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Modeling Galactic History (Part I)

Modeling Galactic History (Part I)

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Over the next two or three posts, I’m going to be going through my reasoning for some of the background assumptions of the Human Destiny setting. This is probably going to come across as being a little stream-of-consciousness. I’m trying to work my way through a logic chain without necessarily knowing where it will need to go before I’m done.

Incidentally, although I’m not going to make any specific references to GURPS in these next few posts, I’ll continue to tag them that way – this kind of thought process is an extension of a few items in the last GURPS Space edition. I’m thinking specifically of the section titled Mapping the Galaxy, on pages 67-72. GURPS referees might find this process of some interest as a worked example.

So let’s get started.

Assumption #1: In the solar neighborhood, there exists about one stellar system for every 300 cubic light-years of space.

Commentary: The usual figure given for the stellar density in the solar neighborhood is about 0.004 stars per cubic light-year. Applying this to the 10-parsec-radius neighborhood of my map, we would predict the presence of about 580 stars. Clearly, the HIPPARCOS data set I work from is missing a lot of stars, since it only has 327 stars in that space (about 56% of the predicted number). As a cross-check, a 5-parsec radius should have about 72.5 stars, but the HIPPARCOS data set shows 64 in that space (about 88% of the predicted number). As expected, we’re missing more and more stars as we get further away from Sol.

On the other hand, we can expect that most “missing” stars are among the smallest and least luminous red dwarfs, increasingly difficult to observe with any significant distance. Even in the HIPPARCOS data set, there’s plenty of evidence that our data on individual red dwarfs becomes very poor well within the 10-parsec radius. Many stars have no other catalog designation, their spectral class isn’t at all certain, and so on. While proceeding through this analysis, I’ll bear in mind that very few of the “missing” stars are likely to have habitable planets.

Close to Sol, stellar systems seem to have an average of 1.2 stars each. This gives us an average density of about one stellar system for every 300 cubic light-years. That indicates about 480 stellar systems in the 10-parsec solar neighborhood.

Assumption #2: Within 10 parsecs of Sol, there exist about 30 habitable worlds.

Commentary: Applying the Architect of Worlds design sequence, I ended up with 28 habitable worlds in that space. Twenty of these (about 70%) appeared in stellar systems that included at least one K-class or more luminous star. None of them had a primary star of less than about 0.15 solar masses (about an M4 or M5). That suggests that habitable worlds circling the very dimmest red dwarfs – the ones by far most likely to be “missing” from the HIPPARCOS data set – are going to be very rare. I therefore assume that there might be one or two habitable worlds that I’ve overlooked, but no more than that.

Combining this assumption with the implications of Assumption #1, we get about one habitable world for every 4800 cubic light-years, or about one habitable world for every 16 stellar systems. We can apply this as a rough estimate for most regions of the galaxy.

Assumption #3: Habitable worlds that currently carry native tool-using civilizations (defined as capable of basic cultivation agriculture at a minimum) are very rare, about one in 40,000 habitable worlds.

This assumption breaks into two sub-assumptions:

  • Assumption #3A: A world will give rise to a tool-using civilization about once in every 500 million years of its habitable lifespan.
  • Assumption #3B: Almost all tool-using civilizations have a finite lifespan averaging about 12,500 years, after which they succumb to natural or sentient-made disaster, without ever developing interstellar capability.

Commentary: The computation is straightforward – divide the average lifespan of a typical civilization into the rate of their occurrence.

Naturally, both parameters are taken from the history of Earth. It should be noted that we have a very limited capability to identify other tool-using civilizations that may have occurred on Earth in the distant path. Still, we’ve never seen any evidence of prior non-human civilizations here, since the post-Cambrian appearance of complex land-based ecologies roughly 500 million years ago.

Meanwhile, humans have engaged in basic agriculture for about 11,500 years at this point. A more dramatic way of stating #3B is that, left to our own devices, we humans will drive ourselves into barbarism and extinction within another millennium or so. That gives us a total lifespan of about 12,500 years, which I’ll take as an average.

