Well, my brainstorming for planets kick continues, now with a little tool for calculating planetary equilibrium temperature (we need better worldbuilding software, after all…), and also with some ideas for a planetary system that was discovered early on but is now often overlooked: Upsilon Andromedae. What secrets might it hold?
First, the fundamentals: Upsilon Andromedae consists of just three known planets, but all of them are massive, falling deep into the so-called super-Jupiter category. The inner planet (“b”) is 1.7 Jupiter masses, the middle planet (“c”) is a whopping 13.98 Jupiter masses, and the outer planet (“d”) is 10.25 Jupiter masses. All three of these gas giants orbit relatively close in, and this combined with the relatively small distance to Earth (44 light-years) is why Upsilon Andromedae b was among the first planets beyond our sun to be discovered, quickly followed by planets “c” and “d”; indeed, Upsilon Andromedae was the first main-sequence star discovered to host a multi-planetary system! 1996 was the discovery date for planet “b”, just barely behind 51 Pegasi b (the first planet found around another main-sequence star) in 1995, and the true multi-planetary nature of the system was discovered in 1999.
The inner planet orbits very close in: at just 0.0594 AU, it whirls around its sun in just five days; since this is an F-type star, which is 3.1 times as luminous as even our own sun is, you’d expect scorching temperatures at this range, and indeed the equilibrium temperature for a planet with Earth-like albedo (how much light it reflects vs absorbs) would be 1400 kelvin: hot enough to melt rock. So life as we know it could not exist here, or on any hypothetical moons (which, alas, have likely been lost due to tidal effects; the giant planet is surely tidally locked at this close range).
The middle planet is more clement, orbiting at 0.829 AU, though orbital eccentricity of 0.26 brings it in as close as 0.61 AU and as far as 1.04 AU. As one might guess for a star three times as luminous as the sun, the prospects of Earth-like life at this range is still questionable: equilibrium temperatures for a planet with Earth-like albedo (0.3) would range from 432 kelvin at perihelion to 331 kelvin at aphelion: even in midwinter, any Earth-style moons of this giant world would still be over 100 degrees Fahrenheit…even without any greenhouse effect. Summers would be literally boiling.
In principle, if a planetary-mass moon were covered by reflective material, the albedo could rise to perhaps 0.8, in which case winter temperatures with a rarefied atmosphere with little greenhouse effect could drop to as low as 242 kelvin…or -24 Fahrenheit. One problem with this scenario is among the most reflective materials is…fresh snow and fresh ice. Which would melt off in the summer: even with an albedo of 0.8 summer equlibrium temperatures would rise to 109 degrees Fahrenheit. Something like reflective salt flats might work though, but in that case you’re talking about a desert planet. Habitability: possible, even inward of the classical habitable zone here, but not very likely.
Though mass helps here: at 14 times the mass of Jupiter, it’s possible and even likely that this planet has equally massive moons. Jupiter’s four moons are each perhaps 0.02 Earth masses; scaling them up for this 14x Jupiter, you could end up with four moons that are each 0.28 Earth masses, twice the mass of Mars. Especially with tidal heating and flexing, they could be dynamic worlds unto themselves with atmospheres, possibly even life. For all we know the proto-planetary disk around this world (similar to that which is proposed to have existed around our system’s gas giants) might have been proportionally larger…or instead of four moons of equal size one dominant moon could have formed (there could easily be enough material for an Earth-mass body!).
But the real star attraction of Upsilon Andromedae is the outer planet, planet “d”: it’s smaller, but still 10.25 times the mass of Jupiter. And it orbits at 2.53 AU; with Upsilon Andromedae being the luminous white star that it is, the equilibrium temperature for an Earth-like world here would be 212 kelvin. That’s -77 degrees Fahrenheit, which is chilly, but not drastically so compared to Earth (Earth would be around 0 Fahrenheit without the greenhouse effect). The planet’s orbit is eccentric (0.299), so it swings out as far as 3.29 AU, where temperatures would get chillier: equilibrium temperature drops to 186 kelvin, or -124 Fahrenheit. Chilly. On the flip side, however, at perihelion it swings in as close as 1.77 AU, where equilibrium temperatures rise to 253 kelvin, or -1 Fahrenheit…almost identical to Earth!
What sort of worlds might Upsilon Andromedae d have orbiting it? At 10 times the mass of Jupiter, you’re in the same genre as with planet “c”: a set of moons proportional to Jupiter’s would be like miniature Earths in terms of atmospheric retention and how dynamic their geology could get. One dominant moon gets you to about one Earth mass. And the temperature range with even a modest greenhouse effect is suitable for liquid water.
In my science-fiction universe, humanity has already explored ocean planets in the form of Proxima Centauri, which hosts intelligent life, a world of desert salt flats orbiting Barnard’s Star, the spooky towers of Sirius B’s planet, and an even spookier Earth twin of a moon that’s like the mother planet filtered through an ancient dream, belonging to Mu Cassiopiae. All of these worlds are much closer than Upsilon Andromedae, and hence in my stories will have been reached and explored earlier. So for Upsilon Andromedae, I want something special. Consider also that this is the closest known system to Earth containing multiple super-Jupiters, and in particular a super-Jupiter in the habitable zone that might host hospitable moons.
