How I Guesstimate World Data

[robject]

Here's how I script a first-approximation. It ain't pretty, but it's something. I have more complicated code for better guesstimates.



1. Assume density = 1.
1a. Alternately, 0.1 + (atm + flux())/10, but meh.

2. Gravity = density * siz / 8

3. Orbital radius is based on some generic star assumptions and an orbital temperature gradient:

Star Assumptions:
   'A' =>  { r => 5, mass => 2.6,  sqrtmass => 1.6,  au => 2.4, heat => 2250 },
   'F' =>  { r => 4, mass => 1.6,  sqrtmass => 1.3,  au => 1.6, heat => 1500 },
   'G' =>  { r => 3, mass => 1.08, sqrtmass => 1.04, au => 1.0, heat =>  950 },
   'K' =>  { r => 2, mass => 0.83, sqrtmass => 0.91, au => 0.7, heat =>  680 },
   'M' =>  { r => 1, mass => 0.47, sqrtmass => 0.68, au => 0.4, heat =>  450 },

The Orbital Temperature Gradient is an array with 17 elements, numbered 0 thru 16:

[1, 0.5, 0.4, 0.33, 0.25, 0.2, 0.16, 0.1, 0.07, 0.05, 0.04, 0.03, 0.02, 0.015, 0.012, 0.01, 0.009]

3a. temperature subdivision (call it Ts) = 1 - (star subdivision / 30).
3b. mass-squared = star's sqrt mass * Ts
3c. orbit = star's AU * Ts
3d. year = 365 * sqrt( orbit^3 ) / mass-squared, in standard days
3e. mass = star mass * Ts
3f. day = mass / orbit

4. Insolation ( = Analbedo * Greenhouse Effect )
= atm / 20

5. Average Temperature
5a. gradient = orbital temperature gradient[ star's "r" value ]
5b. heat = star's heat index * Ts
5c. tbb = heat * gradient
5d. average temp = tbb * insolation - 273
5e. latitude = srqt( day / siz ) * sqrt( sqrt( tbb/29 ) )
5f. "term1" = sqrt( heat * gradient )
5g. "term2" = mass / orbit
5h. latitude2 = 21.5 * sqrt( term1 * term2 ) / sqrt( siz )


"Density" is in Earths.
"Gravity" is in Gs.
"term1" is the Hadley Cell.
"term2" is the Hadley2 Cell.

 

[shaunhilburn]

Here's how I script a first-approximation. It ain't pretty, but it's something. I have more complicated code for better guesstimates.



1. Assume density = 1.
1a. Alternately, 0.1 + (atm + flux())/10, but meh.

2. Gravity = density * siz / 8

For solid terrestrial planets:

r = SizeDigit / 7.9175

Mass m    =  r^3 * 2^r/2  = sqrt(m/g)
Density d =  2^r/2        =  m/r^3
Gravity g =  r * 2^r/2    =  m/r^2

 

[kilemall]

Woudn't density work backwards from atmosphere vs. size?

In other words a small world with a standard atmo would be more dense and have higher Gs above normal for it's size, and a large world with thin atmosphere would be less dense and less Gs?

 

[shaunhilburn]

Woudn't density work backwards from atmosphere vs. size?

In other words a small world with a standard atmo would be more dense and have higher Gs above normal for it's size, and a large world with thin atmosphere would be less dense and less Gs?

The heaviness of an atmosphere weakly correlates to size, whereas mass, density, and gravity strongly correlate to size. Generally, a world larger than Earth will have a heavier/thicker atmosphere than Earth, and a smaller world will have a lighter/thinner one, but this is not absolute.

Venus is slightly less massive than Earth but has a much heavier atmosphere. Titan being much less massive also has a much more massive atmosphere than Earth. Ganymede, being roughly the same size/mass as Titan, has practically no atmosphere.

 

[Carlobrand]

Woudn't density work backwards from atmosphere vs. size?

In other words a small world with a standard atmo would be more dense and have higher Gs above normal for it's size, and a large world with thin atmosphere would be less dense and less Gs?

Important factors:

Planetary size/density and the composition of the core
Amount of radioactives in that core
Amount of energy received from the local sun/s
Amount of outgassing, 'cause you can compensate for loss to some degree by spitting out more gasses
The composition of the atmosphere
Effects of nearby gravitational sources
Age of the planet, because that affects a lot of the considerations above
And probably other stuff I don't know about - of which there's a lot - or we haven't discovered yet

Venus is roughly Earthlike in size and planetary composition but lies on the too-hot side of the zone of goodness and niceness, so suffers from a runaway greenhouse effect.

Mars is smaller, but at one time in its history it had a magnetic field and enough outgassing to support a decent atmosphere. A younger Mars-like world in the Traveller setting might still have a decent atmosphere and might harbor life.

A small world with a standard atmosphere might be more dense. Or, it might be farther from its primary and therefore colder, with a surface temperature that keeps atmospheric temperatures down where constituent molecules have a lower average velocity and are therefore less likely to escape to space. Or, it might be young and undergoing a lot of volcanism and outgassing - which incidentally could be behind a lot of those atmospheric taints we keep running into. Or, it might be older but still undergoing a lot of volcanism as a result of a higher than normal percentage of radioactives keeping the core hot, or as a result of gravitational effects from orbiting a jovian planet.

I've read articles suggesting Earth's atmosphere was more dense at various periods due to increased volcanism and might have been perhaps half as dense 2.7 billion years ago when the younger sun was putting out less energy. Much is speculative and hotly debated because there are a lot of variables involved, but it's pretty clear that Earth's atmosphere was a lot different in the past. That does leave a lot of room to add color to the world you're designing.