Having recently discussed some possible SF solutions to the vexing problems posed by red dwarf stars, it makes a certain amount of sense to consider the various star systems that have served as popular settings for some classic science fiction—even if science has more or less put the kibosh on any real hope of finding a habitable planet in the bunch.
In olden days, back before we had anything like the wealth of information about exoplanets we have now1, SF authors playing it safe often decided to exclude the systems of pesky low-mass stars (M class) and short lived high-mass stars (O, B, and A) as potential abodes of life. A list of promising nearby stars might have looked a bit like this2…
|Star System||Distance from Sol
|Alpha Centauri A & B||4.3||G2V & K1V||We do not speak of C|
|Procyon A & B||11.4||F5V – IV & DA|
|61 Cygni A & B||11.4||K5V & K7V|
After Tau Ceti, there’s something of a dearth of K to F class stars until one reaches 40 Eridani at about 16 light-years, about which more later. And because it is a named star with which readers might be familiar, sometimes stories were set in the unpromising Sirius system; more about it later, as well.
There are a lot of SF novels, particularly ones of a certain vintage, that feature that particular set of stars. If one is of that vintage (as I am), Alpha Centauri, Epsilon Indi, Epsilon Eridani, Procyon, and Tau Ceti are old friends, familiar faces about whom one might comment favourably when it turns out, for example, that they are orbited by a pair of brown dwarfs or feature an unusually well-stocked Oort cloud. “What splendid asteroid belts Epsilon Eridani has,” one might observe loudly, in the confident tone of a person who never has any trouble finding a seat by themselves on the bus.
In fiction, Procyon is home to L. Sprague de Camp’s Osiris, Larry Niven’s We Made It, and Gordon R. Dickson’s Mara and Kultis, to name just a few planets. Regrettably, Procyon A should never ever have been tagged as “possesses potentially habitable worlds.” Two reasons: solar orbits and Procyon B’s DA classification.
Procyon is a binary star system. The larger star, Procyon A, is a main-sequence white star; its companion, Procyon B, is a faint white dwarf star. The two stars orbit around each other, at a distance that varies between 9 and 21 Astronomical Units (AU).
Procyon A is brighter than the Sun, and its habitable zone may lie at distance between 2 and 4 AU. That is two to four times as far from Procyon A as the Earth is from our Sun.
Procyon B is hilariously dim, but it has a very respectable mass, roughly 60% that of our Sun. If Procyon A were to have a planet, it would be strongly affected by B’s gravitational influence. Perhaps that would put a hypothetical terrestrial world into an eccentric (albeit plot-friendly) orbit…or perhaps it would send a planet careening outside the system entirely.
But of course a hypothetical planet would not be human- or plot-friendly. B is a white dwarf. It may seem like a harmless wee thing3, but its very existence suggests that the whole system has had a tumultuous history. White dwarfs start off as regular medium-mass stars, use up their accessible fusion fuel, expand into red giants, shed a surprisingly large fraction of their mass (B may be less massive than A now but the fact that B and not A is a white dwarf tells us that it used to be far more massive than it is now), and then settle down into a long senility as a slowly-cooling white dwarf.
None of this would have been good for a terrestrial world. Pre-red giant B would have had an even stronger, less predictable effect on our hypothetical world’s orbit. Even if the world had by some chance survived in a Goldilocks orbit, B would have scorched it.
This makes me sad. Procyon is, as I said, an old friend.
[I’ve thought of a dodge to salvage the notion of a potentially habitable world in the Procyon System. Take a cue from Phobetor and imagine a planet orbiting the white dwarf, rather than orbiting the main(ish) sequence star. We now know that there are worlds orbiting post-stellar remnants. This imaginary world would have to be very close to Procyon B if it is to be warm enough for life, which would mean a fast orbit. It would have a year about 40 hours long. It would be very, very tide-locked and you’d have to terraform it. Not promising. Still, on the plus side, the planet will be far too tightly
bound to B for A’s mass to perturb it much. Better than nothing—and much better than the clinkers that may orbit A.]
A more reasonable approach might be to abandon Procyon as a bad bet all round and look for a similar system whose history is not quite as apocalyptic.
It’s not Sirius. Everything that is true of Procyon A and B is true for Sirius A and B as well, in spades. Say goodbye to Niven’s Jinx: if Sirius B didn’t flick it into deep space like a bleb of snot, it would have cinderized and evaporated the entire planet.
But…40 Eridani is also comparatively nearby. It is a triple star system, with a K, an M and a DA star. Unlike Procyon, however, B (the white dwarf) and C (the red dwarf) orbit each other 400+ AU from the interesting K class star. Where the presence of nearby Procyon B spells complete annihilation for any world around Procyon A, 40 Eridani B might only have caused a nightmarish apocalypse of sorts. The red giant might have pushed any existing world around A from ice age into a Carnian Pluvial Event, but it would not have gone full Joan of Arc on the planet. The shedding of the red giant’s outer layers might have stripped some of the hypothetical world’s atmosphere…but perhaps not all of it? The planet might have been turned from a volatile rich world into a desert, but life might have survived—it’s the kind of planetary backstory Andre Norton might have used.
1: We had Peter Van de Kamp’s claims about planets orbiting Barnard’s Star, Lalande 21185, 61 Cygni, and others but those failed to pan out.
2: With slightly different values for distance and type, but I don’t have any of my outdated texts handy. Also, ha ha, none of the sources I had back then ever mentioned the ages of the various systems, which (as it turns out) matter. Earth, after all, was an uninhabitable armpit for most of its existence, its atmosphere unbreathable by us. The ink is barely dry on Epsilon Indi and Epsilon Eridani. Don’t think Cretaceous Earth: think early Hadean.
3: Unless you know what a Type 1a supernova is.
In the words of Wikipedia editor TexasAndroid, prolific book reviewer and perennial Darwin Award nominee James Davis Nicoll is of “questionable notability.” His work has appeared in Publishers Weekly and Romantic Times as well as on his own websites, James Nicoll Reviewsand Young People Read Old SFF (where he is assisted by editor Karen Lofstrom and web person Adrienne L. Travis). He is surprisingly flammable.