I've come up with a couple of exoplanets for a story, and have reached the limits of my knowledge. I've googled around, and I had a space enthusiast friend look at them, but he freely admits that he's just guessing for most of this, especially the atmospheric makeup.
The other planet is here

This terrestrial moon orbits a 21 earth-mass ice giant that orbits a somewhat unstable main-sequence white star at a distance of 4.5 to 6.3 AU. Iyutha's mass is 0.064 earths and its radius is 0.6 earths, with a surface gravity of 0.178g. Surface temperatures range from -130 to -35 C. Its relatively thick atmosphere has a surface pressure of 1.31 atms, and is primarily helium, nitrogen, and carbon, with moderate quantities of hydrogen and trace amounts of fluorine, chlorine, and sulfur. It has large quantities of hydrocarbons, many of which involve the trace elements. Its planet's atmosphere is typical for an ice giant, though with significantly more hydrocarbons and fluorine, giving it a moderate bluish-green color, with storms being darker. The ice giant has 42 other moons, ranging in size from 0.06 earths to 0.8 earths, many of which have accreted moderate-to-thick atmospheres.

Atmospheric and magnetospheric interactions with interstellar hydrogen and helium, as well as solar wind cause frequent auroras that cover large portions of the moon. This applies to many other moons, and even the planet itself, often filling the sky with coruscating colors. Iyutha has moderate volcanic activity, which works with dimly-understood processes in the mantle to free fluorine and chlorine from molecular bonds, periodically releasing it in great plumes rich in hydrogen fluoride and hydrogen chloride that roil through the atmosphere, reacting with everything around them. The surface abounds in intricate geological formations caused by millenia of these plumes and the volcanic activity. Pockets of radioactive minerals are also sometimes freed by these eruptions.

Planetary System
Its planetary system is quite young; it has not yet finished forming terrestrial planets. It coalesced from the remains of several supernovas, which had remarkably high quantities of fluorine and chlorine. All bodies in the system possess notable levels of both of these elements. The recent formation of a pulsar in a neighboring system irradiated much of the system, as well as sending a super-Jupiter hurtling at it. The super-Jupiter collided with the system's largest gas giant on its way out of the system, spreading much of its victim's mass across the system and disturbing the orbits of most of the planets and protoplanets.

More details on the system are here.

How plausible is it that this moon could have developed naturally? If it's implausible, what changes would make it more realistic?

A few specific concerns I had:

  • I had to do a bit of a cop-out in order to get the release of acids. I obviously can't provide scientific support for the freeing of fluorine and chlorine, but is it completely implausible for something like this to occur?

My general goal is to create an interesting exoplanet planet that is thoroughly intimidating to biological life but could reasonably be permanently colonized by a robotic civilization with technology that functions best in the -200 - 200 C range, that has access to smart materials, self-repairing buildings, and nanofabrication. Their ideal gravity range is 0g - 2g, but they can function in up to 4g. Colonization would likely be underground.

The important unique aspects of this planet are that it is a planet with a near-constant radioactive aurora with quantities of liquid and gaseous acids that are far above the norm. Any suggestions for changes to any other aspects that would make the core aspects more plausible are greatly appreciated.

EDIT: Thucydides's post made me realize that my original version is unworkable. I've replaced it with a modified version.

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    $\begingroup$ Reactive elements like oxygen, fluorine and chlorine need some method of replenishment in order to last for geological ages in the atmosphere of a planet. Otherwise you end up with something like Mars, with all the oxygen bound to the iron in the rocks. Fluorine and chlorine are even more reactive, and will be chemically bound in a very small amount of time (as atmospheric gasses, they can even react with each other and other gasses). Unless there is some exotic alien life or geochemical process which can replenish these gasses, they will be ton in just a few million years at most. $\endgroup$ – Thucydides May 4 '16 at 17:55
  • $\begingroup$ Then I could probably reduce the frequency of fluorine and have it be a product of either the supernova or the formation of the white dwarf, and be seeded throughout the interstellar medium. That way it's being steadily replenished. Chlorine and oxygen could be produced, or at least freed via natural processes. I know that chlorine can be released from brine and other salts via electrical current, so maybe frequent lightning storms could be the impetus for it being freed. $\endgroup$ – emo bob May 4 '16 at 18:29

Much of your setup is problematic.

Looking at the history of the system, you state the star has ended its reg giant phase and is now reduced to a white dwarf star. During the red giant phase, all the inner planets will have been "cooked" and the volatile elements will have been lost. Consider that when the Sun reaches its red giant phase, it is estimated that the moons of the outer planets may become inhabitable due to the increased insolation. Saturn, for example is 10 AU from the Sun, twice as far as your postulated planet. If Titan becomes warm and balmy, a planet half the distance will be extremely hot.

