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Very small worlds with dense atmospheres
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| Diakonov | Date: Sunday, 10.01.2016, 03:04 | Message # 1 |
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| Hi, I'm using Space Engine for a long time and observed, when I create some worlds around ficticeous stars (main sequence stars), some of them appear to be very small to have dense atmospheres. I observed that some worlds even very small, with very high temperatures, could have atmospheres way denser than Earth's. Is that correct? Can even small worlds, being very hot, have air denser than Earth or Mars? Also observed that some massive rocky planets have a very thin atmosphere. It's possible for a super-earth to have an atmosphere way thinner than earth's?
This is only for some generated values. For example, when I create a planet without atmosphere code. It appears that Space Engine create some unrealistic worlds.
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| Diakonov | Date: Sunday, 10.01.2016, 23:03 | Message # 2 |
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Astronaut
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| Here are some examples of unrealistic generated planets:
Earthlike gravity but almost no atmosphere.
Marslike gravity but very dense atmosphere.
Too much hidrogen for a planet with earthlike gravity.
If I remove the gas giant, things start to be more realistic though.
But here, still too much atmosphere for such low gravity.
There appears Space Engine has a serious problem with it's calculations...
This time is an extreme moon... More than 1000K, 0.15 G, smaller than Pluto and still several times the pressure of Mars! :o
And now, one of the most irrealistic of all:
Very low grav, high pressure and very high temperature. And the planet itself evaporating!!!
Or Space Engine needs to be revised or Space Engine knows more about the universe than anyone on Earth! 
Edited by Diakonov - Sunday, 10.01.2016, 23:09 |
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| HarbingerDawn | Date: Monday, 11.01.2016, 01:04 | Message # 3 |
 Cosmic Curator
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| Quote Diakonov (  ) Earthlike gravity but almost no atmosphere.
What's unrealistic about that?
Quote Diakonov ( ) Too much hidrogen for a planet with earthlike gravity.
Incorrect. See Saturn and Uranus and Neptune. Surface gravity isn't as important as mass.
Quote Diakonov ( ) Very low grav, high pressure and very high temperature. And the planet itself evaporating!!!
Again, how is that unrealistic? At the very least, it makes sense that the planet would be evaporating. You do know that evaporating planets are real, right?
All forum users, please read this!
My SE mods and addons
Phenom II X6 1090T 3.2 GHz, 16 GB DDR3 RAM, GTX 970 3584 MB VRAM
Edited by HarbingerDawn - Monday, 11.01.2016, 01:06 |
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| Watsisname | Date: Monday, 11.01.2016, 01:55 | Message # 4 |
 Galaxy Architect
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| Diakonov, you're making the same error that I made once. Don't use surface gravity when considering atmospheric loss. Use escape speed. Escape speed determines if a particle will escape, and it depends on the mass and size of the planet differently than the surface gravity does. A massive and low density planet can have a low surface gravity but a large escape speed!
SE does use appropriate formulas for generating atmospheres and calculating their loss. Granted, it's not perfect, but most of what I see here does not seem too suspicious. In general, thermal atmospheric loss depends on molar masses of the gases, escape speed of the planet, and temperature at the exobase.
Quote Diakonov ( ) Earthlike gravity but almost no atmosphere.
This is not necessarily unrealistic. Many processes can promote atmospheric loss, like sputtering from solar wind, dissociation and heating by UV (especially close to M stars), or even direct ejection by large impacts.
Quote Marslike gravity but very dense atmosphere.
It has 15% of Earth's mass, and 61% the size, so the escape speed is half of Earth's. Low, but enough to retain heavy molecules like CO2. What is the molar mass of the atmosphere?
Quote Diakonov ( ) Too much hidrogen for a planet with earthlike gravity.
This is 10 times the mass of Earth! With 2.8 times the diameter, that makes the escape speed 90% greater. In reality, this is approaching ice giant territory.
Quote This time is an extreme moon... More than 1000K, 0.15 G, smaller than Pluto and still several times the pressure of Mars!
This does sound less plausible. But what is the molar mass? At those temperatures you may be dealing with a sodium-oxygen atmosphere.
Quote Very low grav, high pressure and very high temperature. And the planet itself evaporating!!!
Same idea as above.
