World Habitability in a binary system

[aramis]

An object in a planet's lagrange points is not a satellite of the planet (it isn't orbiting the planet); it would be a co-orbital instead.

Technically, it's gravitationally bound to both objects, and can be said (except for L3) to be a part of the "system" of both, but only orbiting body 1... (L3 being opposite body 1 from body 2).

L1 and L2 are in fact orbits of body 2 which happen also to be resonant with a matching orbit of body 1.

 

[fusor]

Technically, it's gravitationally bound to both objects

"The Lagrange Points mark positions where the gravitational pull of the two large masses precisely cancels the centripetal acceleration required to rotate with them." ( http://en.wikipedia.org/wiki/Lagrangian_point )

See also: http://www.esa.int/esaSC/SEMM17XJD1E_index_0.html

The OP was asking about a satellite of the gas giant. An object in a Lagrange Point is not a satellite of the planet (and it cannot be. The L1 and L2 Lagrange points are by definition at the edge of a planet's Hill Sphere, and satellites only have stable orbits within the closest third or half of the hill sphere distance). This is obvious when considering the L3, L4 and L5 points, which are Lagrange Points but can in no way be considered to be "orbits around the planet" (this is also shown very clearly for L1/L2/L3 in the ESA link).

Furthermore, L1, L2 and L3 are unstable. Natural objects will not remain there; unless their position at the Lagrangian point is maintained by thrusters or other artificial means, any external force acting on them will disturb them and cause them to eventually collide with the planet or be ejected from the system.

 

[altasilvapuer]

FreeTrav is actually correct in his assumption. I was not paying attention and improperly worded that when I posted it. I realized the mistake when explaining the situation to a friend at work and hadn't corrected it, yet. I had a feeling such was the case, after I realized my mistake, but figured it was worth the mental aerobics in either case.

As to the highly eccentric orbit, I think I follow your reasoning and that's a good point. Although, what if you had a system like Pluto/Neptune around a star hot enough that that range was the HZ. Think it still ends up being the same issue, or no? I think I'd have to scribble out some orbit numbers, first; might do that later tonight.

-asp

 

[FreeTrav]

It's actually quite plausible... Trojan/Lagrange points.


But I dont know that your interpretatin is actually what he meant.

Lagrange/Trojans, yes, but if I read the question correctly he was actually asking about the satellite always being behind the planet relative to the (companion) system primary. That would mean L2, if I remember the Lagrange numbers correctly, and as I recall only L4 and L5 are what I'd call "passive stable" - the others require some sort of minimal station-keeping action, or they'll drift away from the point.

Also note that being in a Lagrange point would make it a satellite of the primary (companion star), not the GG.

 

[McPerth]

I've not run the numbers but, could a world so close to the star (in gravitiy terms) have satellites?

I'm talking totally intuitivelly, and maybe what I say is quite silly, but I guess the accleration received when aproaching the main star, joimed with the decceleration received when mves away would make the orbit quite unstable quite soon.

As I said, I don't know if it's possible, but in our system (the only one we know in any detail by now, AFAIK), the truth is inner systems have no satellites...

 

[HG_B]

I've not run the numbers but, could a world so close to the star (in gravitiy terms) have satellites?

Sure, no problem. We have a satellite orbiting Mercury.

 

[McPerth]

Sure, no problem. We have a satellite orbiting Mercury.

I meant natural satellites. Artificial ones have a very sort livespan in comparison and may have some station keeping capability. Natural ones must have equilibred its orbit in centuries, and I gess for every one in stable orbit, several have fallen to the orbited body due to decaying orbits.

 

[fusor]

There can't be natural satellites if the planet is tidelocked to the star. Even if the planet is not tidelocked, the effect of solar tides on the satellite's orbit could be problematic.

 

[Peter Schutze]

the easiest way I can see for a moon to occur that close in would be something captured, probably by something fairly big in the range of a "super earth" or a small "hot jupiter"

Im not sure how a *tidelocked* gas giant would fare in that situation though

 

[McPerth]

the easiest way I can see for a moon to occur that close in would be something captured, probably by something fairly big in the range of a "super earth" or a small "hot jupiter"

Im not sure how a *tidelocked* gas giant would fare in that situation though

And less so a 'tidelocked' companion star...If that may exist (frankly, I'd nevert thought about this).

 

[altasilvapuer]

Now there's another interesting thought: a tidally-locked star. I'm curious what would happen to it, as well.

Theoretically such an orbital situation would exist, yes? And likely at close orbit. So now I am brought to wonder if the output of the primary will have any appreciable heating effect on at least the surface of the companion. Because if it does, then that will only happen on one side, leaving the far side at the temperature expected based solely on the output of the companion's own production, I think.

I wonder if that would cause enough of a difference to mess with the convection and circulation around the star at all, and whether such would appreciably alter any of the properties of the star's output.