Indeed, so compelling is the system from the perspective of early exoplanetary astronomy, I’ve wondered if in my alternate timeline it might actually be the first to be discovered. In real life 51 Pegasi b beat it to the punch, but only by a year, and while 51 Pegasi b gives off among the strongest radial-velocity signals of any extrasolar planet, Upsilon Andromedae’s worlds actually send a stronger signal. Hence why it was discovered so early: a combination of massive planets orbiting close in and close to Earth around a quiet star is ideal for teasing out the wobbles in these stars that betray the presence of a companion body (such as a planet).
In my alternate-history science-fiction universe, computer technology is perhaps half a century ahead of real life, and as things stood, the computers were the bottleneck: not until the 1980s did the technology really start to be there to even start attempting a radial-velocity-based search for extrasolar planets. How early could it be pushed? Well…with computer technology half a century ahead, you’re dealing with the technology to tease out these signals from the noise being available from the 1930s onward. Which is wild…so wild it makes one wonder if telescopes would have been good enough. The answer, surprisingly, seems to be yes: the telescopes of the time, and indeed considerably earlier, were capable. All you’d need would have been for a visionary astronomer to develop the radial-velocity technique, actually mount a search, and use the necessary computer technology to tease out the signals. With enough vision, I think you could push it back to around 1920, maybe even a bit earlier.
But in my alternate timeline I have Percival Lowell himself discover Pluto in 1916 (rather than dying without having found his Planet X…), followed by the discovery of the other Kuiper Belt objects in subsequent decades (space telescopes launching from the 1940s onward will help a lot; more on that in a future post). So a visionary astronomer who no doubt everyone dismisses as crazy discovering incontrovertible evidence of an unseen planet orbiting Upsilon Andromedae in 1919 fits into the picture beautifully. The true multiple nature of the planetary system will be discovered soon thereafter, and boom: as early as the 1920s you have a huge planet known to be in the habitable zone and potentially hosting moons, granting this system the spirit-animal status of “the first” for mankind.
When mankind finally reaches the system and sees it in person, what will they find? Something more exotic than the standard-issue desert or ocean world is called for for Upsilon Andromedae d if you ask me, and frankly the very mass of the planet provides the perfect answer: what if the world was located right in the middle of the radiation belts? It’s not unprecedented: Io is located right next to Jupiter’s radiation belts, and we can have little doubt a planet 10 times Jupiter’s mass would have massive belts, particularly since it’s orbiting a star considerably more vigorous than our sun.
If the world were roughly Earth-sized, there’s still the possibility if the primordial disk were proportionally a bit more massive than Jupiter’s that there could still be other moons, orbiting further away; let’s say two other moons, locked in a classic Laplace resonance with the big inner moon, to the tune of 1:2:4.
If this planetary-mass moon (henceforth “planet”) is volatile-rich and has a lot of water, then this radiation would tend to strip the water content and break it down into hydrogen (which escapes) and oxygen (which is retained), leading to enormous free oxygen build-up. I’ve explored this in more milquetoast form with my world Thalassa, which has dozens of times Earth’s pressure, all pure oxygen, at the surface, but research has suggested that volatile-rich worlds could build up free oxygen atmospheres of up to 2000 bars. Let’s take that and run with it for this moon: 2000 bars! The full-blown scenario!
With 2000 bars at the surface, there would actually be a layer of breathable air…at perhaps 66 kilometers up. The edge of space by Earth standards, but by this world’s standards you need to be at the edge of space just for the air to be thin enough to breathe!
Water ice clouds are very thick in such an atmosphere, which means albedo is likely high, potentially as high as 0.7…which helps to cool off the moon. Assuming an albedo of 0.7, then at the semi-major axis this planet would have an equilibrium temperature of 173 kelvin, ranging from 201 kelvin at perihelion to 151 kelvin at aphelion. Which is enough for the planet to be frozen year-round. Oops. But wait…what about greenhouse gases? Well…Venus is very reflective, but has an extreme greenhouse effect…but oxygen is an extremely weak greenhouse gas, so that’s not going to be much help. At 2000 bars it will warm the planet, but probably only roughly as much as Earth is warmed by its carbon dioxide and water vapor: perhaps 30 kelvin. Which brings the realistic range of temperatures from 181 kelvin to 230 kelvin (the freezing point, for reference, is 273 kelvin, so this is way below that).
However, there’s a trump card here: like Io, there is also extreme internal heating on this planet, perhaps up to a thousand times the geothermal flux of Earth. On Earth internal heating would be barely enough to warm a subsurface ocean, but multiply it by a thousand times and it starts to make a difference.
