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Obize is an Eden-like moon orbiting the barren terrestrial planet Akaria. This planetary system is special in the fact that the two worlds are complete contrasts to each other. The moon has plentiful liquid water, comfortable temperatures and is perfect for life with conditions (relatively) identical to Earth with native fauna and flora adjusted to the moon's low gravity. The planet it orbits is a dead barren world no different than Mercury.

I am wondering, how could this happen?

Akaria and its moon orbit their parent star smack dab in the middle of its habitable zone. Akaria is roughly 160% the size of Earth while Obize is roughly 60%. The parent star is similar to Earth's sun.

The conditions in the rest of the system and planetary composition is unknown and up to you to decide. What could cause this odd pair to come about? Answers can range from hard scientific to those with high creative liberty. You choose. Right now I would just appreciate any ideas. I can't think of any reasons why the planet would be barren while the moon is paradise like.

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    $\begingroup$ Please note that you can't have conditions identical to Earth on a sphere only 40% the size of Earth (Gravity roughly 0.4G). You could have a moon the size of Earth (and therefore Earth-like) orbiting a Jupiter-sized world.... $\endgroup$ – JBH Aug 29 '18 at 21:48
  • $\begingroup$ @JBH Yeah, I want the parent planet to be terrestrial though without having the moon and planet having some sort of twin planet relationship. So I don't want the moon too big. I guess its conditions aren't 100% identical and the fauna and flora on the moon would be adjusted to 0.4 gravity? $\endgroup$ – Noble Aug 29 '18 at 21:53
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    $\begingroup$ I'm changing my mind. Mars (mass = 0.639E24) is about 10% the mass of Earth (5.972E24) and yet has a thin atmosphere that tempts us to believe we can terraform it (to a degree). At 0.4G (40% the mass of Earth) you'd have perhaps a thin alpine atmosphere. Livable, but think Mongolia. Maybe bump it to 60% and the parent planet something like 300%. That would be believable to me. BUT, note the ratio of our large moon (.00735E24) to our planet: almost 850X. So maybe the primary needs to be 350X or 3500% the size of Earth to make this work. $\endgroup$ – JBH Aug 29 '18 at 21:59
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    $\begingroup$ "The planet it orbits is a dead barren world no different than Mercury." No different in what sense? The question already describes at least three major differences: size, having a moon, being in the habitable zone. $\endgroup$ – Peter Taylor Aug 30 '18 at 7:31
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    $\begingroup$ Are those percentages relative to Earth comparing MASS or DIAMETER? $\endgroup$ – Spencer Aug 30 '18 at 17:57

13 Answers 13

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The barren planet has no magnetic field, and its atmosphere has been stripped away by solar wind.

The creative liberty here would be: how would the larger planet have cooled to the point that its core is no longer generating a magnetic field, while the smaller planet has retained enough heat to keep a magnetic field going? I'm not certain of an obvious hard-science solution to this off the top of my head. Perhaps the moon formed much more recently and then was captured in orbit around the (much older) planet?

[Update, based on further thought and comments in discussion: there seem to be two apparent ways to resolve the problem of the moon having a magnetic field while the planet does not. Both of them could be achieved with the introduction of a rogue body into the system to help form the moon.

  1. The planet's molten core is composed of primarily non-metallic elements, while the moon's molten core contains sufficient metal to give rise to an atmosphere-retaining magnetic field. (thanks to MikeB)

  2. The planet cooled long ago, while the moon was formed more recently by the collision of two bodies. (my thought; similar thought from Rodolvertice)

To the best of my understanding, a rogue body from outside the planet's orbit impacting an already-formed moon would be the most likely/most sensible way for either of these arrangements to occur. If the moon was formed only from the same material that initially formed the planet, it would be difficult to explain a metallic core for the moon and a non-metallic core for the planet. If the moon was formed from the collision of two already existing moons, one would expect the collision to happen much earlier in the lifetime of the system, and it stands to reason that the resulting combined moon would still cool faster than the planet. (Disclaimer: astrophysics/planetology expertise needed to confirm these intuitions)

But introducing a rogue body to collide with a pre-existing moon in orbit around Akaria would avoid these problems and result in a hot-cored, as-metallic-as-you-need it Obize!]

