I already have a frankly rather interesting concept for a planet dominated by sharp relief, to the tune of nigh-Martian highlands and the usual thick-air jungle-y lowlands, with the ocean reservoirs being underground, primarily. “Cascadia Below, Mars Above” I called it; but siting it in the Delta Pavonis system invites the question of what else is out there. I suggested as much in my last post:
Alternatively just siting it by Delta Pavonis, a star that in my sci-fi world I haven’t figured out what to do with yet but is a prominent and nearby sun-like star, is a possibility, but I had in mind an idea for a world at the very limit of breathability and gravity for humans, where as a consequence of the geochemical evolution of the planet a vast original inventory of atmospheric nitrogen was drawn down to ammonium salts, leaving a helium-oxygen atmosphere behind…and having as an interesting result oceans that would have to be chilling down at -40 Fahrenheit, with the climate being tropical in the basic mechanics of it but shifted so snow fell down with the afternoon thunderstorms (as mandated by the cold temperatures). High gravity would impose snake-like locomotion, and the freezing climate would suggest thick fur and blubber as well.
A striking world that I was nevertheless not quite satisfied with; one might think the idea I had for that world as well as this other lush jungle world could be combined, but consider that if the lowlands are already at -40 that implies the highlands are even colder than the -100 or so we’ve been discussing here…and needless to say my original idea for Delta Pavonis did not incorporate a greenhouse effect or a lush but unbreathable lowland. So the two ideas aren’t really compatible…unless we sited them in the same system as two different and distinct but nearby planets. Hmm. Perhaps even as a double planet, both being super-Earths in the same orbit but with rather different evolutionary pathways? That might be fascinating…
Well…I’ve done some brainstorming, and making the “cliffside world” and the “heliox world” a double planet system is really rather contrived. But I really do like the double-planet concept, so my current thinking is three inhabitable worlds. Yes, really. Delta Pavonis has two and a half times the sun’s “metallicity” (abundance of elements heavier than hydrogen or helium), which suggests abundant raw materials for building planets, in particular large heavy terrestrial planets like my two “super-Earths”.
The “cliffside world” would be located further out; around our own sun the strong greenhouse effect means that for the climate to be what I want, it would have to orbit at 2.2 AU (deep in the main asteroid belt; i.e. much further even than Mars…). Around Delta Pavonis, which has modestly greater luminosity than our sun, it would have to orbit at a modestly greater 2.5 AU. For a planet with more or less Earth-like albedo (0.3, i.e. 30% of incoming sunlight is reflected back out), this creates an “equlibrium temperature” of -153 Fahrenheit. Yes, with 5% carbon dioxide by fraction at a pressure at 10 bars, it has to be that cold just in order to not cook from the greenhouse effect; all those greenhouse gases warm it to a rather Earth-like 60 degrees Fahrenheit in the broad lowlands.
The “heliox world” would need to orbit closer in, because its atmosphere was more aggressively drawn down by its biosphere, leading to a near-total lack of a greenhouse effect. Surface temperatures average -40 Fahrenheit, so assuming relatively little greenhouse effect, perhaps 10 degrees (?), we’d want an orbit with an equilibrium temperature of -50 Fahrenheit or so. Which around Delta Pavonis, assuming our planet has an Earth-like albedo (and despite the low temperatures it probably would; it’s supposed to have continents and liquid oceans, since there’s such an enormous quantity of salts keeping it liquid, a la the Antarctic brines of our world…), suggests an orbital distance of 1.4 AU, roughly as far away as Mars is from our sun.
So we have two planets, both envisioned as being roughly 7 Earth masses, one further out and one closer in, both habitable for complex native biospheres but only barely breathable for humans in pockets. With the inner planet being much colder than the outer planet…an amusing state of affairs we might expect to see often in planetary science (as it is Venus is hotter than Mercury, despite its orbital distance and mass suggesting it ought to be more similar to Earth…).