Your planet may not have been consumed by the red giant star, but the surface will have become hot enough to perhaps become molten, and the atmosphere and hydrosphere will have been evaporated into space. Plate tectonics may have ceased as the water lubricating the plates evaporated and the plates have "locked up". If the core remains active, the planet could be wracked by massive earthquakes as huge pressure builds up between plate boundaries and then they move in a sudden burst, rather than creeping along at a few millimetres a year.

All that will be left is a dry crust, with perhaps a thin atmosphere of carbon dioxide released during earthquakes or outgassing from the mantle.

The planet will be quite cold and dark, since the white dwarf will provide little light or heat. One thing a new white dwarf will provide is a shower of deadly ultraviolet radiation, which can penetrate to the planet's surface since there won't be an ozone layer. The carbon dioxide atmosphere might break down under the intense ultraviolet bombardment, releasing some of the oxygen, but it will probably preferentially bind with the carbon ash or with exposed iron and other metallic elements in the rock.

As for the other elements of the question, the planetary nebula formed by the death of the red giant will be very tenuous and the planet will not collect much of an atmosphere by passing through it. What atmosphere there is will be mostly hydrogen enriched with some helium and a few heavier elements. Since this was a smaller star, the fusion reactions stopped at carbon and oxygen, some of which might also be in the gasses of the planetary nebula surrounding the white dwarf.

You also mentioned the plasma from a nearby supernova (this must be the unluckiest planet ever), but it too will be very tenuous after crossing interstellar space, so only traces of heavier elements might be present at the planet from this source.

The planet will certainly be an eerie place to visit, and quite dangerous to boot, suffering from being showered by deadly radiation and suffering from periodic massive earthquakes. It's surface will be a cold, dry plain of basalt and eroded sand drifts, and the daytime sky will be dominated by an extremely bright spark too dangerous to look at but which only casts a pinpoint of light to the surface. A thin, whistling wind of carbon dioxide mixed with a few trace elements will be the atmosphere.

  • $\begingroup$ The white dwarf is the least important aspect of this planet, although the realistic resulting planet sounds really cool too! I might use it elsewhere. The core of what I want to keep is the deadly radioactive aurora and the acid rains (ideally fluorine, but others work too). From what it sounds like (correct me if I'm wrong), the best way to get this is to have a basically habitable planet, other than the presence of so many volatiles and so much radiation. Maybe a planet similar to Titan would work? The abundant hydrocarbons and the thick atmosphere seem like they might work well for this. $\endgroup$ – emo bob May 5 '16 at 14:08
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    $\begingroup$ It seems like the best way to replenish fluorine (and possibly chlorine too) is to have the planet orbit a turbulent AGB red giant. This should have the added benefit of producing enough solar wind to create a strong aurora. The downside is that I have no idea if that could produce the planetary conditions I want, and the solar wind seems like it would be slowly pushing the planet away. Maybe the latter issue could be solved by having the planet orbit between two such giants? $\endgroup$ – emo bob May 5 '16 at 14:08
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    $\begingroup$ The aurora simply needs a strong magnetic field and a lot of energy impinging on it, much like we have on Earth. I have to admit I am stumped as to how you could have the acid rain you want. Even massive volcanic eruptions releasing sulphur dioxide into the atmosphere and creating a sulphuric acid rain doesn't seem to meet the requirements, although that sort of thing is deadly on Earth. $\endgroup$ – Thucydides May 6 '16 at 0:46

Your planet is over 3AU from it's star, thus it won't be tidally locked. That's a good thing because if one side faces the start and the other is in perpetual darkness, you'll end up with a huge temperature difference.

You can than use this calculator and play with mass of the star, the distance of the planet, the albedo and greenhouse effect numbers and see what the resulting surface temperature will be. Quick calculation with the numbers you gave shows surface temperature of around -60C which is in the ballpark of where you want to be.

I think it sounds pretty plausible.

  • $\begingroup$ I'm not sure that this addresses the nitty-gritty details of the question, like the retention of the mentioned gases. $\endgroup$ – HDE 226868 May 5 '16 at 1:28
  • $\begingroup$ @HDE226868 there are 5 separate questions. I think the planet sounds reasonable and I don't see a problem with it existing around a white dwarf, as for the rest, I don't know. $\endgroup$ – ventsyv May 5 '16 at 14:48

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