Voyager Alpha - Worldbuilding IMTU

[tjoneslo]

There are two questions I have about the system after seeing it generate several star systems.

1) Does the metallicity of the star have any effect on the planet generation? It should, as the planetary disk should have the same metallicity as the star around which it forms. I know in the long ago stars were divided into Generation I and Generation II. The older Generation I stars have very few metals, hence would be unlikely to have any rocky planets (gas giants yes, large spheres of Icy water, ok, but no rocks).

2) The difference between the upper range of the Supergiant Planet (1000 to 5000 earth masses) and the lower range of the Type Y stars. More specifically, can the planetary mass accretion process generate (even if rarely) another star in the system?

 

[thalassogen]

1) Yes, metallicity (all stars and the planetary disk have the same Z) has effect:
a) it increases the density/amount of dust available for accretion directly (linearly) and thereby supports larger planets, and
b) it has a little bit (logarithmic) of effect on planet density.

As I researched the accretion process and its connection to stellar mass and metallicity, I learned that without metals accretion of giant planets gets less likely as there is not enough material to reach the critical mass for gas accretion. The accretion process follows a simple rule: stellar mass and metallicity relate to number of planets/planetary mass, i.e. more stellar mass = more planetary mass, higher metallicity = more planetary mass, lower stellar mass = lower planetary mass, lower metallicity = lower planetary mass.

2) No, the planetary mass accretion process cannot create stars and sadly, I wasn't able to generate as Supergiant Planet either. In fact Supergiant Planets should be Brown Dwarfs, but currently there seems to be much discussion where exactly the boundary lies. Some models suggest that Supergiant Planets may exist that don't ignite Deuterium fusion. And there are articles mentioning Brown Dwarfs of masses up to 0.45 solar masses.
I guess, for now I will treat Supergiant Planets as planets and very-low-mass LTY stars as Brown Dwarfs.

If you know some good up to date articles on the either topic I would be happy about any pointers you can provide!

 

[tjoneslo]

If you know some good up to date articles on the either topic I would be happy about any pointers you can provide!

My (admittedly brief) research into brown dwarf stars turned up only the following information:

Hydrogen Burning (fusion) limit: 0.075 solar masses (about 25,000 earth masses)
Lithium Burning limit: 0.06 solar masses (about 20,000 earth masses)
Deuterium Burning limit: 0.012 solar masses (about 4,000 earth masses)

Brown dwarf stars, once formed, then burn through their entire supply of Deuterium in 4-50 million years (smaller take longer). Once their fuel supply is gone, they gradually cool.

This information was condensed from the Wikipedia page, which has several dozen reference papers. I would suggest starting there, and google the author's names for more references.

 

[shaunhilburn]

There are two questions I have about the system after seeing it generate several star systems.

1) Does the metallicity of the star have any effect on the planet generation? It should, as the planetary disk should have the same metallicity as the star around which it forms.

Yes.

I know in the long ago stars were divided into Generation I and Generation II. The older Generation I stars have very few metals, hence would be unlikely to have any rocky planets (gas giants yes, large spheres of Icy water, ok, but no rocks).

There have been metal-poor stars found with transiting rocky planets. "Poor" relative to the sun. The relative lack of refractory elements just means these systems tend to produce smaller terrestrial planets instead of superterrrestrials; they are still capable of hosting habitable planets, contrary to the conventional wisdom.

http://www.spacedaily.com/reports/Metal_poor_stars_are_rich_with_small_planets_999.html

 

[thalassogen]

The relative lack of refractory elements just means these systems tend to produce smaller terrestrial planets instead of superterrrestrials; they are still capable of hosting habitable planets, contrary to the conventional wisdom.

That's exactly what I am trying to simulate. I am just not sure how successful I am at that, but I guess that currently there is not enough data to be sure.

 

[thalassogen]

Thoughts on atmospheres:
- Planets that can hold an atmosphere (subject to gravity and blackbody temperature) normally will hold an atmosphere
- Atmospheric composition will depend on these parameters
- Free oxygen will not normally be present in an atmosphere and will be an indicator of life (I will assume that free oxygen is the result of bacteria/plant-like organisms)

I will also assume that life is carbon-based and that higher lifeforms depend on free oxygen for breathing. More alien forms of life won't be generated and must be dropped in by the referee.

What do you think?