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  • $\begingroup$ Life wouldn't have time to form with the time-based solution. Can a moon 40% the size of Earth, even with a hot core, generate enough magnetic field to protect itself? If so, this is a pretty good idea, but it needs to be proven and/or rationalized. I thought of a second moon that would create gravitic stress to keep the heat going, but that might kill the life on the primary moon. $\endgroup$ – JBH Aug 29 '18 at 21:52
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    $\begingroup$ The tidal stress from the large Planet could also keep its core warmer longer $\endgroup$ – Garret Gang Aug 29 '18 at 23:46
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    $\begingroup$ It's important to note that having a molten core isn't enough to generate a magnetic field; you need a molten metal core. And a significant fraction of the heat in Earth's core is generated by radioactive decay. So you can say that your moon got the requisite metals and radioactive materials to form and maintain a liquid metal core and magnetic field, while the planet did not get enough of these. A plausible explanation for that is that the moon was captured by the planet, not formed from an impact with the planet like Earth's moon. $\endgroup$ – MikeB Aug 30 '18 at 3:04
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    $\begingroup$ How about a planet with 2 moons forming, everything cooling down, but then the 2 moons eventually colliding and warming up, forming a single hot-cored moon? $\endgroup$ – rodolphito Aug 30 '18 at 5:21
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    $\begingroup$ This could happen based on spin. If Akaria suffered a late big impact (after its moon formed) that slowed its spin way down (but not the forming moon's) then its magnetic field might be affected... $\endgroup$ – Sean Raymond Aug 30 '18 at 7:38
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I have two answers which could either stand alone or be combined.

  1. Akaria is much more volcanically active, this makes sense because it is a larger body and cannot cool as fast as Obize. The volcanic activity increases the surface temperature as well as putting gasses into the atmosphere.

  2. Akaria just has more greenhouse gasses in its atmosphere due to it having more gravity. The escape velocity for a gas to leave it is just a lot larger. The atmospheric pressure on Akaria would also be larger.

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    $\begingroup$ An Eden like moon orbiting a hellish planet? How very biblical. $\endgroup$ – Joe Bloggs Aug 30 '18 at 5:39
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    $\begingroup$ Runaway greenhouse effect on the larger planet. I like it. +1 $\endgroup$ – kingledion Aug 30 '18 at 13:02
  • $\begingroup$ So basically Venus with a moon? One that is larger than Luna? $\endgroup$ – hszmv Aug 30 '18 at 16:14
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    $\begingroup$ There's actually no good reason why the two should be very similar, very few moons have similar conditions with the planet they orbit. Usually it's only the elemental composition that is constant. Our moon is a barren wasteland compared to the earth. $\endgroup$ – Mathaddict Aug 30 '18 at 20:41
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Here are a few possibilities that come to mind, based on various aspects of planet evolution:

  1. Akaria is simply in a non present Earth-like state while Obize is. For most of Earth's history the surface did not look like it does today. For instance, trees only came along in the last 10% of Earth's history (see here). Earth as we think of it -- with surface plants and ocean life and large animals -- did not exist for the first 90% of Earth's history. If you found Earth when it was only 3 billion years old it would not look like Earth.

For your story, it's totally plausible to imagine two settings in which one evolved a bit faster than the other. The reason why could be linked with the smaller size of Obize, e.g., becuase its interior cooled faster so there could have been more vigorous tectonics early on. Speculative but not crazy.

1b. On a more common vein, Akaria could be trapped in a "snowball Earth" state whereas Obize is not. It would be an icy desert instead of a hot one, but still not habitable. (e.g., see here).

  1. Akaria got stuck in a prolonged magma ocean phase while Obize did not. When rocky planets form, the heat that is involved in giant impacts creates a very hot (basically molten) surface. Planets farther from the sun cool off faster than close-in planets (see here). Large planets also cool slower than small ones. So, you could imagine that Obize escaped this phase early on and retained a large fraction of its water but Akaria had a long (~100 million years or longer) phase that led to complete planetary dehydration and a desert planet.

  2. Akaria is actually a Neptune-like planet while Obize is a terrestrial planet. Statistical analyses of exoplanets of different sizes have found that the division between "super-Earths" (large rocky planets) and "mini-Neptunes" (scaled down versions of Neptune) is at about 1.5-2 times Earth's size (see here). So it's plausible that Obize is simply more like Earth and Akaria is more like Neptune. If you want to go that direction....

Interesting question. Those answers are what jumped to mind....

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You're not going to avoid a planetary atmosphere. The planet by definition is heavier than its moon; it will have collected and kept more gases.

That doesn't mean it's anywhere livable. Think Venus, nor Mercury. The planet won't be as hot as Venus, but thick sulphuric acid clouds are just lethal when they're near 300K.

If you need it to be less lethal, you might have other gases. SF6(Sulphur hexafluoride) is heavy but not very toxic. It would however displace oxygen, explaining why the surface is inhospitable. And as it's a greenhouse gas, the planetary surface would be quite a bit hotter than the moon's surface, despite being at the same distance to their sun.

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Could Akaria always have the same side turned toward the parent star, but not Obize also? This would make all of the water only the side turned toward the parent star turn to vapor, which then freezes permanently on the side turned away.