Combining this assumption with the others, we determine that at any given time in the natural steady state, tool-using civilizations appear about once in every 192 million cubic light-years, or once for every 640,000 stellar systems. That suggests the average distance between neighboring civilizations (using one of the formulae on page 72 of GURPS Space) is well over 600 light-years!

The natural state of the galaxy is many thousands of primitive cultures in existence at any given time, separated from each other by gulfs of hundreds of light-years, unable ever to see the slightest sign of each other’s presence.

Now, that assumes that no civilization ever attains interstellar capability. What happens if a few of them do?

Assumption #4: Given the possibility of interstellar (FTL) travel, as soon as one interstellar-capable civilization appears, it will no longer be subject to quick extinction and will fill the galaxy in a trivial amount of time.

Commentary: It seems reasonable to assume that an FTL civilization will no longer be subject to all the forces which drive a planet-bound culture to extinction. Only the very largest-scale natural disasters (enormous gamma-ray bursters, galactic core explosions, and so on) could destroy an FTL-capable culture. Such a culture might conceivably destroy itself through internecine warfare, but it seems reasonable to assume any culture likely to do such a thing would have done it before attaining FTL.

Meanwhile, an FTL-capable culture whose numbers expanded at even a modest rate would fill up the galaxy in a short time. Assume an annual rate of expansion as low as 0.01% – very slow, given FTL – then the Milky Way is filled up in as little as 250,000 years.

The question arises, then: given this event occurred, when?

The oldest stars in the galaxy formed about 13.5 billion years ago, but the environment for high-technology civilizations in such an early galaxy was probably very poor. Assume a minimum of four billion years for the first life-bearing planets to give rise to complex land-based ecologies. Then assume a further delay of about two billion years, for early civilizations to overcome the disadvantages of a metal-poor environment, and for more high-technology civilizations to appear at any given time. Then the first FTL-capable cultures may have appeared about 7.5 billion years in the past.

As it happens, this is after the formation of the galactic disk and spiral arms, and after a lengthy period of relatively slow star formation. If we assume one or more FTL-capable cultures appeared about then, they would have had a newly formed spiral galaxy to expand into, and a new flood of young stars to explore and colonize. These Precursor cultures might have hurried the process along, engaging in large-scale terraforming projects to create more habitable worlds.

(By an odd coincidence, the current Architect of Worlds design sequence yields that any potentially habitable world that is at least 7.6 billion years old, if its primary is not a class-IV subgiant, is guaranteed to have a complex biosphere. I didn’t plan that, but it fits! We can imagine that lots of small, cool stars that have been around since before the Precursor era were seeded with their favored ecologies back then.)

Any new FTL-capable cultures that arose during this period would have found the galaxy already full and busy. The Precursors may have been benevolent toward newcomers, or they may have been cruel and aggressive. In either case, newcomers would have had little chance to repeat the Precursors’ success. They would have been forced to survive in the margins of the elder galactic cultures.

We probably can set aside any concept of a unified Galactic Empire. Even with FTL, the natural unit of government is going to be no larger than the single star system. Such a system, if densely populated and developed, is likely to be economically self-sufficient and almost impossible to conquer.

The Precursor era was likely one of many millions of local civilizations, all in constant contact with one another, all of them rising and falling over time. Many single-system cultures may well have collapsed back into barbarism from time to time. Even whole regions of the galaxy might have fallen victim to some disaster or another. Alistair Reynolds’s concept of the “churn” (from his novel, House of Suns) seems likely to be appropriate here. Even so, the galactic association of cultures would have endured, possibly for a very long time.

Now, clearly this isn’t the situation we see now. The galaxy appears to be a wilderness. Something brought this Precursor era to an end, and something is preventing the galaxy from returning to that state today.

Assumption #5: The Precursor era was the only point in galactic history at which nearly every habitable world was occupied by high-technology civilization. Since then, the expansion of new FTL-capable cultures has been strictly limited.