 

[Ishmael]

Have you taken into consideration the relationship between water, the cold trap, and the carbon cycle to determine if a planet is Venus-like or Mars-like in their atmospheric pressures and compositions?

http://www.physics.sdsu.edu/~johnson/writing/climate.html
http://burro.astr.cwru.edu/Academics/Astr221/SolarSys/CompPlanet/venusmars.html

 

[thalassogen]

No, I hadn't. Thanks for the links!

 

[thalassogen]

I am currently getting a headache; I am trying to calculate the minimum molecular weight (MMWR) retained assuming that it would be a good starting point for what will be present in an atmosphere. While researching I found several other escape mechanisms besides thermal (aka Jeans) escape, but those will have to wait - back to MMWR: calculating escape velocity and molecule velocities, keeping in mind that molecules may sometimes be a lot faster than those values and using a suggested factor of 6 (for billion years of retention) I always end with earth having an MMWR of about 2.12 - just above H2 and well below He, but earth did not retain (much) helium! And the seldom earth-MMWR is given as about 5.5. But nowhere is an explanation to be found on how this value is derived!

Anyone around to enlighten me? Please?!

 

[shaunhilburn]

I am currently getting a headache; I am trying to calculate the minimum molecular weight (MMWR) retained assuming that it would be a good starting point for what will be present in an atmosphere. While researching I found several other escape mechanisms besides thermal (aka Jeans) escape, but those will have to wait - back to MMWR: calculating escape velocity and molecule velocities, keeping in mind that molecules may sometimes be a lot faster than those values and using a suggested factor of 6 (for billion years of retention) I always end with earth having an MMWR of about 2.12 - just above H2 and well below He, but earth did not retain (much) helium! And the seldom earth-MMWR is given as about 5.5. But nowhere is an explanation to be found on how this value is derived!

Anyone around to enlighten me? Please?!

I posted about this on the Mongoose board the other day:

"... the MMWR is generally proportional to the radius of a planet and inversely proportional to its mass, e.g., a planet with twice the mass-radius ratio as Earth retains molecules two times lighter than in Earth's atmosphere. A planet with half the mass-radius ratio retains gases two times heavier than Earth (for the same exobase altitude and temp).

Earth's exobase temp @ 600 km is ~ 1275 K and MMWR (for Jeans escape only) = 9.9 g/mol

Say King's M-R ratio is 5.571 = 10 earth masses / 1.795 earth radii.
So 9.9 g/mol divided by 5.571 = MMWR 1.777 g/mol approximately.

For more precise results, you need to calculate the escape velocity at the planet's exobase altitude and compare it to the mean kinetic velocity of various gas species at that altitude - a task best done with a spreadsheet. Mass-Radius ratio is much easier and almost as accurate."

Average speed of a gas molecule (m/s) = sqrt(3×k×T/[m×amu]), where k is the Boltzman constant, T the kelvin temperature, m is the molecular weight of the gas, and amu is the kg weight of a hydrogen atom (atomic mass unit). An atmospheric gas will decrease to 1/e (36.5%) in 100 Myr life IF average speed < 20% escape velocity. The atmosphere is permanent if IF average speed < 17% escape velocity.

The usual rule of thumb is that the planet can hold an atmosphere made of a certain type of particle over geologic time if the average particle velocity in the exosphere is less than the escape velocity by a factor of about six.

You can cut through all of that and directly calculate the escape velocity required to retain a gas against thermal escape = 0.15794×sqrt(T/mw)×6 km/s

 

[shaunhilburn]

You can cut through all of that and directly calculate the escape velocity required to retain a gas against thermal escape = 0.15794×sqrt(T/mw)×6 km/s

If forgot. By working that formula in reverse, you can derive the MMWR.

MMWR = T/sqrt((V/6/0.157935)^4) where V is the body's exobase escape velocity (slightly lower than the surface escape velocity)

 

[thalassogen]

Thanks for your explanation! Now I know that I did my math right. What I didn't know is that I need to use the exosphere temperature for that calculation. Is there a way to calculate that? Or is it "safe" to assume that it is about 5 times that of the blackbody temperature? Plugging in those values in your (or my) formulas does not yield that ominous 5.5 - any idea how this was generated?

 

[shaunhilburn]

Thanks for your explanation! Now I know that I did my math right. What I didn't know is that I need to use the exosphere temperature for that calculation. Is there a way to calculate that?

Exosphere temperatures are kinetic temperature (particle speeds), not thermal temperature. The region is essentially vacuum. Barometric formula and gas laws no longer apply, so there's no way I know to calculate the temperature. They temperatures are directly measured.

