Gliese 876: coplanarity

[ajtribick]

Bean and Seifahrt, arXiv, "The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c"
Combined analysis of radial velocity and astrometric measurements indicates that the planets b and c are in (nearly) coplanar orbits. This also resolves the previous discrepancy between the astrometric analysis which favoured an inclination of ~80 degrees, and dynamical radial velocity fitting which favoured an inclination of ~50 degrees.

Here's a .ssc file you can drop in your extras folder which implements the orbits given in Table 2. Note that due to strong planet-planet interactions in this system, the use of Keplerian orbits isn't a very good representation at all.

Code: Select all

Modify "b" "Gliese 876"
{
   EllipticalOrbit {
      Epoch 2452490.0
      Period 0.16649
      SemiMajorAxis 0.20700
      Eccentricity 0.0363
      Inclination 130.4
      AscendingNode 253.8
      ArgOfPericenter 194.5
      MeanAnomaly 163.1
   }
}

Modify "c" "Gliese 876"
{
   EllipticalOrbit {
      Epoch 2452490.0
      Period 0.083452
      SemiMajorAxis 0.13062
      Eccentricity 0.2683
      Inclination 134.4
      AscendingNode 251.3
      ArgOfPericenter 203.1
      MeanAnomaly 309.1
   }
}

Modify "d" "Gliese 876"
{
   EllipticalOrbit {
      Epoch 2452490.0
      Period 0.005305563
      SemiMajorAxis 0.0208069
      Eccentricity 0
      Inclination 130.4
      AscendingNode 253.8
      ArgOfPericenter 6.3
      MeanAnomaly 312.2
   }
}

[Hungry4info]

Thanks for this =).

I have written a short .stc bit implementing the RV effects of all three planets on the host star, as well as aligning the rotation of the star with the orbits of the planets (not an unreasonable assumption, except for one planet, all planets for which the Rossiter McLaughlin effect has been measured are found to be aligned with the star's rotation.

Code: Select all

# Gliese 876
Barycenter "GJ 876"
{
RA         343.3197246
Dec        -14.2637006
Distance   15.3347
}
   Barycenter "gj876b_barycenter"  # Show effects of b.
   {
      OrbitBarycenter "GJ 876"

      EllipticalOrbit {
      Period          0.166844626968
      SemiMajorAxis   0.00166079
      Eccentricity    0.0363
      Inclination   130.4
      AscendingNode   253.8
      MeanAnomaly   163.1
      }
   }

   Barycenter "gj876c_barycenter"  # Show effects of c.
   {
      OrbitBarycenter "gj876b_barycenter"

      EllipticalOrbit {
      Period          0.082409308693
      SemiMajorAxis   0.00030871
      Eccentricity    0.2683
      Inclination   134.4
      AscendingNode   251.3
      MeanAnomaly   309.1
      }
   }

   113020 "Gliese 876"
   {
   OrbitBarycenter "gj876c_barycenter" # Show effects of d.
   SpectralType "M4V"
   AppMag 10.17

        EllipticalOrbit {
   Period     0.005305297741
        SemiMajorAxis   0.00000168199
        Eccentricity   0
   Inclination   134.4
   AscendingNode   251.3
        MeanAnomaly        309.1
        }
   Obliquity   134.4 # guess, to match inclination
   EquatorAscendingNode  251.3 # guess, to match ascending node
   }


[t00fri]

Bean and Seifahrt, arXiv, "The architecture of the GJ876 planetary system. Masses and orbital coplanarity for planets b and c"

It's a nice first determination of exoplanetary orbital inclinations, but the result is not too surprising. Like in our Solar system, the planets were apparently created in a circumstellar disk such that the individual angular momentum vectors are pretty much aligned...

Fridger

[Hungry4info]

It's a nice first determination of exoplanetary orbital inclinations, but the result is not too surprising. Like in our Solar system, the planets were apparently created in a circumstellar disk such that the individual angular momentum vectors are pretty much aligned...

True. I think it was reported that one of the Upsilon Andromedae planets (d, specifically) are inclined by 30 degrees to the system. But I don't know where that was reported, or if it's even true.

If so, interactions via the Kozai Mechanism between Ups And d and Ups And B (the dM star companion) might have been the cause. I wonder if planetary systems in multi-stellar systems will be found to be over a wide range of mutual inclinations.

[ajtribick]

Well given the apparent regular behaviour of the b+c pair, I suspect it should be possible to represent the system using periodic functions in a ScriptedOrbit.

Pity we don't have a ScriptedEllipticalOrbit so we could see the orbits precessing...

[Hungry4info]

Pity we don't have a ScriptedEllipticalOrbit so we could see the orbits precessing...

That could be useful in a lot of areas. Mercury, HMXB systems, etc.

[chris]

Well given the apparent regular behaviour of the b+c pair, I suspect it should be possible to represent the system using periodic functions in a ScriptedOrbit.

Pity we don't have a ScriptedEllipticalOrbit so we could see the orbits precessing...

ScriptedOrbit is general enough to do the job. But I suppose it would be easier to implement if Celestia's elliptical orbit calculation were available from Lua.

--Chris

[PlutonianEmpire]

I take it Earthlike (habitable by Human standards) moons are impossible in this system?

[ajtribick]

There may be good reasons to believe that the satellites of gas giant planets cannot grow much larger than a few lunar masses without falling victim to rapid type I migration and falling into the planet. Habitable moons may well be impossible in general.

[Hungry4info]

Assuming, of course, that such a moon wasn't captured.

If Neptune were orbiting at 1 AU, I could imagine Triton to be a candidate for habitability (it's low mass aside). But of course, Triton's orbit isn't permanently stable either.