Commentary: I choose to assume that in this setting, the galactic Precursor culture eventually fell victim to a massive conflict, driven by disagreements over several major issues. Among others:

  • Many local civilizations came under the domination of powerful AI, becoming machine cultures. These tended to replace their biological antecedents, through benevolent “mandatory pampering,” through non-violent competition, or through violent extermination. Naturally, civilizations which remained largely biological often regarded this development with alarm.
  • Some local civilizations found ways to “ascend” to new styles of life, often esoteric and incomprehensible to those who remained. Often this was associated with a shift to machine-culture status, as the “ascending” biological sentients abandoned their machine servants and guardians. Such “ascension” meant effectively dropping out of the galactic churn, often vanishing entirely to leave behind apparently empty worlds. Some cultural movements asserted that such “ascension” was the natural outcome and implied purpose of any sentient community. Other cultures rejected any such idea with horror.
  • Some local civilizations became concerned that the galactic community suffered from a lack of variety. They argued that ever since a single original civilization had given rise to the galactic community, all newcomers had been crippled, forced into an unnatural accommodation with that one dominant society. Over time, this became regarded as a fundamental question of justice.

Over millions of years, disputes over these issues gave rise to an epic series of wars. While a star-system community was a difficult thing to conquer, a sustained effort could sometimes do the trick. Of course, such a community was much easier to destroy. A barrage of relativistic kinetic-kill missiles, directed at every inhabited planet and space habitat in the target system, was one of the less destructive methods applied. Over time, the Precursor community collapsed across most of the galaxy, and high-technology culture was nearly eradicated.

Near the end of the conflict, an alliance of local cultures formed to defend what remained of the community, and to impose a specific solution on the galaxy:

  • Certain forms of “ascension” were accepted as the ultimate end of any sentient culture. One of the galactic community’s goals was to facilitate safe methods for such evolution, and to protect elder cultures as they proceeded toward it.
  • As elder cultures “ascended,” this would naturally make room for new biological cultures to arise, thus providing the galactic community with much-needed variety it needed. A second goal for the community was to protect such newcomers, helping them to survive the transition to FTL-capable status, and integrating them into galactic society. This specifically required leaving large volumes of space “fallow,” preventing any one culture from expanding too quickly or too far at the expense of others.
  • Powerful AI, and the machine cultures they tended to create, had a clear role in the galactic community, but they could not be permitted to harm or crowd out organically evolved cultures. Strict limits were placed on the use of AI by non-ascended civilizations. The creation of self-replicating AI was specifically forbidden.

This post-war settlement remains in effect, down to the present. Somewhere in the galaxy, very far from the solar neighborhood, a very powerful network of beings still works tirelessly to manage the galaxy, as if it were a vast garden. This network is called the Synarchy.

The Synarchy manages the galaxy by:

  • Intervening at certain points in the history of developing civilizations, helping them to avoid self-destruction and move toward readiness for participation in the interstellar community. This intervention is usually subtle but may involve overt conquest if necessary.
  • Enforcing certain foundational laws designed to prevent any one culture from overrunning the galaxy. Notably, no one civilization may claim or occupy more than a small fraction of the galaxy, and no civilization may build independent or self-replicating AI. Civilizations which break these laws may be brought into line by force.
  • Preserving knowledge and making it available to all participants in good standing in the galactic community, through the promulgation of a galactic Library. The evolutionary pathways that end with “ascension” are specifically revealed to all interstellar cultures at a certain level of maturity.

Much of the Synarchy’s work is done through proxies. These “mature interstellar empires” have generally been in existence for at least a few million years, have a good record of adherence to the Synarchy’s law, and have exhibited the ability to coexist smoothly with younger civilizations. The Synarchy deputizes such cultures to manage their areas of the galaxy, generally concealing its own existence from less mature civilizations.

So, what does an area of the galaxy overseen by one of the Synarchy’s proxies look like?

