Gliese 876 redux

[ajtribick]

A while back I made an add-on which gave trajectories to the planets of Gliese 876 which included planet-planet gravitational interactions. Since then an additional planet has been discovered in the system, making Gliese 876 the second known example of a system in a Laplace resonance (planets c, b and e are in a 1:2:4 resonance), the first being Jupiter's Galilean moons Io, Europa and Ganymede. As a result of this and various potential issues with coordinate conventions, I asked for the original version to be removed from the Motherlode.

For the remake I'm going to do more than just add the trajectories for the planets, I'm going to try at having a go with planet and star textures to make this a more complete add-on, though I haven't got anything usable yet. Any info on simulations of red dwarf star surface patterns would be nice (I've come across simulations of red giants and supergiants, but not the dwarfs). At present I've rewritten the integrator code in C and generated a 20 year trajectory for the star and the 4 planets, relative to the system barycentre. This is a different setup to the first add-on, where the orbits were given relative to the star: this is made possible using reference frames.

I've attached some screenshots from the current state of the add-on. More to come soon!

[Hungry4info]

Very much looking forward to this!

[ajtribick]

So Gliese 876 c is a tricky one to come up with a texture for. The close proximity of the star means it is probably tidally locked, and we don't have a gas giant that rotates that slowly in our solar system. There are various computer simulations of hot Jupiters (for example this paper) but these planets receive far more energy from the star and rotate several times faster. The slowly-rotating planets with thick atmospheres in our solar system are Venus and Titan, so I'm going to base my rendering on those.

As a starter, here's a very simple texture made by using a logarithmic spiral pattern in the xy-plane, texture generated with POV-Ray. Looks a bit like the Y-feature in the Venusian atmosphere.

[ajtribick]

Further evolution of the Gliese 876c texture. Here I am using the logarithmic spiral as a displacement on a noise function. This is an extremely simple texture definition in POV-Ray and it is something of a re-learning experience for me, I haven't actually used POV-Ray for quite a while.

Basic scene set up in POV-Ray is to set up a spherical camera at the origin, and put a sphere around the camera. Here's the basic scene file I use:

Code: Select all

#version 3.6;

// make all textures self-luminous
#default {
    finish {
        diffuse 0
        ambient 1
    }
}

// camera, set up so z-axis points north
camera {
    spherical
    angle 360 180
    location 0
    look_at x
    sky z
}

// sphere of radius 1 located at the origin
sphere {
    0, 1
    // put texture definitions here
}

This naturally produces textures that have the correct sphere mapping to use in Celestia.

[ajtribick]

So in the previous pictures I have been depicting clouds as white and the cloudless parts of the planet as blue. However Gliese 876 is a cooler star than the Sun, so things might turn out a bit differently. I created a small application (also to try out a few WPF techniques, yeah yeah Windows is evil etc. etc. whatever) to see what the results of integrating different spectra gave, with the star colour given by a blackbody and the sky based on multiplying the blackbody spectrum by 1/??. I used the CIE1931 (modified by Judd and Vos) colour matching functions from this website to generate the XYZ colours, and converted these values to sRGB.

For a 5780 K blackbody, the sky colour comes out as a nice blue, which is fairly reassuring that the program is not doing anything too ridiculous.

Temperature of Gliese 876 is quite hard to find a good reference for. The quoted stellar parameters in the Rivera et al. (2010) paper I linked in the first post imply a temperature of around 3500 K, which suggests a fairly pale blue sky. The Extrasolar Planets Encyclopaedia gives a value of 3350 K, which results in a pale greenish colour for the sky. Then there's Casagrande et al. (2008), who give 3076 K, giving a pale orange sky. Not sure I entirely believe such a low value though.

(Of course the spectra are not actually blackbodies, but it works as a first approximation)

So I think the final texture (still under development) is going to be fairly pale...

(Incidentally because of the ongoing horrendously slow performance of shatters.net I may migrate this thread over to Celestial Matters...)

[t00fri]

...
(Incidentally because of the ongoing horrendously slow performance of shatters.net I am may migrate this thread over to Celestial Matters...)

Yes, please do I'd love to see it at CM. This sort of analysis fits very well.

Fridger

[ajtribick]

Because of continued problems with the shatters.net website I have started a new thread on the Celestial Matters forums, starting with updates to the sky colour predictions from my previous post here.