But as I drilled down into the Delta Pavonis system of my imagination, I started to wonder about satellites…and this is where the double-planet aspect comes back into prominence. Around the outer planet, we already have a red sky like Mars, despite the conditions being rather more Earth-like in the lowlands (the reason is all that rusty dust blowing off the plateaus…). Do we see a moon like Earth’s? My first thought was it might be more interesting to take a page out of classic science fiction by siting multiple planetary-mass moons around the planet…nothing large enough to have an atmosphere, but big enough to appear quite similar to Earth’s moon in the sky, only with multiple instances of it. Always cool.
The obliquity of my planets, i.e. how much the axis of rotation is tilted relative to their orbits, is another question. I might not want either to be Earth-like, so we’re dealing with either lower or higher tilt than Earth’s familiar 23 degrees. The climate of the inner planet is a bit more fine-tuned, at perhaps -40 year-round, since I don’t really want massive sea ice to form or for rains to ever fall in the summer (it’s too cool to have snow that melts on contact without rain occurring…ever). So coupled with the thick atmosphere, a low obliquity seems appropriate. Which then suggests, in the interest of dramatic contrast, a higher obliquity for the outer planet. I’m thinking something like 60 or 70 degrees; not quite “tilted on its side” like Uranus, but almost; certainly the seasons on such a world would seem rather alien, since the tropical circles are poleward of the polar circles, i.e. the sun would at certain times of the year be directly overhead in the same areas that experience midnight sun and “polar night” at different times of the year.
This would even make some measure of sense, in as much as the outer planet might have been knocked around by impact events early in its history, hence the large moons, and also hence also the unusually rapid rotation, even for a super-Earth (days might be 10-12 hours long or so), and also the crustal fracturing which leads to topography that looks more Miranda-like than Earth-like.
Meanwhile, the inner planet I envision as experiencing slower rotation than Earth’s, in the interest of dramatic contrast. And since it was a super-Earth that presumably had faster rotation to begin with, something had to have slowed its rotation down, with the most reliable mechanism for this being tidal deceleration from a large moon. The bigger the better. Earth’s own rotation has slowed substantially since the Moon formed 4 billion years ago, and our moon is only 1% of the mass of its primary planet. Charon, by contrast, is 12% as massive as its primary, Pluto, and the two are in a mutual tidal lock (they each show the same face to the other body). Pluto and Charon also rotate slower than the Earth does: 6.4 days.
Let’s suppose the inner planet (“heliox world”, for those keeping track) has a moon as large as Charon, proportionally. This might sound extreme, but it’s well within the range of what giant impact events can produce (our moon might be a bit on the light side, actually). Assuming the primary is 7 Earth masses, then 12% of that adds up to…0.84 Earth masses. Wow. That’s a big world. Of course that’s approaching the mass of the Earth, but it might also be worth noting that’s actually slightly heavier than Venus (which is 0.81 Earth masses).
Like Pluto and Charon each body would likely orbit a center of mass located outside the surface of the primary, thus making the system a double planet by the usual definition that’s applied. In any event, an Earth-sized or so world located at 1.4 AU from Delta Pavonis would surely have an atmosphere. It likely was originally much like the Earth: unlike the hulking-huge primary, it wouldn’t have had enough mass to have retained helium or any kind of extremely thick atmosphere. The primary difference is likely its orbital position: remember, at 1.4 AU, it’s far enough out for the equilibrium temperature to be -51 Fahrenheit. That’s 50 degrees colder than Earth.
Earth’s original climate is uncertain, but it’s known that we had liquid water oceans that likely were global in extent very early, by around 4 billion years ago…which is particularly striking since the sun was 30% dimmer back then, and so climate models have trouble resolving how the oceans could be liquid at all instead of frozen. It’s assumed that there must have been a strong greenhouse effect, but even then our climate was likely temperate to cool worldwide. Think less the steambath of popular imagining and more a maritime body like Kamino from “Star Wars”. To wit, it probably wasn’t that far above the freezing point. So our Earth-analogue moon would likely have been frozen over originally.