Assumption #6: In an area of space currently governed by a Synarchy proxy civilization, habitable worlds that currently serve as the home-worlds of native tool-using civilizations are much more common, about one in 400 habitable worlds.

This assumption derives from Assumption #3A, and from the following sub-assumptions:

  • Assumption #6A: About one in four tool-using civilizations survives long enough to develop a high-technology culture that will require intervention.
  • Assumption #6B: After intervention and emergence into the galactic community, civilizations have a finite lifespan averaging about 5 million years, after which they either voluntarily die out, or they “ascend” to the Synarchy and beyond.

Commentary: These assumptions imply that the average lifespan of a tool-using civilization is about 1.25 million years. Dividing this into the rate of occurrence of new civilizations (about once in 500 million years per habitable planet) gives us about one civilization per 400 habitable planets.

It should be noted that a Synarchy proxy could apply a different strategy, giving rise to a much higher density of high-tech civilizations. For example, a proxy could locate and intervene in the development of even pre-industrial cultures. Or it could even seek out promising pre-sentient species for “uplift” and civilization. I’ll assume that the Synarchy discourages such intense interventionism, possibly because it would lead the intervening civilization to force its clients into too restrictive a cultural mold. This would lead to a loss in the variety that the Synarchy values.

Without assuming anything (yet) about the shape or configuration of any volume of space governed by a Synarchy proxy, let’s examine how that space might be populated. If a given proxy governs space that includes N habitable worlds, then:

  • On the average, a new tool-using civilization will appear in that space every 500 million divided by N years.
  • On the average, a new high-technology civilization will appear, ready for intervention, every 2 billion divided by N years. This is also the rate at which established civilizations within the proxy’s sphere of influence will vanish into voluntary extinction or “ascension,” maintaining a steady state.
  • The current population of that space at any given time will be about N divided by 400 FTL-capable civilizations.

Suppose each FTL-capable civilization is allocated about 100 habitable worlds to colonize and occupy throughout its lifespan. If the space containing these worlds is compact, that implies a volume of about 480,000 cubic light-years, or a sphere with radius of about 48.5 light-years.

About 75% of the habitable worlds in a proxy’s volume will be left “fallow” at any given time. This should allow plenty of space for likely candidate species that might give rise to high-technology civilizations over the next million years or so. Potential colony worlds can be allocated to minimize the probability that a new civilization will appear on a world that’s already occupied.

The question arises: just how much space will a given Synarchy proxy be able to govern? Suppose a proxy can last much longer than the average of 5 million years for a full FTL-capable culture? Does it continue to grow, accepting responsibility for more and more space? Will the collective of all the Synarchy’s proxies fill the galaxy, or will there be “empty” space?

These questions are important, since we’re modeling a setting that needs to be consistent with what we’ve seen so far of the real universe. Results which indicate that Sol and Earth should have been visited and colonized many times in the past will mean that something has gone wrong.

I’ll examine some of these questions in the next post.

Human Destiny Reference Map Complete!

Human Destiny Reference Map Complete!

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Okay, after several weeks of effort, I’ve finished my project to use the Architect of Worlds design sequence and place habitable worlds throughout the “solar neighborhood.” I’ve also finished producing a map of the region, based on those data.

The Human Destiny setting ended up with 28 more-or-less habitable worlds, and two colonized star systems without habitable worlds, in that ten-parsec radius from Sol. That’s out of roughly 328 stars that make up 265 star systems, indicating an average of one habitable planet for every nine or ten star systems. A bit more than I expected when I got started, but it’s a figure I can work with.

Here’s a thumbnail for the final draft map:

It’s a pretty huge file, so you might do better to download it and view it locally. Alternatively, here’s a link to the map’s page in my DeviantArt gallery.

At this point, I have a couple of things to publish here over the next few days. One is a review of the large-scale galactic situation in the Human Destiny setting (how common interstellar civilizations are, how they are likely to be structured and so on). Now that I have a plausible count of Earth-like worlds, I can finish those notes.

