.