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| Diakonov | Date: Tuesday, 12.01.2016, 00:20 | Message # 5 |
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| Ok, understood. Here's what I found about molar mass and escape velocity:
Escape Velocity: http://hyperphysics.phy-astr.gsu.edu/hbase/vesc.html
Molar mass: http://hyperphysics.phy-astr.gsu.edu/hbase/kinetic/kintem.html
Blackbody Radiation: http://hyperphysics.phy-astr.gsu.edu/hbase/wien.html
Elements and states of matter: http://www.ptable.com/
And now some comparisons:
World 1 (hot selena):
MEarth: 0.01
rEarth: 0.274
Temperature: 1450 °C
Vescape: 2135 m/s
World 2 (cold selena):
MEarth: 0.01
rEarth: 0.274
Temperature: -150 °C
Vescape: 2135 m/s
World 3 (scorching subearth):
MEarth: 0.25
rEarth: 0.61
Temperature: 750 °C
Vescape: 7157 m/s
World 4 (warm subearth):
MEarth: 0.25
rEarth: 0.61
Temperature: 50 °C
Vescape: 7157 m/s
World 5 (scorched superearth):
MEarth: 2.5
rEarth: 1.37
Temperature: 2000 °C
Vescape: 15102 m/s
World 6 (gasdwarf):
MEarth: 10
rEarth: 2.82
Temperature: 0 °C
Vescape: 21053 m/s
Not let's compare...
World 1 has similar mass to our moon, but is scorching. For N2 Ve is ~1141 km/s. For elemental sodium is ~1259 km/s. For CO2 Ve is ~ 910 km/s. Still inferior to world Ve, but very next to it and also this world is very next to the star, making the solar wind extreme violent, destroying any dense atmosphere there. I expect at least a thin atmosphere.
World 2 is much colder, but still similar to the moon. For N2 Ve is ~336 km/s. For CH4 is ~403 km/s. Weak solar wind, so probably will be more similar to Titan. Even more if it's a moon around a massive gas giant and next to it, making it very geologic active and so through volcanism creating a more dense atmosphere.
World 3 is a scorching subearth. N2 Ve is ~879 km/s. CH4 Ve is ~1163 km/s. For a planet that mass, probably a thinner atmosphere than ours, about 0.25atm. But I will not expect a 100atm in such world, unless it's made most of gases like CO2 or SO2, but even not near compared to Venus. Probably 25atm at most.
World 4 is the warm version of the other, like a warm marslike. N2 Ve is ~494 km/s. CH4 Ve is ~653 km/s and CO2 is ~394 km/s. It can still have a dense atmosphere, even more if it orbits a massive gas giant. Imagine a marslike world a bit bigger orbiting Jupiter closer to sun. It could be a heaven for flying species, dense air + low gravity!
World 5 in an experimental scorched superearth to it's limits. I expect a very dense atmosphere, even that hot! And obviously a bright comet-tail, an evaporating planet! Would you go there? Want to know how hell is? Visit world 5!
World 6 is a gasdwarf at temperate temperature. Looks like it can hold a lot of hydrogen, but obviously not as a much colder version of it such as Neptune. And I expect this to have a comet-tail.
Didn't expect some small worlds, even hot could hold so much atmosphere. As it's observed in our solar system, most of small worlds have thin atmosphere and bigger ones a dense atmosphere, with Venus and Titan as two exceptions. Venus because of its heavy gases and Titan because of its low temperature. It's strange that a massive world can have thin atmosphere, maybe if it's a dead planet, a planet orbiting a pulsar or one that had violent impact hits.
Now it means that in the system I'm making (future add-on) I can make small moons with dense air without having to worry they are unrealistic. Maybe even a big asteroid moon with a thin atmosphere near a massive planet... 
But I will not make any big planet with thin air in that system, that's guaranteed. 
Edited by Diakonov - Tuesday, 12.01.2016, 00:25 |
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| Watsisname | Date: Tuesday, 12.01.2016, 02:51 | Message # 6 |
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Galaxy Architect
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| Diakonov, I made an excel sheet with formulas if you want to use it to easily calculate lifetime of atmospheres for your planets. You can download it here. Simply enter the mass and radius of the planet (in units of Earths), temperature at the exobase (in Kelvin, and as reported in SE's info tab > atmosphere parameters), and molar mass of the gas (in g/mol).
The spreadsheet will calculate the escape speed, average particle speed, and the ratio of these (Ve/Vo). If the ratio is less than 4, the atmosphere escapes rapidly. If 4, the lifetime is thousands of years. 5, millions of years, and if 6, billions of years (lifetime of solar systems.)
I put in some solar system planets and your own for you, though with your surface temperatures in place of the exobase. Generally the exobase is hotter.
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| Diakonov | Date: Tuesday, 12.01.2016, 13:16 | Message # 7 |
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| Thanks, this will help to make my own system more realistic.
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| Diakonov | Date: Tuesday, 12.01.2016, 16:55 | Message # 8 |
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| Now, this is a proposital impossible planet!
A planet very similar to Earth, with some kind of liquid (possible liquid sulfur that is black at those temperatures) and green vegetation based of carbon and sulfur compounds. It is very small and the air is more pressurized than Venus. It also has a comet tail to make things even more impossible. This is a very wrong planet indeed.
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| Diakonov | Date: Wednesday, 13.01.2016, 13:24 | Message # 9 |
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| Space Engine allows you to make those miracles...