It also occurs to me that I now have a list of interesting worlds from the new map – I should draw up some capsule descriptions for those. I seem to be converging toward being able to publish a mini-worldbook in GURPS terms for this setting.

More long-term projects: now that I’ve given the Architect of Worlds system a thorough test drive, I need to go ahead and polish up and upload the working draft of the planetary-system design chapter. I also have a whole sheaf of case studies with which to develop and test a new section, on the design of individual worlds. I think I’m also prepared to produce a new draft of the next Aminata Ndoye story, a novella titled In the House of War, which will be the next item to get published. Busy, busy – but at least I’m continuing to work through my Gantt chart.

Status Report (21 August 2018)

Status Report (21 August 2018)

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Still slogging along through the HIPPARCOS catalog – every day, I work through a dozen or so stars (and find myself wishing I had just written a C program for this already). At the moment I seem to have gotten through 276 entries in the database, out of a total of 327 reaching to the ten-parsec radius. Out of those stars, 23 have at least one planet with a complex biosphere, and at least a few systems have two each. It’s looking like a trend of about one in ten to twelve stars will have a more-or-less-Earthlike. I’m not bothering to count the “pre-garden” worlds, with liquid-water oceans but too young to have developed a post-Cambrian biosphere. There are quite a few of those.

Today I sat down for a few hours and started drawing a map of nearby space, including all stars of K class and above, and those few M-class stars that have Earthlike worlds. I’m using the same techniques that I once applied to this map of the solar neighborhood, and I imagine the end result will look similar.

I’m using a galactic coordinate system this time, rather than the usual equatorial coordinates, so a lot of stars will look like they’re in the wrong place if you’re accustomed to the maps from (e.g.) the 2300 AD or Universe tabletop games. I’m planning to include the appropriate coordinate transform in the Architect of Worlds draft, when I get around to writing the “using real astronomical data” section.

I’m also marking down tentative names for Earthlike worlds, instead of an abstract “resource value.” My vision for the Human Destiny setting has evolved quite a bit over the past few years. Today I’m assuming that the dominant interstellar civilizations won’t spend all that much time or effort exploiting star systems that don’t host complex biospheres. So the systems of greatest interest are going to be the ones that humans (eventually) settle.

If anyone’s interested in glancing at the work in progress, here’s a link to the appropriate entry in my Scraps folder. Only about twenty or so stars placed so far, or a little under one-third of the way through my data set. This is slow work, but it’s starting to come together.

Meanwhile, I’ve been working on a revision to my old notes about the density and structure of interstellar civilizations. Here’s a link to an article I wrote a few years ago, which lays out an argument about the limits to an interstellar civilization’s growth. (That article is also one of my few contributions to Winchell Chung’s Atomic Rockets website, in fact.) The Human Destiny setting incorporates that notion into its basic assumptions. I’ll probably publish those notes here within a few days.

Status Report (11 August 2018)

Status Report (11 August 2018)

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Still working through my data pull from the HIPPARCOS data set. I haven’t found any more planetary systems that the draft Architect of Worlds model simply won’t fit, although the famous Gliese 667 C system came close.

One thing I have discovered is that my assumption about red dwarf stars seems to have been premature. A little further research tells me that the photosynthesis problem isn’t an absolute deal-breaker. The problem isn’t that photosynthesis is impossible under red-dwarf starlight, it’s that an early photosynthetic organism would have to adapt to long periods of visible-light scarcity, punctuated by the nasty stellar flares young red dwarfs tend to generate. One might imagine mats or colonies of photosynthetic microbes that drift to the surface of a planet’s ocean to take in the sunlight, then submerge to ride it out when flare weather sets in. Eventually, most red dwarf stars seem to settle in and stop producing major flares, so if their planets can give rise to life at all, evolution to complex biospheres seems at least possible.