It’s entirely possible that as Delta Pavonis brightened over the billions of years after its formation, and assuming the world still had its greenhouse effect, that our moon thawed out, exposing global oceans, perhaps peppered with continental landmasses as well (like its primary, as well as like Earth). But this would have made the evolution of photosynthesis all but inevitable, a process that draws down carbon dioxide, and thus the greenhouse effect. On Earth this is thought to have already tipped us into periods of global glaciation, but we had 50 extra degrees of solar warming to help us thaw back out. On this world? Once the greenhouse gases are drawn down and it’s globally glaciated, it would all but assuredly stay that way.
Consider that ice is far more reflective than dry land and liquid ocean (it’s bright and white). Fresh snow reflects 80-90% of the sunlight that reaches it, with even mature ice still being 50-70% reflective. Thus the Bond albedo of a “snowball Earth” would be as high as 0.7, not the 0.3 of an Earth like we see today. Equilibrium temperatures are already -51 Fahrenheit with Earth-like albedo. With an albedo of 0.7? Equilibrium temperature drops all the way to -130 Fahrenheit. Even if greenhouse warming were as powerful as Earth’s is today (60 degrees Fahrenheit or so), you’re still only up to -70 Fahrenheit or so.
At the tropical and subtropical latitudes during the summer it would be warmer, of course, so life could still thrive if the ice became thin enough to let in light to the oceanic depths below, and especially if the ice broke up and formed polynyas: areas of open water surrounded by contiguous sea ice, opened up by mechanics of ice flow rather than by warm temperatures. There is a limit below which polynyas will not form (Europa’s ice crust, for instance, is thought to be dozens of miles thick, and so we never see open water there), but on a good day temperatures might reach -20 on the warmest parts of the moon, which is starting to get within range of where polynyas can form on Earth. The kicker though, is the tidal forcing: although the moon and its primary are mutually tidally locked, the least bit of orbital eccentricity will induce “libration”, which could easily flex the ice enough at this point to crack it open with regularity, thus we should expect a polynya season on this world, permitting summertime blooms of photosynthetic life and a feeding frenzy. The place might be surprisingly lush and vibrant despite the frigid temperatures…
As such, the atmosphere of this world could be surprisingly Earth-like. Photosynthesis would have oxygenated the atmosphere to begin with, and once in the atmosphere in a glaciated planet, oxygen will not tend to be removed. Compared to when the oceans were liquid at the surface, photosynthesis will slow, but it will not cease. Consider that Delta Pavonis is 6 billion years old compared to the Earth’s 4 billion, so oxygen buildup continues. The atmosphere may well be at a modestly lower pressure than Earth’s, but with a higher oxygen fraction, perhaps 50% or so. The climate would not be comfortable, but you could go out there with an arctic parka on and breathe the air unaided. Probably easier than you could on the other two worlds.
So we have a picture here of not just two but three worlds, all of which have breathable areas, but also all of which are cold. Habitable, and Earth-like, all three of them, but none of them being true Earth twins. That honor in my universe is reserved for Mu Cassiopeiae, which itself has its own peculiarities.
That’s not to say Delta Pavonis doesn’t have fascinating possibilities. Sure, an intelligence with a civilization like ours doesn’t really fit into my science-fiction universe here, but then again such a discovery is unlikely anyway. But intelligence, as such? If there’s any system with an alien we could talk to nearby, certainly a system with three Earth-like planets should be a strong candidate. And the environments at play are so fascinating, I’m wondering how, as a worldbuilder, I could resist peppering the whole system with aliens. That, I think, is the next task in this latest bit of science-fictional caprice…stay tuned. 😉
This post’s featured image is an artistic depiction of Earth during the Huronian glaciation by Oleg Kuznetsov (2020). CC-BY-SA 4.0.
