Planetary density?

[Leitz]

The Earth has a planetary density of 5.514 g/cm3. It makes sense that thinner or more dense atmospheres result from a higher gravity, but planetary mass is based on density and size. So smaller worlds have to be more dense to have relatively Earth normal gravity, etc.

What makes planets more dense, or less? Would it just be a larger core or higher amount of metals in the crust? Or what?

Along with that; what levels of gravity relative to Earth are human compatible for extended periods?

 

[atpollard]

Along with that; what levels of gravity relative to Earth are human compatible for extended periods?

This one is easy ... no one knows. We have data on exactly two points: 1G and 0G. People comfortably live their whole lives at 1G and bad things happen at prolonged 0G.

 

[Timerover51]

Stephen Dole, in his book, Habitable Planets for Man, looks at a set of parameters for other planets besides Earth that would be readily habitable by man. He assumes that the maximum gravity that men could tolerate for the long term would be 1.5 gravities, or assuming the planet has a density the same as Earth, a planet 1.25 times the size of Earth, or about 10,000 miles in diameter. He then looks at what he sets as the smallest planet that would be able to retain an adequate atmosphere to live outdoors without extensive equipment, and based on his parameters, assume that would be a planet of 0.75 times the Earth, or about 6000 miles diameter. That would give a surface gravity of 0.68 that of Earth. That does assume a density the same as Earth.

I understand there are those who view Dole's work as a bit out of date, but I view it as still quite usable. If I need a planet a bit smaller than Earth with similar gravity, or am a dealing with one that is in say, the Spinward Marches, and has a standard atmosphere albeit smaller than Earth, I assume a denser planet with either a higher concentration of metals or a greater core to diameter ratio.

A world like Titan, which is smaller than Earth but has a very dense atmosphere, gets that atmosphere by being very cold, with a corresponding lower upper atmosphere velocity of gases meaning less gas escapes from the planet by solar heating. Then there is also the escape velocity of Saturn at Titan's orbital position to consider as well. As for Venus, that planet is a bit of an anomaly.

 

[Enoki]

With respect to the atmosphere, one of the key requirements is a magnetic field. A stronger field will allow a planet to retain more atmosphere. This is a key reason Mars has lost almost all of its atmosphere.

Size, composition, and rotation rate all effect the strength of this field. There are other concerns, but those for Earth sized planets are probably the most important.

Of course, we have only limited data on this right now, so...

 

[aramis]

This one is easy ... no one knows. We have data on exactly two points: 1G and 0G. People comfortably live their whole lives at 1G and bad things happen at prolonged 0G.

You haven't done enough diggin in nasa's publications. Nor ESA.
We have data for some chickens raised in centrifuges.
IIRC 1.2 G wasn't found to be a problem.

https://www.esa.int/Education/Spin_Your_Thesis/Examples_of_past_experiments2

For more than 40 years most of the gravitational physiology studies on animals have been performed inside centrifuges. Initially mice, rats, dogs and chickens were investigated, but toads, rabbits, snakes and crickets later became subjects of study. Nearly all body systems were investigated and influenced by the increase in weight. In the last decade there has been increased interest in the application of chronic acceleration in the study of neuroplasticity, general animal behaviour and cognitive functions. [van Loon, Jack J.W. A.; Tanck, Esther; vanNieuwenhoven, Frans A.; Snoecks, Luc H. E. H.; deJong, Herman A. A.; Wubbels, Rene J.; Journal of Gravitational Physiology, Volume 12, Number 1; July 2005, pp. P5-P10]

Chickens in the centrifuge in Jan '64 Popular Science Magazine on Google

 

[vegas]

What makes planets more dense, or less? Would it just be a larger core or higher amount of metals in the crust? Or what?

As a 0th order estimate, density is going to depend on the mix of water (1g/cm^3), rock (~2.5g/cm^3), and iron (7.9g/cm^3) on the planet.

Weiss and Marcy developed this pair of estimates:

for planets with R < 1.5 Re (Earth radii)

density = 2.43 + 3.39 (R/Re) g/cm^3

for planets with R > 1.5 Re

M/Me = 2.69 (R/Re)^0.93

Small sample size, just 65 exoplanets and the solar system terrestrials, but interesting stuff. I think Traveller needs more high G planets.

 

[Leitz]

There's a 2007 copy of Dole's book that should arrive by Tuesday. I'm posting the code in my repo and will expand it with notes as I figure things out. I'd like to use existing UWP data to influence the density range, we'll see how it goes.

 

[McPerth]

You haven't done enough diggin in nasa's publications. Nor ESA.
We have data for some chickens raised in centrifuges.
IIRC 1.2 G wasn't found to be a problem.

https://www.esa.int/Education/Spin_Your_Thesis/Examples_of_past_experiments2


Chickens in the centrifuge in Jan '64 Popular Science Magazine on Google

See that this may serve for over 1G, but we have no idea (at least AFAIK) about lesser Gs, as would be in a Moon base or a spin hábitat with less than 1G (as many spaceships in 2300AD).

