What Atmosphere Would a Metal Planet Have?

[TheBlacksmith]

Alright, I'm not really a physics major or a scientist, but I do have a basic understanding of physics. In fact, I'm actually a writer and right now I'm trying to research the physics of what this thread is titled as, a metal planet. Problem is since I'm not a physics major or scientist, I sort of have to rely on psuedo-science and so I found this site through google in hopes that some of you could help give me a better understanding of what I'm trying to build.

Now, let me go into details about this metal planet...

- It's an iron-based planet, made mostly of a bio-organic metal earth/dirt.
- It's about the size of Mars, maybe a bit smaller.
- There isn't a lot of water besides the steam that rises out of huge valleys in-between continents. (Think of an ocean valley in-between north America and Europe.)

- There are also some measurements I put together by looking at graphs of other planets.

Distance from the Sun in its Solar System: 73.6 million miles [118 km/s]
Orbital period: 144.3 Earth Days [One Year]
Avg. orbital speed: 7.84046 mph
Mean radius: 3,385.2 ± 0.1 km [Half the size of earth]

- There's two small moons.

(As for the measurements, they may be a bit sketchy as my math is rusty, lol, so please correct me on any of that.)

Okay, on the subject at hand, what I'm wondering is for this metal planet, what kind of atmosphere would it have? What kind of material would life be based off of (humans being carbon based)? Also, what other information would be required to answer these questions?

P.S. - This is all hypothetical.
Thanks.

 

[russ_watters]

Well first it is a solar system, not a galaxy. Second, a mile and a km/s are not the same type of units. Third, how did you calculate those numbers? Just looking at Venus's orbit (closer to the sun but longer period) tells me that those numbers are not possible.

This link has a graph of the equation for distance vs period: http://www.astro.washington.edu/larson/Astro150u/CoursePak/cp06_orbitaldistanceorbitalperiod.pdf

If the planet is roughly the size of Mars and not too much heavier, it wouldn't be able to have much more of an atmosphere than Mars, and it would be mostly carbon dioxide.

 

[marcus]

you don't mean [118 km/s]
you meant to say [118 million km]
Why not correct it while you can still edit your post. I think there is a 24 hour limit on editing.


Also you haven't said what the rotational period of the planet is.
You need to say, in terms of some Earth unit (like an Earth second or Earth hour) so we can picture how rapidly it turns.

And when you say the orbital period is 144.3 days, what days do you mean? Earth days, or the planet's own days (which please define how long.)

You probably will be recalculating the orbital speed of the planet, so don't worry about that now. First tell us how long the year lasts in terms of some time familiar Earth time unit (unless you mean 144.3 Earth days, which would be fine.)

 

[TheBlacksmith]

Well first it is a solar system, not a galaxy. Second, a mile and a km/s are not the same type of units. Third, how did you calculate those numbers? Just looking at Venus's orbit (closer to the sun but longer period) tells me that those numbers are not possible.

This link has a graph of the equation for distance vs period: http://www.astro.washington.edu/larson/Astro150u/CoursePak/cp06_orbitaldistanceorbitalperiod.pdf

If the planet is roughly the size of Mars and not too much heavier, it wouldn't be able to have much more of an atmosphere than Mars, and it would be mostly carbon dioxide.

I came up with those numbers by looking at the numbers of Earth and mar's size, distance, and speed, and then altered them to fit the kind of planet I need. I mean I made the planet to be that size after considering the weight of it would be incredible, so a smaller planet and a slower rotation on its axis would (again, hypothetically) make the gravity lighter.

Anyway, could you explain how the size of the planet determines whether it has an atmosphere by the way?

you don't mean [118 km/s]
you meant to say [118 million km]
Why not correct it while you can still edit your post. I think there is a 24 hour limit on editing.

Also you haven't said what the rotational period of the planet is.
You need to say, in terms of some Earth unit (like an Earth second or Earth hour) so we can picture how rapidly it turns.

And when you say the orbital period is 144.3 days, what days do you mean? Earth days, or the planet's own days (which please define how long.)

You probably will be recalculating the orbital speed of the planet, so don't worry about that now. First tell us how long the year lasts in terms of some time familiar Earth time unit (unless you mean 144.3 Earth days, which would be fine.)

Ah, thanks, I make the edits accordingly.

Yeah, I had trouble with figuring out the rotational period of this planet. Considering that I was trying to more or less copy Earth's conditions, the rotational period would probably be twice as much as earth's. I don't know. If I had to be honest, I'd shamefully admit I kind of threw some of it together. However, yeah, I did mean Earth days.

 

[Janus]

I'd have to say that your period to distance doesn't work unless your Sun was some 3 times more massive than our Sun, and if that was the case, due to the relationship between mass and luminosity, it would be nearly 47 times brighter/hotter than our sun.

for a Star of our Sun's mass, I get a orbital period of 255.13 days and an orbital velocity of 33.63 km/sec. (75669 mph)

 

[Janus]

Anyway, could you explain how the size of the planet determines whether it has an atmosphere by the way?

All other things being equal, the thickness of your atmosphere depends on escape velocity. For the surface of the Earth, escape velocity is 11.2 km/sec. For a planet the size of Mars, but dense enough to have a surface gravity equal to that of the Earth, the escape velocity would be only 8.16 km/sec, which means that it will be harder for this planet to hold on to its air.

 

[LURCH]

It should be added, however, that the determining factors for gas retention (I mean the planet's; not my own) are not well understood. Venus, for example, has an escape velocity of only 10.4 km/s, yet it has a much thicker atmosphere than Earth's. Magnetic fields are also believed to play an important role, and your metal planet should have that in spades. This will prevent Solar winds (well, stellar winds, since the planet is not orbiting Sol) from blowing away the gasses that form the atmosphere.

As to the questions at the end of the OP; the composition of both the atmosphere and the life-forms is completely up to you. I mean, just because it is a metal world doesen't mean there are no non-metallic ellements, right? After all, Earth is considered a "rocky" world, yet we have hellium and oxygen and nitrogen and all this other great stuff.

You could try basing the life-forms on metal, as life is likely to be made of the most abundant materials available, but it's a fairly tricky proposition. You'd have to make up a whole new biochemestry scheme (maybe using complex sulfuric acid molecules to replace DNA), and determine how it's powered and everything. I suppose the base of the food chain could be photovoltaic cells that take the place of photosythysis, and then be consumed by more complex metal organisms.

 

[Savant13]

I came up with those numbers by looking at the numbers of Earth and mar's size, distance, and speed, and then altered them to fit the kind of planet I need. I mean I made the planet to be that size after considering the weight of it would be incredible, so a smaller planet and a slower rotation on its axis would (again, hypothetically) make the gravity lighter.

Unless it is spinning fast enough that centrifugal effects become important (REALLY fast), its rotational period will have no discernable effect on perceived gravity.