Plausible Exoplanets? (Need to Pick Everyone's Brains)

[Sauron]

So, I'm thinking of setting a story in an alternate solar system (so assume the star is Sol by some other name). I'm trying to come up with a different collection of planets and I want to do a sanity check, get some thoughts (with rigorous scientific backup insofar as it's possible), and brainstorm a bit.

In particular, I want two relatively nearby planets that are plausibly visitable by human explorers.

Currently I'm thinking of the following 3 and I want to check my thoughts on 3 and 5 in particular:

1,2: Hot, tidally locked balls of rock

3: A warmer planet closer to the sun? Not a hothouse like Venus? Something whose atmosphere reflects enough sunlight to keep things (kind of) habitable with some difficulty. How hot? What would a planet like this look like anyway?

4: Earth Analogue

5: Rocky with a thick sheet of water ice and a thin atmosphere (~.1 atm) (made of N2? CO2?). Aproximately Mars gravity with subsurface oceans, volcanism, and very, very dangerous weather (what would that look like?). About as easy to reach as Mars. Temperatures on the surface might occasionally break freezing--more factors for weather? This planet would be the main destination--ISRU is very easy and very good potential for extraterrestrial life

6: 1 sheppard gas giant. More massive than Jupiter with a few more massive moons (icy and organic-compound rich)--more like saturn than Jupiter. Some bodies with volcanoes and/or subsurface oceans

7,8,9: 3 ice giants with various icy moons

10: 1 large, cold gas giant (saturn-sized?) that migrated to a very odd orbit. Lots of interesting stuff but impossibly far out.

Plus some random dwarf planets

[crubs]

All sounds theoretically possible to me, and very interesting also! I'd be curious to hear more about this story. The only problem I have is you describe your Saturn-like ice giant to have an "odd" orbit. What exactly does "odd" mean? Highly eccentric? Highly inclined? both? You could make it a captured planet with both, and have it's inclination close to 90 degrees. That would make it extremely difficult to explore.

[MeticulousMitch]

What you need to do is head right on over to http://en.spaceengine.org/, and download SpaceEngine for free.

After a couple hours of exploring the universe you should have a grasp on what's plausible, and a whole bunch of new ideas for exoplanets too.

Most of what you've described, I've seen somewhere in SpaceEngine.

[Scotius]

It does sound interesting Only thing that seems wrong is requirement for severe weather on your Mars-analogue. Coldness is not a problem. As our own Antarctica shows us, storms in such environment can be real killers. But thin atmosphere...at 0.1 of Earth's air pressure winds will be rather weak. Still cold, but without much punch.

[Tommygun]

Number 3; could be a Dune like world where only the polar areas are reasonable liveable.

The other areas of the planet would be survivable, but very difficult and the equator would be outright lethal.

Number 5; I could be wrong, but with a thin atmosphere, cold and low gravity, I think the weather would tend to be weak?

[-Velocity-]

Planet 5- probably a CO2 atmosphere. If the atmosphere is thin and underwent a lot of erosion, then CO2 is more likely to remain than nitrogen, since it is heavier.

How massive is your planet 3? I'm not sure that I can think of any plausible way to get a thick atmosphere that does not cause significant warming. Venus is a highly reflective planet- if I remember correctly, its albedo is around 60%- but even with all that reflection, intense global warming far outstrips reflection.

How about this- your planet #3 is only kept habitable by a mysterious fleet of ancient, self-maintaining robotic solar shades that hover at the Lagrangian point between the planet and the host star, blocking a large percentage of the star's light. These robotic solar shades were set up by some long-gone, ancient alien civilization. (This is also a plausible way to maintain the habitability of Earth into the distant future, and is far more practical than the only competing idea I know of- expanding Earth's orbit with millions of asteroid flybys.)

Edited September 9, 2014 by |Velocity|

[Streetwind]

Take a look at this video:

It summarizes in a handful of minutes the trends we're currently seeing in other solar systems compared to our own. There's some unexpected stuff in there...