 

[aramis]

See that this may serve for over 1G, but we have no idea (at least AFAIK) about lesser Gs, as would be in a Moon base or a spin hábitat with less than 1G (as many spaceships in 2300AD).

The thing is, given enough data points, a curve can be estimated. We've got multiple data points above 1G.

Especially interesting is the chicken studies that had multi-generational chickens raised in up to 1.5 G's, and how they differed when put in 1G. One of the quirks is that the chickens kept doing somersaults for some time.

 

[TheDS]

Mass attracts mass, so it follows that larger planets are more dense than smaller planets. This would then narrow the diameter range of worlds we would find bearable. Frex, rather than the 6000-10,000 mile range suggested above, it might be 6500-9500 miles.

Obviously, planetary formation is chaotic, and large worlds might be smashed to form small, dense worlds, or small, light worlds might coalesce into a large, light world.

 

[dr fre0n]

What makes planets more dense, or less? Would it just be a larger core or higher amount of metals in the crust? Or what?

Mercury is almost as dense as Earth - a bit less (5.427 g/cm³).

Mercury's planetary body is more compressed than Earth's. If we take the uncompressed density of the respective bodies and we compare, Mercury's uncompressed density is higher than Earth's, explained by the nature of their interior.

Mercury have a (dense) iron core which occupies 42% of its volume for a radius of 1830 km. Earth's solid (dense) iron core is in the range of 1200 km radius, thus not only smaller than Mercury, but very much smaller proportionally.

Here is an excerpt from Faure 2007 (Table 3.4)

Planet	Bulk	Uncompressed
Mercury	5.44	5.4
Earth	5.52	4.2

I think it show how the composition of the body have a role in its density.

Further, bulk densities of planets in our solar system decrease in regular pattern when the distance from Sol increase, explained by their respective chemical composition. (Earth is an anomaly in this context). Indeed, Pluto, as a solid body, is only slightly more dense (2ish) than the typical gas giant (1.7ish).

To summarize, the interior chemical composition is important to determine density among other factors in addition to the position of the planet from its star - which determine which elements were likely 'kept' by that planetary body during the planetary system formation.

 

[starship_captain62]

Don't forget low density super earth's. These seem to be super earth's which have attracted a higher proportion of 'volities.'. Essentially waterworks - somewhere between a large terrestrial world and a mini Neptune. Larger diameter but lower density and suprisingly, similar surface gravity to Earth.

There are a lot of different world types out there that we didn't know about a couple of decades ago.

I suggest that the World generation tables in Traveller could do with an overhaul.

 

[Keklas Rekobah]

Hmm ... a quick look at planetary data tables would seem to indicate that worlds with densities above approximately 3,200 kg/m^3 would likely be "Terrestrial" worlds, while worlds below approximately 2,200 kg/m^3 would likely be "Gas Giants".

The densest uncompressed element is Osmium, at 22,590 kg/m^3. This would seem to set an upper limit on ordinary matter.

I won't pretend to know all of the physics involved in planetary construction; but there must be some kind of "roll 2D + 6 and multiply by 1,000" rule somewhere for determining planetary density.

(Sorry, my copy of World-Builders Guide is on loan to a local professor.)

 

[aramis]

I won't pretend to know all of the physics involved in planetary construction; but there must be some kind of "roll 2D + 6 and multiply by 1,000" rule somewhere for determining planetary density.

There are several... but they don't all agree!

TNE's is a table based

Roll	World Density
6	High
2-4	standard
1	low

Size	Low	Std	High
1	0.1	0.15	0.3
2	0.15	0.25	0.5
3	0.3	0.4	0.8
4	0.35	0.5	1.0
5	0.45	0.6	1.2
6	0.55	0.8	1.6
7	0.6	0.9	1.8
8	0.7	1.0	2.
9	0.7	1.1	2.2
A	0.8	1.2	2.6

Right approach. It has some serious issues, tho', in the details.
1) (Mass)/(distance in radii of earth)²
2) Mass is radius³ * density.
3) density trends slightly higher as radius increases, more so in more liquid envirnments
so, given 1-3...
4) G_s=R_w/R_e * (D_w/D_e) is our reduction, given R as radius and D as density, w as world and e as earth

A reasonable range...
Hmm.
Earth is roughly sg 5.5
Asteroids typically run around "Krasinsky et al. gives calculations for the mean densities of C, S, and M class asteroids as 1.38, 2.71, and 5.32 g/cm3"
So, for metalbody worlds, we can assume a minimum density of about 5. Silicate of about 2.5.
Mars is about 4.
dwarf planets all run about sg 2-3
Earth is the highest known, but we know heavier cores (

So,
a 2d6 table:

Roll	Density (Earths)	SG
12	1.5	8.25
11	1.4	7.7
10	1.3	7.15
9	1.2	6.6
8	1	5.5
7	0.9	4.95
6	0.8	4.4
5	0.7	3.85
4	0.6	3.3
3	0.5	0.275
2	0.4	1.1
1	0.3	1.65
0	0.2	1.1
[tr][tc=3]DM -1 if outer zone DM-1 if size ≤2[/tc]

[td]

Multiply Density by Size/8 to get G's at surface.