So, rather than forbid red dwarfs from having garden worlds at all, I’ve decided to impose a penalty, requiring them to take a lot longer to develop complex biospheres. Even so, since red dwarfs burn so steadily over many billions of years, an ocean planet has plenty of time to work on the problem. Red dwarfs that are at least as old as Sol, certainly the ones that are a few billion years older, are possible candidates.

I worked out a set of criteria to determine whether I should work out a red dwarf star’s planetary system at all: at least as old as Sol, bright enough that the habitable zone falls out where the inner planets are likely to orbit, and with metallicity high enough to permit terrestrial planets at least one-quarter as massive as Earth. I’d say maybe one out of three red dwarfs in the solar neighborhood have fit the criteria well enough for me to break out the calculator, spreadsheet, and dice.

Now another facet of the new model comes into play. The draft model often generates systems of planets whose orbits are more tightly packed than one would expect, just looking at our own system. Which in turn significantly increases the probability that at least one planet will sit in the liquid-water habitable zone. In fact, sometimes I’m getting two planets in the zone in the same system. That’s not a result that the GURPS Space 4/e model would have produced very often, if ever.

The upshot is that although any given red dwarf is unlikely to host a garden world, there are so many red dwarfs that I’m getting a significant number of them. Lots of “eyeball planets” out there, it seems; possibly as many as the more Earth-like worlds with reasonable day-night cycles.

So far, I’ve worked out planetary systems to about 25 light-years from Sol, including all the K-class and hotter stars, now also including all the red dwarfs that seem to be plausible hosts for garden worlds. 168 lines in the HIPPARCOS database, although a handful of those aren’t actual stars, and 16 stars that have complex biospheres present. Looks like roughly one out of ten stars is giving me at least one garden world. More than I expected, actually, but it’s a result I can live with.

Status Report (5 August 2018)

Status Report (5 August 2018)

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Most of my effort over the last few days has been directed toward two tasks. First, continuing to test the Architect of Worlds model for planetary systems by generating collections of worlds for stars close to Sol. Second, using those results to motivate the first definitions for the next stage of the design sequence: determining the physical properties of an individual world.

The first is going as well as can be expected. So far, I’ve only found one star system that I flatly can’t model properly (the HR 8832 system, about 21 light-years from here, which is believed to have an even stranger collection of super-Earths and close-in gas giants than usual). Otherwise, I’m getting a very plausible set of planetary systems, a significant improvement over the results I would have gotten from the old GURPS Space 4/e design sequence.

As far as the second task goes, I’ve had something of a breakthrough: I’ve found a model I can live with to help the user decide whether a given planet is tide-locked to its primary star or not. It’s a horrible kludge – but the question of how long it takes a planet to tide-lock is very complex, and there’s no consensus in the literature about it. If a planet could be modeled as a uniform and perfectly elastic body, the math simplifies pretty well, but planets just aren’t like that. The equation I’ve come up with seems at least plausible, in the forty or so star systems for which I’ve generated data so far.

Right now, I’m wrestling with how to decide whether a given planet (or moon) has a substantial atmosphere or not, and whether it has oceans or not.

In GURPS Space 4/e, I kind of took a backwards approach – I had the user decide which of several categories a world fell into, and then he generated the world’s mass, density, and so on to fit. I think that was slightly more useful for the gaming context, but the math was kind of annoying (not least because SJG editorial policy forbade me from using SI units, so I tried scaling everything to Earth and the Sun, with weird outcomes). The math is a bit more straightforward doing it the other way – define a planet’s mass and density, then figure out what its surface environment will be like.

Of course, now I have to wrestle with questions like why Mars has almost no atmosphere despite being massive enough to retain molecular nitrogen and carbon dioxide (and it can’t just be because Mars has no magnetic field to speak of, because Venus doesn’t either, and it has a very thick atmosphere). Or, say, why Titan has a substantial atmosphere when the almost identical Ganymede has none.

Slowly, a classification scheme is emerging, but it will probably be a few more days before I’m happy with it.