Earth's high exosphere temperature is the result of excitation of oxygen by ultraviolet rays. Mars and Venus having no free oxygen, have much lower values, probably half.

The wikipedia article has some equations. Maybe you can rework one to solve for T.

Or is it "safe" to assume that it is about 5 times that of the blackbody temperature?

Don't think so.

Plugging in those values in your (or my) formulas does not yield that ominous 5.5 - any idea how this was generated?

No idea.

 

[thalassogen]

OK, I finally got it: MMWR=5.5 for earth assumes a retention-multiple of 100 (10) instead of 36 (6), and is based on the blackbody temperature of the planet - or the other way round: uses the 36 (6) retention-multiple and equates the exosphere temperature to about 2.8 times the world's blackbody temperature.

As the exosphere temperature is difficult to calculate and obviously higher if free oxygen is available, I will go with the above method for retention during the early days of the world.

 

[shaunhilburn]

equates the exosphere temperature to about 2.8 times the world's blackbody temperature.

Exosphere temperature has no correlation at all to surface conditions. It's caused by to the interaction of gas atoms (in space) with solar radiation. Venus and Mars have similar exobase temps but wildly different surface temperatures.

I could see some connection if a planet were molten/incandescent or strongly magnetic.

 

[thalassogen]

Exosphere temperature has no correlation at all to surface conditions. It's caused by to the interaction of gas atoms (in space) with solar radiation. Venus and Mars have similar exobase temps but wildly different surface temperatures.

And that interaction is dependent on the kind of gas - different gases interact differently with different wavelength, right?


BTW, I updated the website with MMWR output.

For White Dwarfs the MMWR is based on the star's highest luminosity, even if that lasts only for days. I hope to account for the expelled planetary nebula passing the planets by using the highest luminosity. Currently I don't have any data to base White Dwarf/Planetary Nebula effects on, so this may be subject to change later.

 

[shaunhilburn]

And that interaction is dependent on the kind of gas - different gases interact differently with different wavelength, right?

Correct. Each gas in the exosphere responds differently to ionizing radiation and has its own average particle speed, hence its own kinetic temperature. Hydrogen atoms tend to zoom around faster and have a higher "temperature" in aggregate than nitrogen atoms .

The exosphere temperature is the average of all particle velocities in the exosphere.

 

[thalassogen]

I am currently working on finalizing atmosphere composition and pressure, and converting the result into a UWP digit. Most "atmospheres" result in 0 and A (and a lot less in B or 1-9). That's boring - not the fact that these are "real" atmospheres but the fact that there is only one UWP Atm Code for the most common atmospheres (A).

Has anyone ever tried to create a new UWP coding scheme that classifies atmospheres in more detail? A - exotic, e.g., may be everything from Very Thin to Very Dense... (not even speaking of composition)

 

[whulorigan]

I am currently working on finalizing atmosphere composition and pressure, and converting the result into a UWP digit. Most "atmospheres" result in 0 and A (and a lot less in B or 1-9). That's boring - not the fact that these are "real" atmospheres but the fact that there is only one UWP Atm Code for the most common atmospheres (A).

Has anyone ever tried to create a new UWP coding scheme that classifies atmospheres in more detail? A - exotic, e.g., may be everything from Very Thin to Very Dense... (not even speaking of composition)

It was briefly discussed in this thread:
http://www.travellerrpg.com/CotI/Discuss/showthread.php?t=30998

I would add composition as an extended UWP digit indexed to a list of common gas mixes.
Special Supplement 2: Exotic Atmospheres in JTAS #17 might be some help as well, though I do not remember if they had a coding scheme.

 

[Ishmael]

The UWP appears to focus mainly on pressure with composition being an afterthought. But then, the UWP also seems to have originally assumed that the main world was in the habitable zone with liquid water and breathable atmospheres primarily.

I just treat it as relating to pressure where the (Atm^2)/49 gives the surface pressure in atmospheres and then use other methods to guesstimate the percentage of O2 such that finding the partial pressure of oxygen, I can determine if people can breathe unassisted.

Outside the habitable zone, atmospheres are mostly CO2 or N2 depending on whether they are inside or outside the snow line.

Of course, I have not used the standard UWP method for a number of years; I made my own a long time ago.
It makes for a considerably smaller number of inhabited worlds than the OTU.