[Sauron]

@Planet 5: .1 atmospheres strike me as enough for weather (in that that's the kind of pressure you'd see within the actual atmosphere)--Perhaps make it a bit denser? With an active core it's possible that planet 5 has enough of a magnetic field to prevent erosion and/or to replace any losses with volcanic gasses (this would lead to a different atmosphere too). But weather doesn't have to be wind--I'm curious about other possibilities here. Also, what the effects of very intermittent above-freezing temperatures (summer flash floods?)

@Planet 3: No aliens here--not really my goal. The possibility of Venus' atmosphere (specifically the thick clouds) reflecting a lot of heat was entertained before we sent probes there. Perhaps a very volcanic world with a modest but very cloudy atmosphere with lots of particulate matter? It doesn't have to be vaguely earthlike, but I don't want it to rain molten metal

@Exoplanet trends: this system is explicitly meant to be a slightly (but not hugely) more exploration-friendly analogue of our solar system. So hot jupiters and the like don't make much sense (unless everything was around a gas giant orbiting in the habitable zone?)

[christok]

3: A high albedo would certainly help. Oceans have a lower albedo than dry land, and dry sand higher than wet sand. Water is also a potent greenhouse gas. You can thus make your planet a little cooler by making it a little drier. Don't make it bone dry. That introduces all sorts of problems on geological timescales, mainly due to water binding atmospheric CO2 to minerals. To further increase the albedo, you can consider either making the crust richer in white minerals or look for something cloud-forming that would add a higher anti-greenhouse effect than greenhouse effect.

5: Volcanism requires a large partially molten metal core in the absence of tidal heating. This could be a little tricky to explain depending the ice sheet, as it should have rapidly grown into an ice giant out beyond the frost line. Perhaps a collision between an icy body and a rocky one would result in something reaccreting with a substantial icy crust, or perhaps its orbit could have been disturbed during formation? Or perhaps the hydrosphere is similar in size to ours, but frozen at the surface? Thin atmosphere and dangerous weather don't sound like a good combination to me. I'd rather make the atmosphere thicker, or subsitute things like toxicity/corrosiveness for dangerous storms. A bit of danger from a (possibly rare) thunderstorm is something you could fit in somewhere. How about a normal atmosphere Snowball Earth instead?

6: Your largest planet should form in the first position after the frost line. This looks fine to me. Volcanism is only really possible at the innermost moon or two unless they're ludicrously oversized. And out far from the planet, they likely won't be. Since this planet must have formed outside the frost line, expect mostly icy moons and captured asteroids outside of the volcanic ones. Subsurface oceans are likely common but very deep down. Smaller moons will be less completely differentiated so the more even mixture of rocky and icy material near the surface is probably more interesting--and more nutritious.

10: I'm thinking about all sorts of potental problems with getting it into that orbit depending what you mean.

I think you're distracting from the story by adding too many planets. I'd probably lose at least 2 and 8-10 but it's your choice.

[-Velocity-]

@Planet 5: .1 atmospheres strike me as enough for weather (in that that's the kind of pressure you'd see within the actual atmosphere)--Perhaps make it a bit denser? With an active core it's possible that planet 5 has enough of a magnetic field to prevent erosion and/or to replace any losses with volcanic gasses (this would lead to a different atmosphere too). But weather doesn't have to be wind--I'm curious about other possibilities here. Also, what the effects of very intermittent above-freezing temperatures (summer flash floods?)

If it's a Mars-sized world, it's not likely to have enough volcanic activity to have a magnetic field unless the system is young. Mars is mostly-dead geologically, at least compared to its initial conditions. It's small and it cooled off long ago. Maybe there are still occasional burps of volcanic activity (which was a real surprise, in fact, when that was discovered), but there is not much. It hasn't been able to sustain a geomagnetic dynamo for billions of years.

0.1 ATM is more than enough atmosphere to support significant weather. Present day Mars has dust storms, cirrus clouds at all latitudes, thicker clouds (stratus?, I donno, but you can look up animated gifs of them) seasonally present around the poles, and even water ice and dry ice precipitation around the poles (the Phoenix lander detected BOTH occurring simultaneously, in fact). Mars is only 0.006 ATM.