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| Watsisname | Date: Thursday, 14.01.2016, 03:00 | Message # 10 |
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Galaxy Architect
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| Sure it does. It will create whatever you tell it to create, and will apply formulas to determine its properties, whether the situation is realistic or not. I'm pretty sure you can even put a comet in an orbit inside of a star if you want.
I guess I just don't quite understand what you're doing? What do you want to create and how are you going about it?
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| Diakonov | Date: Thursday, 14.01.2016, 11:45 | Message # 11 |
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| Ah, I'm creating, I hope, a more realistic planetary system. It's called Teran, a 3 star system. Each star is distant enough to have planets, rocky and gaseous, but not so distant, about 24 AU distant each other, both M2V, and a third M5V orbiting around both, at 72 AU from the barycenter. Teran A had a planetary system more like ours, with many planets (3 of them superearths and 1 ocean planet) one 0,5 jupiter mass planet at a more distant orbit, one 0,375 jupiter mass planet and some more distant icy worlds. Teran B has 3 gas planets (with smaller masses) next to the star and more distant superearths, with the first being the core of an evaporated gas giant, and each of these gas planets have captured moons. In Teran A rocky planets formed where they formed and in Teran B the gas giants migrated inward, pushing the superearths to more distant orbits, and these are rocky worlds, but more lightly built. Both systems are no more than 5 AU in size.
As for Teran C, I'm still deciding what kind of system will have. Soon I'll create a topic about my system.
Edited by Diakonov - Thursday, 14.01.2016, 12:18 |
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| Diakonov | Date: Thursday, 14.01.2016, 14:01 | Message # 12 |
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| There's also a problem I found, about greenhouse. It appears that Space Engine only computes greenhouse values up to 3 atm pressure, above this, the greenhouse value is the same, but as far I know, the more pressurized the atmosphere, the more it traps heat. For example, in the system I'm creating, there are 2 big moons, one which air pressure is 12,5 atm and the other 25 atm, but the greenhouse value is almost the same. Looks like I'll have to put greenhouse values above 3 atm manually. The correct greenhouse value for 12,5 atm should be around 70°C and for 25 atm 122°C, considering atmosphere type and the distance of those moons to the parent star. Because when I put 2atm the value was ~27°C and for 3atm ~31°C.
Edited by Diakonov - Thursday, 14.01.2016, 14:15 |
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| Diakonov | Date: Thursday, 21.01.2016, 16:27 | Message # 13 |
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| In the next version of Space Engine, the greenhouse value should not be limited to up to 3 atm. Certainly a world with 10 atm will have a stronger greenhouse value than 3 atm. It also would be interesting if there was a classification of the gases in such atmospheres. For example, if the atmosphere had more methane in the air, the greenhouse would be much higher than if with the same air pressure it had less methane. Titan should have a stronger greenhouse effect on the sea level than Earth, due to the high amount of methane in the air, even though less heat reach the surface.
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| Watsisname | Date: Friday, 22.01.2016, 04:25 | Message # 14 |
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Galaxy Architect
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| Quote Diakonov ( ) The correct greenhouse value for 12,5 atm should be around 70°C and for 25 atm 122°C, considering atmosphere type and the distance of those moons to the parent star. Because when I put 2atm the value was ~27°C and for 3atm ~31°C.
Huh. Are you assuming the greenhouse effect is linear with pressure (for a constant gas mix)? I'm pretty sure that it should be either a log or root function of some kind. What does SE say is the greenhouse magnitude for 0.5atm and incrementing by 0.5 up to 3?
Quote Diakonov ( ) Titan should have a stronger greenhouse effect on the sea level than Earth, due to the high amount of methane in the air, even though less heat reach the surface.
The greenhouse effect is stronger on Titan, in the sense that that a larger fraction of the outgoing thermal radiation is absorbed by the atmosphere (~90% on Titan vs ~60% on Earth). The greenhouse effect there is also caused by hydrogen and nitrogen, not just the methane. (This seems surprising since normally we don't think of those as greenhouse gases, but they are on Titan because the greater air density induces vibrational modes which let them interact with radiation at those wavelengths.)
However, the magnitude of the warming this causes is only about 10K, or about 1/3 as much as on Earth. This is because there is only 1% as much solar flux, so the equilibrium temperature is much lower (~85K), and there is also a sort of "anti-greenhouse" since the haze layers absorb visible light and efficiently radiate thermal.
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| PlutonianEmpire | Date: Saturday, 23.01.2016, 03:01 | Message # 15 |
 Pioneer
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| Shouldn't the forum be auto-merging Diakonov's triple post? I could've sworn it does this sometimes....
Specs: Dell Inspiron 5547 (Laptop); 8 gigabytes of RAM; Processor: Intel® Core™ i5-4210U CPU @ 1.70GHz (4 CPUs), ~2.4GHz; Operating System: Windows 7 Home Premium 64-bit; Graphics: Intel® HD Graphics 4400 (That's all there is :( )
Edited by PlutonianEmpire - Saturday, 23.01.2016, 03:02 |
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