Meanwhile, the upcoming week is going to be unusually busy at the office. I’m teaching one course, taking a second course, and facing impending deadlines on writing two more courses after that. Generally, my life is not quite that full! I may or may not have a lot of time to play with my worldbuilding over the next few days. We’ll see how things go.

Architect of Worlds – Some Initial Results

Architect of Worlds – Some Initial Results

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Over the past couple of weeks, I’ve been applying the star-system and planetary-system design sequences from the Architect of Worlds draft to generate planetary systems for nearby stars. In a number of cases, this has involved tweaking the parameters of the model to fit strong exoplanet candidates that we already know are there. In other cases, it also involved tweaking the parameters to avoid creating exoplanets that we would reasonably have detected by now, if they were there.

So far, the model has held up surprisingly well. I’ve had to make a few adjustments to make the results more plausible, and to allow for some of the real-world cases. That’s to be expected, but by and large I haven’t had to do any major redesign.

Let me summarize some of the results thus far.

Alpha Centauri

We know of two strong exoplanet candidates for this trinary star system – one close-in planet for each of Alpha Centauri B and Proxima Centauri. I had no difficulty at all fitting either of these candidates to the model. The result was five planets for Alpha Centauri A, nine for Alpha Centauri B, and twelve planets for Proxima. The tight or loose packing of planetary orbits makes a big difference, and of course Proxima has no close companion to block off the outer system for planetary formation. Thus the little red-dwarf companion gets the most extensive family of planets.

The mass of the known candidate for the B-component suggested a lighter protoplanetary disk than usual, but this still yielded two Earth-likes in the habitable zone, generously defined. One of these is the best candidate I’ve generated so far for a habitable world. The mass of Proxima’s known companion suggests a denser than usual protoplanetary disk, so Proxima ended up with a few gas giants up to Saturn size, at fairly wide orbital radii.

Barnard’s Star

Low-mass star, very low metallicity, not much material with which to build massive planets. The system ends up with nine planets, all packed well within one astronomical unit of the star, all but the last of them little Mars-likes. This turns out to be fairly typical of red-dwarf stars, given the assumptions of the model.

Wolf 359

Very small and dim red dwarf, although the metallicity is high and that might help form planets. We end up with ten planets this time, several of them cold super-Earths. The innermost planet is in the habitable zone, but is too small to retain much atmosphere.

Lalande 21185

Another small red dwarf. Interesting in that we have a strong exoplanet candidate here, a super-Earth very close in. The problem is that we don’t seem to see any more heavy planets or super-Jovians further out.

This star caused me to make the first adjustment to the model: I added a rule that permitted a massive, volatiles-heavy “failed core” to appear close to the star in rare cases. This makes sense, given that some of our known exoplanets are both massive and not very dense, suggesting that they’re not rocky “terrestrial” planets, but something rich in water and other light compounds instead. If a gas giant can migrate inward, perhaps a smaller planet can form out past the snow line and barrel in close to the star as well.

Placing the exoplanet candidate as a “failed core” rather than a “terrestrial planet” permitted me to keep the assumed density of the protoplanetary disk to a reasonable value. The rest of the planets turned out to be quite small close in, leading to a few modest-sized gas giants on the outskirts, safely under our current detection level. Ten planets in all, two of them within the generous habitable zone, both of those probably too low-mass to be truly Earth-like.

Luyten 726-8

Two very low-mass red dwarf stars, co-orbiting at a close distance that probably forbids either from having many planets. The model gave me two planets for the A component, one for the B component, all cold “failed cores.”

Sirius

A very hostile star system. The bright A component ended up with nine planets, all packed close in, a mix of gas giants up to sub-Jovian size and a few rocky super-Earths. All of the planets are far too hot for human comfort, the coolest of them running a blackbody temperature over 400 K.

I didn’t bother to generate planets for the white dwarf B component – I need to work out rules for applying the model to white dwarf stars, and in any case there’s no possibility of an Earth-like world in such a planetary system anyway.

