I’ve already done some worldbuilding for my planet “Cerberus” and the surrounding system (Sirius B), but what about the Dog Star itself, Sirius A? We wouldn’t want it to be boring, but we also want to keep it scientifically plausible, yet not duplicative of the other “exciting” possibilities I’m exploring in our stellar neighborhood in my science-fictional-cum-space-operatic universe. That’s tough.
No truly gargantuan planets could exist around Sirius, or else they’d have been detected already, and I’ve already used the concept of an oxygen-drenched ocean world for Proxima Centauri’s planet, and I will be using the concept of a salty ocean world with desertified Devil’s-Tower-esque continents for Sirius B’s planet Cerberus, along with the idea of a rather more Martian but much more volcanic world also for Sirius B, another planet inward of Cerberus. So where does that leave Sirius A in terms of exciting but not repetitive or implausible options?
Sirius B’s orbit imposes limits on how far out a planet could orbit Sirius A and still be stable: Sirius B orbits as close as 8 AU and as far as 31 AU from Sirius A. Sirius A has 25 times the luminosity of our sun, so even at the likely maximally stable orbital distance of 2 AU or so it’d be hot. So for terrestrial possibilities you’re talking about lava worlds (which we already have around Sirius B…), Mercury-like moonscapes, and Venus-like hellscapes. Meh.
But then it struck me that one category of planet we haven’t heard much from from our stellar neighborhood in my world is gas giants. Again, nothing gargantuan could plausibly exist around Sirius in the real world, but Neptune-mass or even heavier worlds? Absolutely, that’s still a possibility…one I might lean into when brainstorming out this system. And there’s no shortage of gas giant worlds that exist at hot temperatures.
But would just another hot Jupiter or hot Neptune really do for this system? After all, any number of systems closer by ours might have your ordinary hydrogen-helium (or ice-giant) worlds that orbit close in. But there is a category of planet that is not found in our solar system, nor would be the most common type to find around your everyday red-dwarf star: a helium planet. Aha.
Unlike a typical gas giant, which is primarily comprised of hydrogen, helium planets, as their name suggests, would have a much higher proportion of helium. The usual reason for this would be heavy stellar radiation stripping their atmospheres of lighter elements…which in my setup, is exactly what happened system-wide, hence why Sirius B’s inner planet has just a wisp of atmosphere and Cerberus, which might have originally been a water-drenched world like Thalassa (Proxima Centauri’s planet), had so much of its water stripped away that dry land was exposed, and so much of even the oxygen that remained after radiation split the water into its constituent elements blown off that what remains is barely enough for humans to breathe at sea level (about a tenth of Earth’s total atmospheric pressure). If similar processes occurred around Sirius A, then absolutely a typical gas giant might be stripped down to being a helium world.
What would such a planet be like? It would be denser than our gas giants, but not drastically so. No, the real change would be felt in terms of the colors and hues of these worlds: while ordinarily gas giants in Sirius A’s solar system (at least beyond the hot Jupiter range of orbits) would be a clear blue, helium scatters light distinctly differently from oxygen, nitrogen, or hydrogen. Most likely such worlds would have skies dominated by white or grey hues, appearing bright yet nearly featureless to the human eye, especially under the harsh bluish glow of a star like Sirius A at such close range.
The abundance of helium and the depletion of constituents like water and methane (remember: they’re hydrogen-rich compounds) would make these worlds rather inhospitable to life, even if they weren’t too hot for life as we know it on Earth to take place. But what about something more exotic? I’m thinking metal-oxide based life that uses liquid silicates (i.e. lava) as a solvent; in my universe microbial life is known from Io that uses volcanic liquids as a solvent instead of water, so in the context of this setting the idea isn’t all that weird.
And airborne life is known from Venus, among other worlds heavy in atmosphere but light on hospitable surfaces: in particular, it’s theorized in this universe that the microbes of Venus evolved on the surface when it was more habitable but then adapted to life in the skies when the planet’s climate catastrophically warmed, rendering the solid surface a hellscape. What if something like this happened in the Sirius A system?
Picture a moon of one of these gas giants that was Io-like, but then it became disrupted, perhaps from a decaying orbit that caused it to break up, leaving no trace but a dark dusty ring system around one of these gas giant planets (most likely the outermost one, since a ring system would be the most stable out there), but seeding the world with these microbes, which thrived in the upper reaches of the atmosphere, dominated by helium gases but with enough trace elements for silicates to condense into clouds and rain down lava drops into the depths (this is known to happen on hot Jupiters in real life, by the way: silicate cloud formation; yes, these worlds are that hot).
These microbes could even have been transported to the other planets in the system via meteorites after the breakup of the moon (or even before, as per the lithopanspermia hypothesis).
Anyway, picture three helium worlds, featureless bright white except for darker greyer silicate clouds raining down droplets of lava, rarefied precipitation leading to periodic blooms of color, perhaps rusty reds if their metal structures are made of or at least enriched with iron oxide. The minerals needed for these microbes’ metabolisms might be very rarefied in the upper reaches of a helium world’s atmosphere, so I’m picturing these life-forms remaining dormant most of the time, blooming most of all when a stray meteor burns up in the atmosphere, temporarily enriching a small volume of the planet with the nutrients they need to thrive…and surprising human explorers, who might assume these worlds are chemically inert to the point of being deader than dead. That’d be a cool little concept for a story…
My preliminary concept calls for three of these gas giants, all of them helium-dominated worlds, but with slight differences. The outer world has a dark dusty extensive ring system, the remnant of the volcanic moon that gave rise to these life-forms in the first place, less active but with life still present in a kind of pre-biotic soup for lava-drinking microbes that find themselves trapped in a helium-dominated atmosphere: very much life, but primitive in form. More complex and perhaps even photosynthesizing life-forms might be present on the inner-most planet (UV light is abundant there, so photosynthesis evolving early and hard is a believable, if speculative, development). That would give the inner planet a lot of distinction; the outer planet of course has the rings, and as for the middle planet…eh, I don’t know yet what distinct characteristics it’d have (if any).
I’m just brainstorming here, after all, and I’m still in the early stages. I don’t even have the orbits nailed down yet, in terms of where I’d like to place these fictional bodies, but I am intrigued by the possibility of just doing a classic Laplace orbital resonance a la the Galilean moons of Jupiter: orbital periods around Sirius A synchronized to the tune of 1:2:4 for each planet.
Now, as for exact details and how that fits into a story, how it ties into Sirius B’s system, and how it ties into a potential follow-up expedition from Sirius to Procyon (which is the next really bright star system accessible from Sirius once you’re there)…I don’t know, especially since Procyon’s setup (a white dwarf combined with a young white giant star) is very similar to Sirius’s! Grr…how is someone to make unique setups for all these solar systems? How is one to decide which targets would plausibly be selected for expeditions that might take decades to complete? Decisions, decisions, but such is the life of a science-fiction worldbuilder…