(Pardon me for the hiatus. Had to fly to Houston to do some flight testing at NASA.)
I spent a pretty good weekend doing some world-building. Since discovering the maps in the first pages of The Lord of the Rings, Redwall, and the like, I have really enjoyed sketching out maps of imaginary worlds and outlining details of the cultures and histories that play out over those maps. My maps started as knockoffs of Tolkien’s (with the bad guys sequestered in a nice, rectangular wall of mountains around some barren lands) or parallel-universe versions of the terrain around my house. Since then, though, I’ve started to inject a lot more realism into the worlds I create. Want to know where the tectonic plates and prevailing winds are on my map of Oghura? I could show you!
Beyond the maps, some of my imagined cultures have fully fleshed-out languages, religions, and customs. Slowly, slowly, I’ve been compiling reference documentation on the Oghuran desert and people, the fantastical Cathedral Galaxy, and the future-universe of the Four Colonies. This weekend I was spending my time in the Cathedral Galaxy, putting together a master list of the major galactic regions and polities, along with distinguishing characteristics. Now I know a bit more about why the Imperium of the Triumvirate is split in three, how the far-from-galactic-center Traders’ Rim came to be populated by merchants and entrepreneurs, and the tumultuous history of conflict between Amseile and Shobah. I’ve also got the beginning of a couple more stories – one concerning an Imperium gladiator’s bid for freedom and another describing the Waygehn people, who evolved to sentience near the death of their star and outlived the event, leaving them homeless in the galaxy. That’s one of the most fun things about deciding to build a universe purely for short stories: I get to invent worlds, and then immediately show them off with snippets of detail!
Though the Cathedral Galaxy has some distinctly space-fantasy elements, I decided early on that it would be a universe based on hard science – though not necessarily our hard science. My short story “Conference” illustrates the point, as it shows that there are technical concepts built upon technical concepts – but at the level that Arthur C. Clarke would have described as “indistinguishable from magic.” I have no idea how the Channel Network could be set up, and building planet-size structures is clearly fantastical. (And none of you know yet what’s in The Cathedral!) But I made sure that the story was relevant to us Earthdwellers, and I lean strongly on plausible concepts to describe things like astronomical bodies or planetary orbits.
For example, take Heliast, the resort world on which much of “Conference” takes place. Here’s the description that conference-goers got of the world:
The tour guide explains how Heliast is an ancient world with a single moon nearly half its own size, and how that has dominated the history of the planet and made it ideal for resort paradises. A billion or so years ago, the planet spun many times under one orbit of the moon, and the energy input of ocean tides among all the planet’s archipelagoes – Heliast is over eighty percent water – gave rise to life. But nowadays, the moon orbits in tidal lockstep with one Heliast day, the prime factor contributing to the perpetual calm of its seas. The small radius of Heliast’s solar orbit leaves the planet with a reasonable day length, while the dimness of its sun places it in the liquid-water zone. Without tides, with a massive moon helping to protect the planet from asteroid impacts, and with barely any eccentricity in its orbit to create seasons, there have been few selective pressures on Heliast’s life forms. Life on the planet thus failed to diversify much, and after millions of years of evolution with few external stressors, there are now only a few ecological niches on the world. Three or four avian species, eight or ten surface-level swimmers, two or three land animals, and about six land plants are all most tourists have the chance to interact with. The rest of the planet is geological beauty for visitors to enjoy.
So, the planet’s “month” equals its “day,” but there are still many days per year and there is much liquid water on the surface. The dynamics shaped the world’s evolution. That was fun to think of! But, more and more, I am completely amazed by the strange worlds that actually exist in our own universe. Many Earth- and space-based observatories keep returning data on new exoplanet candidates, and in the last few years, the galaxy seems a lot more planet-populous than it has in the past.
This past Monday, I went to a fascinating astronomy seminar on the potential climates of Gliese 581g given by Dr. Raymond Pierrehumbert from the University of Chicago. (He’s preparing these climate models for an arXiv preprint.) Besides tying the Gleise 581 system with 55 Cancri for most number of known exoplanets around the same star (5), this planet is interesting because it falls right smack in the middle of the traditional “habitable zone,” the range of orbital radii necessary for planet surface temperatures that could support liquid surface water. Now, of course, the discovery of Gliese 581g has to be confirmed to become official – and there’s some doubt about that! – but it’s at least got scientists thinking about these dwarf-star systems in interesting ways.
Gliese 581g, which the discovering astronomer apparently would like to name “Zarmina,” is a larger-than-Earth world that is most likely tidally locked to its star. “Tidally locked” means that the planet rotates once (one day) for every orbit around the star (one year), similar to the orbital situation of the Moon around the Earth. With a little visualization, it doesn’t take long to realize that a planet with a day equal to its year would always have the same point on its surface pointing towards its sun (the “subsolar point”). So one side of the planet would be hot, and one side would be cold, and the comfortable intermediate temperatures for liquid water – and life! – would be in the narrow band of longitudes around the terminator. (Star Wars fans: this is supposedly the case with Ryloth.) Right?