Ross 154 and Ross 248

These two red dwarf systems each turned out to be barren, with four and five planets respectively. I’m not going to report on any more red-dwarf systems, as they’re all going to be similarly uninteresting.

In fact, my research tells me that the probability of any red dwarf giving rise to an Earth-like world is going to be very low. Any world that’s warm enough will almost certainly be tide-locked, with all the problems that implies. Not to mention that red dwarf stars put out most of their radiation in the infrared range. That means a world that’s warm enough to live on is likely to get so little visible-light insolation that photosynthesis is going to be problematic.

Epsilon Eridani

This was an interesting case – we have at least one strong exoplanet candidate here, a super-Jovian, with a strong indication of a dense asteroid belt just inside that candidate’s orbit. None of this was a problem for my model. The mass of the known gas giant, together with the known density of the current debris disks, suggested a high-density protoplanetary nebula. I was able to generate the rest of the planetary system to match.

I ended up with nine planets, one of them a super-Earth squarely in the middle of the habitable zone. The star system is quite young, well under a billion years old, so that planet is almost certainly a heavily oceanic “pre-garden” world, lacking complex life or a human-breathable atmosphere. Still, maybe a terraforming candidate? Meanwhile, the asteroid belt is in place and I would be comfortable marking that as a rich resource zone. This looks like a star system that people would come to visit, even if there isn’t an Earth-analogue there.

61 Cygni

No surprises here. I got thirteen planets for the A-component, due to tight packing of planetary orbits, and only seven for the B-component. Nothing so massive as to be detectable from Earth, so there’s no sign of Mesklin, alas. Each star ended up with a planet in the habitable zone, but both were too low-mass to be good Earth-like candidates.

Procyon

As expected, this system ended up like Sirius A, but not quite so extreme. Eight planets, all of them rocky worlds, no “failed cores” or gas giants in the mix. The outermost might almost be cool enough for habitability, but it’s far too small, so it’s more like a baked-dry Mars than anything else.

Epsilon Indi

This star gave me a little trouble, and caused me to tweak the model again slightly. The problem is that we have an exoplanet candidate here, but it’s simultaneously very massive (about 2.5 Jupiter masses) and very distant from the primary.

As written when I got to this point, my model permitted massive gas giants to form, but that would tend to require a dense protoplanetary disk, which would in turn force the gas giant to form close in and migrate closer. A gas giant forming much further out would suggest a lighter disk, which would render the planet less massive. A paradox. I solved the problem by adding another size category for gas giants – in rare cases, even a fairly light disk can now give rise to a super-Jupiter. Which makes sense, as this isn’t the only massive gas giant we’ve detected in the cold outer reaches of a star system.

Final result was nine planets, the innermost of which wouldn’t be a bad Earth-like candidate, except that the system is fairly young. Probably another “pre-garden” world here.

Tau Ceti

We have several strong exoplanet candidates here, all of them super-Earths fairly close in to the star. I computed the most likely disk density and proceeded, adding Mars-sized mini-worlds to fill in two gaps left by the known exoplanets. Final result was nine planets, none of them further out than about 6 AU, which gives us room for this star system’s apparently very wide and rich Kuiper belt.

Two of the known super-Earths are close to the habitable zone, but one of them is probably too hot, and the other one is probably only habitable if it’s running a very aggressive greenhouse effect. Probably interesting places to visit, but not somewhere anyone would want to live.

Summing Up

Twenty-seven stars so far, in twenty star systems, and so far I’ve only generated one strong candidate for Earth-like conditions (Alpha Centauri B-V). I’ve also concluded (or, rather, verified for myself) that red dwarf stars are very unlikely to give us Earth-likes. Depending on one’s assumptions, this may mean that such stars are best ignored when building a star map for fictional purposes.

I’m going to continue with this, probably saving myself a bunch of time by skipping over most or all of the M-class stars. Meanwhile, this is enough for me to start building a new version of my solar-neighborhood map. Stay tuned.