Astrobiology, worldbuilding…you see so many red dwarf, orange dwarf, sun-like systems you feel like your eyes are going to roll after a while. Well…I’m going to flip the script: what about a paradise of a planet, orbiting a giant star that’s well off the main sequence? For bonus points, how about multiple giant stars that have evolved off the main sequence? Perhaps one that’s not even that far from Earth, that could fit into my science-fictional universe I’ve worldbuilt out to Upsilon Andromedae?
There is a star system that answers to this description, and it’s a star that’s very prominent in the northern winter sky: Capella, the brightest light of the constellation Auriga. Among the brighter stars in the night sky, giant stars abound, including those that have evolved off the main sequence and are in their death throes, but most are rather far away; Capella is something of an exception, being bright not only because of how intrinsically luminous it is, but also because of sheer proximity to us: just 42.9 light-years distant from Earth.
The Capella system is made up of four stars that are gravitationally bound, but two are red dwarfs that orbit each other far out from the center of the system and are very dim (Capella H and Capella L); the main attraction is the central pair, Capella Aa and Capella Ab, both of which are yellow-white giant stars, orbiting each other every 104 days, at a mutual distance of 0.74 AU, roughly the same distance as between Venus and the Sun. Were these stars dwarf stars no doubt they’d each have compact systems of their own orbiting around them, but their sheer luminosity makes our own sun look like a pipsqueak: already massive, with Capella Aa sporting 2.57 times our sun’s mass and Capella Ab weighing in only slightly lighter at 2.48, the fact they’re in their helium-burning giant phase enhances their output still further, with Capella Aa putting out 78.7 times our sun’s luminosity, and Capella Ab outputting 72.7 times solar luminosity.
Where this gets fancy is that these two stars combine for an output of 151.4 solar luminosities; that’s enough to push the “habitable zone” far out compared to what we’re accustomed to looking at with the dwarf systems that are out there. To wit, if you placed this pair of suns in the middle of our solar system, Earth would have to migrate out to Saturn’s distance to maintain habitability (more precisely, a planet orbiting Capella Aa/Ab at 10 AU would have an equilibrium temperature of 282 kelvin, assuming Earth-like albedo, significantly warmer than Earth).
So any planets in the habitable temperature range would be orbiting Capella Aa/Ab at around that distance, ensuring that any Earth-like worlds around Capella would orbit both suns (a circumbinary orbit), leading to them dancing around each other tightly in the sky, always together; this configuration in fact is not that common in our stellar neighborhood. The vast majority of stars are either singletons, or they’re too widely spaced for a circumbinary planet to be anything other than in the deep freeze, or there’s a great disparity in luminosity between the components: there are few systems that consist of a pair of suns of comparable luminosity that would keep a planet in a circumbinary orbit as warm as the Earth. Capella is one of the closest (the closest?) places where you could have a binary sunset like the iconic scene in “Star Wars”.
So why don’t you hear more about Capella as a site for alien life? Well…one reason is exoplanet detection is easiest for massive, close-in planets around small, quiescent, steady stars, which Capella is basically the diametric opposite of: even a super-Jupiter-sized planet were it orbiting at 10 AU would take decades just to complete one orbit, so the radial-velocity signal or the transit alignment would likely not have been observed yet (we’ve only had the ability to detect exoplanets reliably since maybe 30 years ago; worlds like Neptune or even Saturn are still largely invisible to our instruments).
But the more important reason is that a star like Capella is young: heavier stars burn brighter, yes, but they also burn faster. Capella’s two suns are already exiting the main sequence, and they’re only 590-650 million years old; compare to our own sun, which is 4 billion years older (!). And it took over three billion of those years for a complex biosphere of plants and animals to emerge on Earth. When Earth was half a billion years old, life existed, yes, but as far as we know it was just primitive microbes, rather than anything “iinteresting” from a SETI perspective.
Worse yet, for most of the past 650 million years, Capella Aa and Ab were main-sequence stars, burning far dimmer than they shine today; any planet with Earth-like raw materials orbiting at 10 AU is temperate now, but for most of the past 650 million years it would have been far too cold (at least for surface life). Meanwhile, any planet that was orbiting at a temperate distance during the main-sequence part of Capella’s lifetime is now a sea of magma, utterly inhospitable to any form of life as we know it.