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  • $\begingroup$ It's called beeing tidal locked (from Physics Stack Exchange) with its star, like our Moon with the Earth. $\endgroup$ – Ckankonmange Aug 30 '18 at 9:13
  • $\begingroup$ Good idea. Though in the solar system, none of the tidally locked objects has itself a natural satellite – the close orbits seem to make it unlikely for satellites to be captured. But I do reckon it's possible. $\endgroup$ – leftaroundabout Aug 30 '18 at 12:09
  • $\begingroup$ That would still leave a habitable strip on Akaria, eternally in twilight. With a lot of wind, though. $\endgroup$ – fishinear Aug 30 '18 at 15:02
  • $\begingroup$ This is a nice idea but I'm not sure it works. If tides from the star are strong enough to cause Akaria to tidally lock, then tides would also cause Obize to lock. In this situation tidal evolution generally destabilizes the moon (see here for technical reference: arxiv.org/abs/1206.0334). But destabilization can take billions of years so it's plausible that this system is just on the way to destabilization and has two tidally-locked Eyeball planets -- see wp.me/p3BSYQ-8c and nautil.us/blog/… $\endgroup$ – Sean Raymond Aug 30 '18 at 15:51
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    $\begingroup$ But, given a truly habitable moon, would that strip really be considered habitable? Sure, Antarctica is 'habitable' but it's really not. The moon would outshine the small strip of barely habitabke land. $\endgroup$ – Andon Aug 30 '18 at 16:45
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Akaria took a substantial asteroid impact not too long ago. Life was wiped out (perhaps some deep bacteria survived, perhaps nothing did.)

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Rocky planets are created basically without an atmosphere, the lighter stuff comes later from cometary bombardment. In your system the bombardment was much heavier than ours, Akaria has a thick enough atmosphere that the greenhouse effect ensures there is no liquid water and thus no life. (Think Venus-lite, perhaps without the acid.)

Obize didn't suffer this same fate because Akaria is more massive and drew in the majority of the comets.

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The larger planet traveled inwards until it's orbit is stabilized when it picks up the smaller planet as a moon.

This explains why the two would be so different from each other. You can then accept one of the many reasons why they are different from other answers (i.e. magnetic field, green house gases, volcanic, etc.. etc..)

Since the moon never leaves the Goldilocks zone it retains it's water, magnetic field and other ideal conditions.

The further out the larger planet originates from would yield a more primitive world.

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Obize could very well be a new Eden whose life came from Akaria. Environmental mismanagement could have wiped out anything that would support life on Akaria and just a small bit of life was able to be brought to the moon and take root, and survive. There is argument as to how likely life is to arise even on a planet that would support it as we know it--statistically speaking it is more likely to arise on larger object--the planet Akaria.

Why isn't this Obize developed then if its settlers were able to get there from their planet? The people could have faced any sort of hardships and struggles to wipe them out, or if you want a primitive people perhaps they lost their technology through the hardship. The early stages of "seeding" the moon could have been done hastily with urgency, giving only sparse supplies to keep it going, making it harder to expand as a colony, survive, multiply, and pass on knowledge

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I don't think its a too unreasonable arrangement. Obize could be a captured planet. It developed out on its own, and eventually was captured by the larger Akaria. Saturn's moon Titan may be an example of this.

I do think its unlikely a planet large enough to capture Obize would be without some form of an atmosphere. Being larger, it will of course hold more (and smaller) gasses. Even with a previous large collision, its still likely to have retained something of an atmosphere. So having a different - and poisonous to the residents of Obize - atmosphere wouldn't be unusual, it'd be the expected state.

Also, the equilibrium temperature of the planet will not be much different than the moon's, so you'll need other factors, probably atmosphere, to explain why its so hot.

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World Destruction can come in a variety of forms.

Gray Goo -- run-away nanotech ate all of the good stuff.

Primordial Black Hole -- Suitably size, the planet can be suffering from the ravages of the slow destruction of the planet. Won't affect the moon until the collapse starts spewing too much gamma radiation.

Berserkers -- The came and thourghly destroyed the planet (a consequence of exterminating all of the deep bacteria) -- Life on the moon just happened to appear soon after the Berserkers left.

Global Thermonuclear War -- When the local cold war was insanely out of hand, then went hot. Really tough to kill a planet this way, but when Tsar Bombs are one of the small ones ...

Industrial Waste Dump -- The residents of the next planet over had serious pollution. Not wanting to dump into into the sun (makes it really hard to recover to reprocess the waste) and thinking that the planet looked like a prime dump site, dropped untold tons of mercury, plastics, bio-hazards, radioactives, etc. onto the planet. Why not dump elsewhere? -- Because this was the easiest place to dump and they were not known for their good stewardship.

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At 160% of the mass of the Earth, Akaria is likely to be a water world with an ocean that is nowhere less than 100km deep. That's not a hospitable environment for life to evolve or to gain a foothold by hitching a ride on a meteorite, because there will be no trace elements at the surface, and no sunlight at the bottom.

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Suppose that originally neither body had significant water. The moon gets hit with a water ice asteroid that gives it water.

Or both had water, and the planet gets hit with something that contains enough calcium oxide to react with all the present water.

Note: The ability of a body to hold air is very mass dependent. A small change in mass makes a huge difference in how much atmosphere a planet can hold. I suspect that this dependency will make habitable planets very uncommon.

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