Look up a phase diagram of water to figure out under what temperature ranges fresh liquid water would be stable. It won't be that large.

@Planet 3: No aliens here--not really my goal. The possibility of Venus' atmosphere (specifically the thick clouds) reflecting a lot of heat was entertained before we sent probes there. Perhaps a very volcanic world with a modest but very cloudy atmosphere with lots of particulate matter? It doesn't have to be vaguely earthlike, but I don't want it to rain molten metal

I only suggested aliens because it provides an interesting back story. A good writer will provide an interesting, detailed, and mysterious universe to keep a reader engaged. Sure, that can be done without resorting to aliens, but don't limit the scope of your vision too much.

Anyway, the ideas about a cool Venus was also at a time before we really understood how atmospheres worked. We also didn't realize how ridiculously thick the Venusian atmosphere was. I doubt there is any way to have a dense atmosphere with reflective clouds where global warming would not run amok. You're better off thinning out the atmosphere some.

Also, volcanoes spew massive amounts of greenhouse gases into the atmosphere. After they are done spewing out sunlight-blocking ash, the planet heats up. If you had massive volcanic eruptions happening enough to keep ash suspended in the air, I suspect that the atmosphere would be choked with greenhouse gases, probably enough to overwhelm any cooling by the transient presence of ash.

You're better off just making the planet have a thinner atmosphere with less greenhouse gasses. Either that, or imagine a Venus analog moved far out enough where the surface temperatures start to become bearable. Basically, you might want to move your planet #3 OUTSIDE the orbit of planet #5. Present-day Mars is in the habitable zone, after all; if early Earth were placed where Mars is, it would be quite habitable (though the atmosphere would not be suitable for us). Mars is simply too small to hold a large enough atmosphere for significant global warming.

@Exoplanet trends: this system is explicitly meant to be a slightly (but not hugely) more exploration-friendly analogue of our solar system. So hot jupiters and the like don't make much sense (unless everything was around a gas giant orbiting in the habitable zone?)

Some recent research cast doubt on the habitability of exomoons, I can't remember the reason though. However, I'm sure you could find some contradicting research in favor of habitable exomoons somewhere too. Exomoon habitability is a tough problem to solve. Consider that in determining habitiablity for exomoons you have to take into consideration tidal heating, the magnetic field of the host planet, sunlight reflecting off the host planet, the radiation belts of the host planet, etc. IN ADDITION to all the other factors that you have to consider with any planetary body. We'll likely be even further off the mark with predicting their habitibility than we are with predicting "regular" worlds. It's best to stay away from exomoons, IMO.

[PakledHostage]

It does sound interesting Only thing that seems wrong is requirement for severe weather on your Mars-analogue. Coldness is not a problem. As our own Antarctica shows us, storms in such environment can be real killers. But thin atmosphere...at 0.1 of Earth's air pressure winds will be rather weak. Still cold, but without much punch.

And the fact that you need a mix of warm and cold to get really exciting weather. Look at the stormiest places on our own planet. The interior of Antarctica is pretty much a frozen desert. There's not much in the way of precipitation as the antarctic high dominates. Closer to the coast, where katabatic winds tumble down off the ~3000 metre high central plateau and mix with the relatively warm and moist air over the southern ocean is where you get the exciting weather. Likewise, the north wall of the Gulf Stream in the western Atlantic is a notorious breeding ground for nasty North Atlantic storms. There, the warm moist air over the Gulf Stream mixes with the cooler air sitting over cold water brought south by the Labrador current. There are other examples too, including the Gulf of Alaska in the North Pacific and the "roaring forties" in the southern ocean. They are all locations where cold and warm moist air masses mix.

[peadar1987]

Okay, my thoughts (I've tried not to stray outside of what I know)

So, I'm thinking of setting a story in an alternate solar system (so assume the star is Sol by some other name). I'm trying to come up with a different collection of planets and I want to do a sanity check, get some thoughts (with rigorous scientific backup insofar as it's possible), and brainstorm a bit.

In particular, I want two relatively nearby planets that are plausibly visitable by human explorers.