So there’s a reason you don’t hear much about a star like Capella: there probably just hasn’t been enough time for life to have emerged beyond the microbe stage. “But?”, you might protest, “you were talking about a paradise earlier around Capella?”. Why…yes. Yes, I was.
The fancy part is that when you expand your imagination a bit, you realize that there’s no particular reason a paradise world for humans has to be life-bearing to the point of being lush with native life-forms. For some purposes, it might be more paradisaical if it wasn’t. Such as, for instance, skiing, snowboarding, and winter sports. Sure, tree skiing is fun, but all you really need is the finest powder in the universe. It matters not if there’s a tree or an animal on the whole planet: as long as you have a liquid water ocean, an atmosphere at the right temperature range, and the terrain, you’ll have skiable slopes.
So what would be the most optimal environment for a ski world? Obviously we don’t just need water oceans, we need continents; landmasses, and ideally in a configuration that would strike a human explorer as being something out of a dream. On planets like Earth, continents appear as rafts that converge and diverge, drifting and floating basically at random, but from a terrain point of view, the absolute optimum for skiers would be narrow platforms of mountains that catch the prevailing winds…and nothing else. Ideally, such narrow platforms of mountainous land would be contiguous, forming a great circle around the entire planet. Since winds tend to blow in the direction of planetary rotation, you would want a pole-to-pole configuration.
Such a configuration of continental land would be contrived on a primary planet like Earth…but on a satellite planet, i.e. a moon, it’s a whole different game. For a moon, gravitational effects could easily lead the “rafts” of the continents to “pile up” in specific locations over geological time; one possible configuration, as we see with Iapetus, for example, today in our own solar system, is the terminator, or the limb, being high and rugged compared with both the near sides and the far sides as viewed from the primary planet. This of course assumes the moon is tidally locked to its parent planet, which is certain for any satellite that’s a small fraction of the primary and orbits relatively close (prominent examples include Jupiter’s Galilean moons as well as our own moon).
For Iapetus it just appears as rugged terrain on what is essentially an airless desert, but scale up the mass of such a moon, and it would retain an atmosphere and, if it were in the right temperature range, would collect bodies of liquid water on its surface, so the “lowlands” could easily be filled in with liquid water ocean. Perfection.
Another bonus of this pole-to-pole continental configuration is that there would be variations in climate, though it would be mediated by altitude. Optimal temperatures for certain sorts of skiing would occur at lower altitudes near the poles and at higher altitudes near the equator, but there would be a circumference of the entire planet that’s skiable; with downslope skiing and the development of a life system, you could circumnavigate the world on skis. A project that could easily take years, but what a life to induct young people in. Move over, Earth, there’s a new white paradise, just 43 light-years away…
It gets even fancier when you realize that the continents “pile up” near the terminator of the moon, the edges of the near side as seen from the primary planet; what this means is that from anywhere on dry land on this moon, the primary planet would loom near the horizon, either just above it or just below it. For those areas that are just within the near side, the view of the primary planet would be spectacular, especially when it eclipses the double suns of Capella A. Heck, the eclipses of Capella Aa and Capella Ab would be spectacular as well: the twin suns would regularly appear to merge and the total light received would dim noticeably, an astronomical event utterly unlike anything we’d ever see on Earth.
Skiers and mountaintops would regularly cast two distinct shadows due to the two suns occupying slightly different positions in the sky, and the double nature of this world’s sun, not to mention how it’s orbiting what presumably would be a gas giant planet, possibly even with a spectacularly icy ring system, would make for dramatically alien vistas almost with every picture of the skiers and snowboarders.
One might imagine the atmosphere would be alien, what with the life-forms being limited to early and presumably anaerobic microbes…but you might be wrong. True, Earth’s atmosphere was originally a carbon-dioxide-rich blanket devoid of free oxygen until life evolved a metabolism that dumped enormous quantities of free oxygen into the seas and later into the air as well…but life is not the only source of free oxygen out there. It’s been proposed in scientific studies that stellar radiation could easily split water into its components, hydrogen and oxygen, and in smaller worlds the hydrogen would tend to escape, with the heavier oxygen building up; around some red-dwarf systems especially a water world could easily end up with a free oxygen atmosphere of several thousand times Earth’s pressure. All without life having to be in the picture at all. The upshot of the study in real life is that a huge oxygen atmosphere may well be a false-positive “biosignature”: suggesting life where in fact none exists.