Well, not really, as I found out yesterday. In turns out that global climate models of a world like Zarmina give some interesting results in terms of how water behaves on its surface.
For instance, consider a case in which the planet has water on the surface, but has no atmosphere. Water vapor is a very inefficient greenhouse gas, so the planet surface would radiate energy away into space over almost all of its surface – and any surface ocean would be frozen on top. If the ocean is global, this would be like a super-size version of Europa. However, the surface around the sub-solar point would still receive enough incident radiation that the surface temperature there might be about 270 K. So, the ice layer there might be pretty thin (meters to kilometers), and a relatively small fluctuation in the local temperature could melt the ice and expose liquid water to the surface.
So, the sub-solar point might be more slushy than icy. Now, without an atmosphere above, the exposed water would immediately boil. There might be Enceladus-style geysers blasting water up out of the slush periodically. This slushy area provides a conduit for interactions between the surface and the ocean: a way for energy from sunlight to get down into the ocean, and a way for fresh materials from the ocean to get thrown up onto the ice. Those sorts of exchanges are very important when considering habitability! And it’s pretty cool to think about anyway. What a wild world that would be!
The result is worse for habitability if Gliese 581g has an atmosphere composed mostly on non-greenhouse gases, like N2. The atmospheric dynamics from Dr. Pierrehumbert’s climate models are such that fast high-altitude winds would efficiently distribute the solar heat flux over the entire world. (Surface winds would actually be fairly slow.) But this thermal mixing would actually even out the surface temperatures, so the planet wouldn’t get a slushy sub-solar point. Any ocean would be covered with a kilometers-thick layer of solid ice.
But here’s the totally awesome hypothetical case:
If Gliese 581g has an atmosphere with about 20% greenhouse gas content, CO2 in Dr. Pierrehumbert’s model (which is apparently not unreasonable for known chemistry with silicate rocks), then there could be a circular region around the sub-solar point that supports liquid water. He showed us pictures with a global ocean over the planet, with a thick ice layer over most of the planet surface but exposed liquid water inside a circle extending to about ±45° in latitude and longitude. He called this case “Eyeball Earth.”
The liquid water ocean within the “iris” of the eye would have a surface area about equivalent to the area of Earth’s oceans, since Zarmina is larger than Earth. There could be islands or continents within the ocean, all under a red sun that never moves from its zenith.
Now I feel like my world-building capabilities have been completely put to shame by the laws of physics! Just imagine the mythologies that would arise among cultures on a planet in which the known surface world – with about the same area as our own – would actually be an almost-flat disc bordered by ice. If any adventurous Zarminan heroes pushed out into the ice, they would find themselves trekking across a barren wilderness that gets progressively colder…and darker…until they finally lose sight of their sun entirely and start to freeze. What a world!
Of course, if the surface water distribution isn’t global, then there might not be an ocean completely covering the sub-solar “iris” and there might not be an unbroken icy wall around the ocean. Nor would there be ice over the entire rest of the surface. There might be visible rock. But it would be true that any water more than ~45° away from the sub-solar point would be covered by a thick ice layer. (There’s also a complication in that a completely iced-over world is apparently also a solution to this case. The planet might alternate between an “iceball” state and an “eyeball” state.)
See? The universe is much better at building cool worlds and sci-fi settings that I am. (Makes me wish I was more like Robert Forward!) Even if the detection of Gliese 581g turns out to be spurious, that doesn’t change the fact that the physical models give these results for the same parameters. With all the stars out there, it is very plausible that many worlds like the ones I described exist!
And Zarmina isn’t the only case of the observable universe out-world-building me. Some other interesting dynamical cases arise for tidally locked planets for which the planet’s orbit is a little bit eccentric, as I found out in another astronomy talk last semester. With that slightly eccentric orbit, a person standing on the planet would see the sun rise from the horizon in the morning, reach a low-angle zenith at noon, and then retrace its steps to sink back down to a sunset in the same direction as sunrise!
Then there’s HD 209458 b – a gas planet with “superstorm” winds as fast as 2 kilometers per second rush from the 1000 C side of the planet facing its sun to the other hemisphere. Just to give you an idea how outrageously fast that is, the International Space Station orbits at a velocity of 7 km/s. Escape velocity from the surface of the Earth’s moon is 2.4 km/s. These winds are really flying, and they are driven by the simple physics of a thermal gradient.
And check out this article on Centauri Dreams about brown dwarfs and the concept of the “habitable zone,” in systems where the fading light of the star over time may have implications for evolution – and the evolution of intelligence. In short: it’s possible that, since these planets start off habitable and get progressively less comfortable as time passes, there would be a selective pressure that favors species able to mitigate their planet’s slow death – or even colonize worlds. Woah!
George Lucas couldn’t make these things up. And they can, or do, actually exist.
Edit: Check out Phil Plait’s similar comments about planets orbiting eclipsing binaries!