Currently I'm thinking of the following 3 and I want to check my thoughts on 3 and 5 in particular:

1,2: Hot, tidally locked balls of rock

Tidally locked to each other, or to the sun? That should be possible, I think, I know Mercury has very slow rotation, so closer in, you could easily get tidal locking.

3: A warmer planet closer to the sun? Not a hothouse like Venus? Something whose atmosphere reflects enough sunlight to keep things (kind of) habitable with some difficulty. How hot? What would a planet like this look like anyway?

It doesn't need to be as far in as Venus, if you want it a bit hotter than earth, you can move it in a bit closer than earth. Venus orbits at about 0.7AU, move it out to maybe 0.9AU and it'll be a lot more potentially earthlike, only hotter. If you're worried about gravitational interactions with your earth analogue, maybe move that out to 1.1 or 1.2AU and give it an atmosphere richer in greenhouse gases.

4: Earth Analogue

Earth is possible [citation needed] so this should be possible.

5: Rocky with a thick sheet of water ice and a thin atmosphere (~.1 atm) (made of N2? CO2?). Aproximately Mars gravity with subsurface oceans, volcanism, and very, very dangerous weather (what would that look like?). About as easy to reach as Mars. Temperatures on the surface might occasionally break freezing--more factors for weather? This planet would be the main destination--ISRU is very easy and very good potential for extraterrestrial life

You'd need some way of keeping the core molten. If you're looking at the potential for life, a recent giant impact probably isn't the best explanation. Maybe it has several large moons in orbital resonance with each other, warming the planet by tidal heating? I'd need someone who knows more to comment on the plausibility of that.

6: 1 sheppard gas giant. More massive than Jupiter with a few more massive moons (icy and organic-compound rich)--more like saturn than Jupiter. Some bodies with volcanoes and/or subsurface oceans

7,8,9: 3 ice giants with various icy moons

We already have them, so I can't see a problem

10: 1 large, cold gas giant (saturn-sized?) that migrated to a very odd orbit. Lots of interesting stuff but impossibly far out.

Hmm... How did it get out there? I don't think it could have formed there. Rogue planet gave it a gravitational slingshot? Or a mini black hole? Saturn might be too big, maybe a Neptune-sized body would fit better?

Plus some random dwarf planets

Who doesn't love a dwarf planet?

[Sauron]

And the fact that you need a mix of warm and cold to get really exciting weather. Look at the stormiest places on our own planet. The interior of Antarctica is pretty much a frozen desert. There's not much in the way of precipitation as the antarctic high dominates. Closer to the coast, where katabatic winds tumble down off the ~3000 metre high central plateau and mix with the relatively warm and moist air over the southern ocean is where you get the exciting weather. Likewise, the north wall of the Gulf Stream in the western Atlantic is a notorious breeding ground for nasty North Atlantic storms. There, the warm moist air over the Gulf Stream mixes with the cooler air sitting over cold water brought south by the Labrador current. There are other examples too, including the Gulf of Alaska in the North Pacific and the "roaring forties" in the southern ocean. They are all locations where cold and warm moist air masses mix.

With .1-.2 atm you get plenty of variation in temperature, I'd think? (Like Earth, the difference between the equator and poles and like Mars weather patterns based on the thermal properties of the ground).

Perhaps make this planet larger than Mars (~.5G?)--but not hugely--and posit a particularly large amount of radioactive elements in the core or a large moon.

As to the second planet? Not sure.

But in general, besides the Earth-analogue, these planets aren't really meant to be habitable for humans--just slightly more promising candidates for (simple) life and human exploration than, say, Venus.

As to the second gas giant, I imagine it was hurled out into a very wide, eccentric orbit early in the planet formation (it formed closer in, presumably).

[Tommygun]

Also, what the effects of very intermittent above-freezing temperatures (summer flash floods?)

Summer could trigger large ice flows with big slabs of pack ice causing a lot of havoc.

This made me think of this story of how rocks are moved around in Death Valley.

Just imagine this on a planetary scale around the equator with much bigger rocks.

http://www.latimes.com/local/lanow/la-me-ln-rocks-move-death-valley-lake-bed-20140827-story.html