But for our purposes, we could imagine that this skiers’ paradise of a world had a composition not too unlike our outer planets’ icy moons before it was warmed enough to melt, and over time enough of the primordial water has been split off to build up oxygen. It’s not even too far-fetched: as it is Europa’s exosphere is actually primarily oxygen, because of the same process, it’s just that it’s far more tenuous than these exoplanets’ atmospheres might be.
For skiers, therefore, the possibility exists that an atmosphere that happens to be pure oxygen might be waiting for them, perhaps even in the right pressure range to be breathable. The absolute optimum amount of oxygen is probably higher than Earth’s but not too much higher; perhaps a partial pressure of 0.25-0.5 bars, similar to or perhaps double Earth’s oxygen content. Enough to make you feel invigorated on the slopes but not so much as to induce oxygen toxicity (yes, that’s a thing). The lower ambient pressure (due to oxygen being the only atmospheric component) would also make the sky on this world a deep blue color, with it being even darker and deeper at higher altitudes, perhaps even enough to permit some of the brightest stars to be visible in daylight. Very crisp, as befits a skier’s paradise.
Climatologically, there’s no particular obstacle to the weather being like something out of a dream: snow, snow, and snow some more, with the oceans functioning as a never-ending lake-effect conveyor belt. Now, in real life sea-effect snow shuts down after the temperatures equilibriate, so it tends to be seasonal, but on this world the possibility exists for year-round sea-effect snow. How is that possible? The lunar nature of the world, that’s how it’s possible. Consider that moons are subjected to tidal heating from their parent body as well as any orbital resonances with other moons (as we see in dramatic fashion with Io), so even as cold winds harvest the heat of the ocean to make sea-effect snow squalls, the internal energy from deep within the moon replenishes the heat content. So the lake-effect snow machine never shuts down. Fresh powder falls, falls, and falls some more. Forever.
Consider also that the youth of this world, at just 600 million years, would be a decisive advantage: at that stage in Earth’s development radiogenic heat was much more powerful, to the point of uranium still having enough fissile content to create naturally-occurring nuclear fission reactors (yes, really). Georeactors could drive localized heating, enough to create hot-spring-like lakes in the mountains reminiscent of Iceland’s geothermal pools, with georeactors under the ocean leading to pronounced “hot spots” that would create enhanced sea-effect snow zones as well. It might not be a world with complex native life, but it’s a very dynamic environment.
As for tidal heating, for it to have this much of an effect on the climate, ideally we’d want to maximize the effect, which means the moon needs to orbit its primary very close in, perhaps even close in enough to approach the “Roche limit”, where a planetary body would be gravitationally disrupted and break apart. Inside the Roche limit planets are shredded to create rings (this is thought to be how Saturn’s rings formed: an icy moon that migrated too close for its own good…), but just outside the Roche limit planets remain intact, but become distorted and elongated into a more oblate (i.e. egg) shape, rather than spheres.
This would help stimulate the continental pile-up near the limb (i.e. near the boundary between the near side and the far side), since ocean water level would be higher at each tidal bulge (pointing directly toward and directly away from the planet), owing to liquid water being more responsive to tidal pull than solid rock (this is the source of Earth’s tides, by the way: land flexes too, not just water, but water, being a more flexible material, flexes more, hence why it washes over and under rocks on the beach depending on the cycle).
So extreme could this effect be that gravity could be noticeably lighter at each tidal pole, and highest near the limb, i.e. where all the land is; as you stray toward the edge of the land, where the primary looms higher over the horizon, gravity might be noticeably less, though even here the effect won’t be drastic, owing to how narrow the continental landmass is.
Though one gravitational effect will be pronounced: the fact that even the baseline gravitational pull is unlike Earth. Keep in mind this is a skiers’ paradise, and so we don’t want gravity making things feel too heavy or strenuous; we want enough so they’re not hopping with every step like on the Moon, but modestly lower gravity than Earth, perhaps half as much, would be plenty enough to keep skiers anchored while also putting an extra floaty spring into every movement through that dreamily deep powder. Combined with the boost from the extra oxygen it might feel like the place is too easy, almost like a winter sportsman’s version of heaven.
Lower gravity also implies smaller mass, which helps; the formation of smaller moons is more likely than bigger ones, and we don’t really need a hulking huge massive body of a moon here. How massive?