Greater quantity of sunspots on the far side? Fewer sunspots overall on the whole star? I'm kinda curious, now, if the mathheads have any thoughts on the matter.

 

[Enoki]

Another problem is solar wind and the two sun's magnetic fields. If the two are close enough it is also probable that their magnetic fields (although weak at that distance) interact to a small degree and there could be an exchange of solar material, sort of a continious solar flare, moving between the two. A planet passing through this region would be massively bombarded by charged particles and radiation.... That might be a small problem.....

 

[altasilvapuer]

Another problem is solar wind and the two sun's magnetic fields. [...] there could be an exchange of solar material, sort of a continious solar flare, moving between the two. A planet passing through this region would be massively bombarded by charged particles and radiation.... That might be a small problem.....

Actually, that's a very good point. A little bit of an amusing understatement, too.

Perhaps this would make a good place to force-drop a Research and/or Scout base (outside either star's orbit) and declare a Red or Amber zone. Ideas..

-asp

 

[Enoki]

I don't think that this warrants a red or amber zone rating but, rather it limits the world placement such that the main world might have to be outside the habitable zone or even placed as a satellite of a gas giant (definitely a possibility in itself particularly for smaller worlds). As we are finding out with astromomy today weird is what planetary systems are all about.....

 

[altasilvapuer]

So I was rolling up a few more systems tonight and came upon another interesting situation that I thought I'd share.

[Distances to linear scale; world/star sizes figurative only - not to scale]

Primary: A6V

• Max. Orbit: 6 (3 Effective)

Companion: DK (Orbit 6)

• Max Orbit: 7 (3 Effective)

A quick look through the Habitability charts shows that the habitable zone for an A5V is Orbit 6, right where this companion dwarf happened to fall. A DK has no habitable zone at all, though, which at first glance obliterates any habitable zones in the system.

I had a thought, though. Orbit #0 is only 0.2AU from its star, so would Companion orbit #0 be close enough to be marginally in the habitable zone of the Primary? I can see such an orbit having rather extreme summers and winters as it orbits the companion and moves through the inner and outer reaches of the primary's habitable zone.

Just some food for thought tonight.
-asp

 

[BytePro]

As long as the 0.2AU is not too hot from DK (or with combination of primary's temp).

As the original post - one could just make the 'shield' part of the planet... i.e. the planet rotates around its axis such that only one side always faces the central star (making for long nights, etc., unless orbit around primary is very fast). That side is significantly higher in elevation such that the planets atmo only exists on the other, depressed side. The heating of the uninhabitable side drives the wind and seasons on the depressed side as the atmo circulates around the heated 'walls'.

I could poke all kinds of holes in that - but it fits the 'shield' concept without being a displaced shield, with all its inherent complexities, so I thought I'd throw it out there. This could also be a terraforming concept (smashing rocks and iceballs to build up one side, while changing the orbital revolution and also providing depression and atmo). Why? - maybe for the security of the jump shadow...

 

[Icosahedron]

It occurs to me that ALL of the companion's planets are in Orbit 6 of the Primary - they're not in Orbit 5, are they? The 'habitable zone' will be fairly wide. Companion-2 looks good to me - still reasonably within Orbit-6 of the primary, but far enough from the secondary to avoid gravity and radiation problems..

 

[fusor]

A quick look through the Habitability charts shows that the habitable zone for an A5V is Orbit 6, right where this companion dwarf happened to fall. A DK has no habitable zone at all, though, which at first glance obliterates any habitable zones in the system.

The Dwarf star obliterated any habitable zones in the system, period.

Think about what a white dwarf is - it's the end result of a star's evolution. A few million years ago, that star that was a red giant, probably about 1-2 AU in radius and very luminous. It certainly wouldn't have any planets orbiting it at the distance you show, it would have consumed them all and roasted the planets around the A6 V star for good measure.

 

[aramis]

Fusor, your timescale is off by a few orders of magnitude. To be a K... that's at the cold end. Either it was rather small (≤0.5 solar masses) or very old. It may even be a capture situation. Dwarves are often billions of years old

 

[altasilvapuer]

Well, I don't suppose it would apply much here, given that this is a K-type and likely too hot for it, but for theory's sake, what about a Brown Dwarf?

I could be wrong, but I think I remember reading once that at least some brown dwarfs were theorized to be stars that didn't have sufficient mass to perpetuate stellar fusion, like super-massive Jovian planets. Or am I mistaken and they're currently theorized to be more like white dwarfs - stellar remnants following novae, et al?

On that note, is it theoretically tenable that the companion could actually be a super-massive gas giant that never achieved stellar fusion and instead is a form of brown dwarf? These orbiting 'planets,' then were more like satellites of the GG-brown dwarf.

Food for thought.
-asp