An Impossible Moon

[Contada]

I've been out on a short exploration trip when I came across something of an anomaly, a gas giant orbiting a Y dwarf. It's not often that a moon is larger than its primary body. At first glance, it seems like it's too big to be a moon, and that it should rather be a barycentric system.

Spoiler

So I decided to do the math. A system will have a barycentric orbit if the the distance between the primary body and the shared center of the orbit exceeds the radius of the primary body itself. A quick trip to Wikipedia says that the distance to the barycenter for the primary body is r = d/(1+m1/m2).

From the system map, we get:

Y Dwarf
mass (m1) = .0195 Solar Mass = 3.88*10^28 kg
radius = .0939 Solar Radius = 6.54*10^7 m

Gas Giant
mass (m2) = 942 Earth Mass = 5.63*10^27 kg
semi-major axis (d) = .2 AU = 2.99*10^10 m

r = 2.99*10^10 m / (1 + 3.88*10^28 kg/5.63*10^27 kg) = 3.79*10^9 m

So we have a distance from the Y Dwarf to the barycenter that exceeds the radius of star itself, in other words, the gas giant shouldn't be represented as a moon. Am I missing something, or was the system map/random system generator wrong? This irked me to the point that I made a forum account just to ask this.

 

[Heavy Johnson]

Stellar Forge, the background system, often does not care about such things. This also causes things like Wolf-Rayet stars with a temperature of 200 kelvin.

 

[Straha Yeagar]

I suspect that the system map simply does not display stellar bodies and planetary bodies in barycentric orbits. Thee only way to tell for sure would be to actually observe the bodies over time, and there, I suspect that they would show a barycentric orbii.

 

[Sapyx]

+1 to the OP for doing the math. For thinking of doing the math, even.

Planets and moons are depicted in barycentric orbits all the time; the general ED rule is that if a barycentre is physically outside the sphere of the larger body, then it's depicted as barycentric. This one is not, for some reason.

Have you included the masses of the rings in the calculations? I assume they wouldn't make much difference, but you never know...

 

[marx]

Yeah, I was about to say that you didn't include the masses of the rings, but given that they are over an order of magnitude below the body masses, they don't change the conclusion.

Mind sharing a screenshot of the system's main star though? That might be the cause. The Forge seems to have some small errors when it comes to moons (although I didn't know that these might even affect orbits) , so edge cases like this might come into being by that.

 

[Straha Yeagar]

+1 to the OP for doing the math. For thinking of doing the math, even.

Planets and moons are depicted in barycentric orbits all the time; the general ED rule is that if a barycentre is physically outside the sphere of the larger body, then it's depicted as barycentric. This one is not, for some reason.

Have you included the masses of the rings in the calculations? I assume they wouldn't make much difference, but you never know...

We do see barycentric orbits depicted quite frequently, but I only remember seeing them depicted as stellar bodies with stellar bodies, and planetary bodies with planetary bodies. So I am wondering if something is flagged as a star, that the system map will only show a barycentric orbit if the other body(ies) are also stars.

For purposes of definition, planetary bodies being the gas giants, MR, HMC, R, I RI, WW, AW, ELW.

 

[MattG]

We do see barycentric orbits depicted quite frequently, but I only remember seeing them depicted as stellar bodies with stellar bodies, and planetary bodies with planetary bodies. So I am wondering if something is flagged as a star, that the system map will only show a barycentric orbit if the other body(ies) are also stars

I don't think this is the case. For starters, that 50G world I posted a few days back (a rocky world itself) is in a barycentric orbit with a T Tauri, and although I can't find one currently I'm certain I've seen gas giants in barycentric orbits with gas giants.

I think what's more likely is that even if the barycenter is outside a body, if it's substantially closer to one of the bodies it will be shown as orbiting it. In fact, I ran the numbers. In ~80% of cases where a gas giant is presented as the first moon of a solar body, the barycenter lies outside the solar body. But if r1 + r2 = sma, then r1 never
[*] exceeds 23.5% of sma. I don't promise my calculations are correct though - it is Monday, after all.


[*] based on eddb data from 6th Oct

Edit: so I checked cases where a gas giant is presented as the first moon of a non-solar body too. There are far less occurrences of this, but still in over 63% of these cases the barycenter exceeds the radius of the parent body. The highest r1 here seems to be just over 20.2% of sma.

 

[marx]

To OP: I asked for a screenshot of the main star before, but on second thought, could you please share bodies 1-3 as well, or easier, just upload the system data to EDSM and/or EGO? The program EDDiscovery can take care of that quickly for you. I'm asking because I'd like to recreate the system, and see how the orbits would end up. (And if Elite's provided data would even be stable.)

 

[Valije]

+1 rep for finding those bodies, noticing something wrong (instead of just checking they are NOT ELW) and then doing the maths to check the actual mass of said bodies.

 

[Contada]

For anyone who wants to see the rest of the system, as well as a selfie with the gas balls in question.

Spoiler

I suspect that the system map simply does not display stellar bodies and planetary bodies in barycentric orbits. Thee only way to tell for sure would be to actually observe the bodies over time, and there, I suspect that they would show a barycentric orbii.

There is an example of a stellar body and a planetary body with a barycentric orbit in the same system.

To OP: I asked for a screenshot of the main star before, but on second thought, could you please share bodies 1-3 as well, or easier, just upload the system data to EDSM and/or EGO? The program EDDiscovery can take care of that quickly for you. I'm asking because I'd like to recreate the system, and see how the orbits would end up. (And if Elite's provided data would even be stable.)

The system data should be on edsm.

I don't think this is the case. For starters, that 50G world I posted a few days back (a rocky world itself) is in a barycentric orbit with a T Tauri, and although I can't find one currently I'm certain I've seen gas giants in barycentric orbits with gas giants.

I think what's more likely is that even if the barycenter is outside a body, if it's substantially closer to one of the bodies it will be shown as orbiting it. In fact, I ran the numbers. In ~80% of cases where a gas giant is presented as the first moon of a solar body, the barycenter lies outside the solar body. But if r1 + r2 = sma, then r1 never
[*] exceeds 23.5% of sma. I don't promise my calculations are correct though - it is Monday, after all.


[*] based on eddb data from 6th Oct

Edit: so I checked cases where a gas giant is presented as the first moon of a non-solar body too. There are far less occurrences of this, but still in over 63% of these cases the barycenter exceeds the radius of the parent body. The highest r1 here seems to be just over 20.2% of sma.

This seems like a good explanation. If this issue is present throughout the game, I can see how it would be easy to miss. I seem to recall that high mass gas giants don't become much larger in volume past a certain point. The Y dwarf in question is about 7 times the mass of the gas giant, but both have a fairly similar radius. So assuming it's not a fluke, it makes sense that a pair like this would be the most visible example of this backend optimization as is possible.

 

[GunnerBill]

That's no moon!

 

[duckofdeath]

I have been looking for "reverse orbits" myself, but it is a rather narrow (impossible?) window to find it in, the smallest stars we get are 0,0078 solar masses (2,6 million earth masses) and the largest procedurally generated gas giants seems to top off around 4 million earth masses. I'm still hoping there would be other 15-ish million earth mass gas giants like, what I presume is, the early procedural generation model with more relaxed limits we have around the populated bubble. They just doesn't seem to exist. Y class or T Tauris sharing a barycentre with gas giants does exist in our galaxy. I haven't checked the numbers where our galactic forge sets the limit for that to happen, though.