I’m not decided yet, but to keep it simple, a body with half the mass of Earth but the same radius would of course have half the surface gravity; half of Earth’s density, which this planet would have, equals 2.74 grams per cubic centimeter, which actually is in the plausible range for a water-rich body (note: in this case, the numbers imply that the oceans are extremely deep; think thousands of miles, rather than the half dozen or so that Earth’s water depth tops out at). I’d change the numbers, of course, so it’s not exactly the same radius as the Earth and exactly half an Earth mass, but it does give a rough idea of what we’d be looking at.
Keep in mind that lower mass for a moon might be an advantage, in as much as if a moon like this was even as massive as the Earth, it might become geologically too vigorous due to tidal heating, becoming a world more similar to Io or (more to the point) Mustafar from “Star Wars”.
And as mentioned before, this moon and its parent planet would need to orbit Capella Aa and Ab at a very wide separation compared to what we’re used to dealing with with sun-like stars to red dwarf type systems. Playing around with my very own planetary equilibrium temperature calculator, the sweet spot (for plausible albedo ranges, from an Earth-like 0.3 to an icier 0.6 (ice is more reflective than most terrestrial surfaces)) seems to be 10-12 AU from Capella Aa/Ab. For reference, this is actually slightly greater than the distance Saturn orbits our sun. In such an orbit, the planet would take over 18 years to orbit Capella, which means seasons like Earth has would last for 18 times as long.
Though I don’t envision this world as having seasons, really. The climatological setup should be perpetually perfect for skiing weather, so there should be low axial tilt and low eccentricity, keeping solar input relatively constant across the entire planet. It might seem contrived compared to what we’re used to on Earth, but it’s plausible enough: Jupiter has low eccentricity and low axial tilt, for instance. In this case the primary temperature variation on this skiers’ paradise would come between day and night: as it whips around the primary, different parts of the planet would be illuminated or obscured, just like our own moon has phases. In our own moon’s case it orbits so far out that the days and the nights are long: on our moon there are 14 days of sunlight and 14 days of darkness. It need not be so lethargic on our skiers’ paradise, though; in fact it probably wouldn’t be. As a body approaching the Roche limit, its orbit, and hence its rotation period (remember: it’s tidally locked) would be relatively quick. Io orbits Jupiter in less than two days.
One pitfall to an Io-like orbit is that a moon analogous to Io would be right in the middle of the primary planet’s strongest radiation belts…which means any skiers would start to get sick, lose their hair, and drop dead within days. Oops. But the Roche-limit-approaching orbit solves that problem; in such a close orbit, our world would be safely outside the radiation belts. Indeed, our skier’s paradise of a moon might even be in an orbital resonance with a moon of similar mass in an Io-like orbit, perhaps icy like the skier’s paradise in terms of composition, but it would receive less tidal heating, and in any case it would be less subjected to the continental pile-up factor, making it less likely that any land breaches the water surface, and on top of that it could easily be right in the middle of the deadliest radiation belts. Think a world bathed in radiation that looks like the arctic sea ice writ large. Interesting, perhaps, but not the main attraction of the system, and certainly not a target for human colonization.
The radiation belts might be intense compared even to Jupiter, because such massive moons, at more than ten times the Galilean satellites’ masses (!), implies that the primary is likely also massive, i.e. that it would be in the “super-Jupiter” class. Plausible, considering that Capella is a massive system. And while such a world is in principle easy for our instruments to detect on a mass basis, the orbit is so widely separated that any radial-velocity signal would take 18 years to complete and then another 18 years to repeat, making detection difficult, especially with a star system as variable as Capella: even if there was such a monster world and we were detecting the signal, it would probably be dismissed as random noise.
Indeed, with such low axial tilt and low eccentricity, the variation in stellar output due to the suns’ own “weather” would likely be the primary source of “seasonal” variation, which is unlike anything in our solar system. Capella is in an interesting place where it’s stable enough to maintain habitability if the orbital distance is just right, but it’s variable enough for changes in stellar output to noticeably affect temperatures on these hypothetical planets.
So overall…a compelling world. One attractive to humans and a dreamy target for shirt-sleeve colonization…without it being a suitable habitat for the evolution of native life like we see on Earth. Capella’s skier’s paradise, as I will include in my science-fictional universe as a target for exploration and colonization, might be an extreme example of a heaven-in-space, but I bet a lot of planets are out there in real life that answer to that general description: the best places to go might not be “another Earth”, but rather something even better…