The biggest difference would actually be in the interior, with the core: the core-mantle boundary could easily reach temperatures of 6000 kelvin, with pressures being such that silicon dioxide (the primary constituent in magma) would not only be liquid but past its critical point, which is poorly studied. But the core would not be solid at any point: it would be all supercritical fluid. And the water would also be supercritical deep down, under such extreme temperatures and pressures; there would not only not be a solid core, but there would also be no distinct boundary between the deep water ocean and the magma core. Rather there would be a supercritical water-magma slurry in the deepest interior of the planet, grading smoothly from more water-rich to more magma-rich as you descend.
Massive volcanism in the form of black-smoker-style hydrothermal-esque plumes would erupt from the supercritical boundary worldwide and stream upward toward the surface. Which would make a very dynamic environment for life.
Interestingly, all the carbon dioxide and so forth put out by the volcanoes would be liquid under 2000 bars worth of atmosphere and would not be a greenhouse gas: it’d just rain out in the low altitudes. Indeed, about the only constituent that could survive the severely oxidizing high-pressure environment would be nitrogen (N2), perhaps adding up to several bars at the surface (more vigorous volcanism means more outgassing)…but even then, given the sheer thickness of the atmosphere even 2 bars worth of it only adds up to 0.1% of the total atmosphere. Even supercharged volcanism is no match for the sheer drenching mass of oxygen.
It is a match for the cold though: dozens of kelvin worth of warming at the surface could easily be achieved from this level of internal heating. So with 40 kelvins worth of internal heating (which is about what we’d probably get), temperatures at the surface range from 220 kelvin to 270 kelvin, which approaches the freezing point. Keep in mind also that deep water has extreme thermal inertia and seasonal variations would be smoothed out a great deal. So instead of a 50 kelvin range there might be more like a 10 or 20 kelvin range annually, despite the long year.
These figures are still below freezing! But consider that a planet and a primary with this strong of a magnetic field would harbor extreme aurora borealis, quite possibly reaching daylight levels of illumination on the night side on a routine basis; most of the light output from aurora is indeed in the visible and near-infrared parts of the spectrum, so there could be an additional 20 kelvin or so worth of heating from that alone.
Add it all up and this moon’s oceans might not freeze, despite the distant orbit and lack of greenhouse gases. Certainly summer open seas are plausible, with even water staying liquid at the surface in winter a possibility. Keep in mind also the entire planet would have about the same temperature due to the thick atmosphere redistributing heat.
The extreme ambient radiation would also drive Cherenkov radiation in the water and even in the thick air (!), creating ghostly blue flashes lasting for several seconds and ripple-like glows in the first several hundred meters of the ocean surface, leading to additional illumination. Photosynthesis is plausible, since at high noon on a good day the surface of the water could experience illumination levels similar to civil twilight (oxygen is a bright nigh-transparent blue, but 2000 bars adds up, so it’ll be relatively dark down there…).
The extreme oxygen levels mean that once aerobic respiration evolves on this world, the native life would have access to truly extreme levels of energy, leading to truly enormous body sizes (think wingspans of over a mile), and potentially being enough to loft themselves high into the atmosphere, evolve biological jets and rockets, and even launch themselves into space. Picture airborne reefs that are lofted straight up into the edge of space, and then beyond into orbit, native bird-like forms gradually adapting to the ultimate high ground, which would be evolutionarily advantageous. Space-hardiness would be an advantage, and radiation resistance to extreme levels would be a given. Once complex life gets going there could be a Cambrian explosion, which could have occurred recently in this world’s geological past.
Life-forms could photosynthesize and radiosynthesize even in space, if they were hardy enough, which they may well be, considering how gradual adaptation to the rigors of the edge of space is both possible and advantageous. Consider also that the extreme ambient electromagnetic energy courtesy of the magnetic fields could drive electrical impulses, which drive neural and brain systems like in humans. So by tapping into this source, far bigger and more sophisticated neural activity and brains could be sustained than at the comparable stage of evolution on Earth.
Space-dwelling primary producers in the radiation belts would be primely positioned for this, but all the life could have stunningly large and complex neurology for what it is. Since the Cambrian explosion of this world only occurred recently then intelligence could have arisen but only geologically recently, and because they’re dependent on the outside power source of the electromagnetic fields for their brains and nervous systems to function (which they evolved perhaps to navigate magnetic currents and perhaps as a kind of runaway sexual selection) they wouldn’t easily be able to leave their home environment without industrial-age style technology…which would be very difficult for them to develop in the first place. So a very extended primitive age despite their smarts is likely.
This vast network of life extending flush into orbit introduces the possibility of electrical signals being used to communicate directly even between bodies, a la how it’s used inside the human body itself in the brain and nervous system, with these life-forms being attuned not to sound or even light but rather to magnetic fields and electricity. Life forms in deep water could use that as a medium to propagate electrical signals effectively as well, and god knows what goes on in the supercritical layer.
So there could be a sapient intelligence emerging in orbit, most likely as metallic-coated fractal sphere-formed organisms (spheres are identical in all directions, useful in a zero-g setting, and fractal maximizes the surface area to collect radiation with), in communication with the deep-sea neural layer of the biosphere, which they could even start to learn to manipulate from afar to create a bio-formed industrial biosphere that they could harness for resources, for manufacturing, and so forth, but all this is in the extremely early stages as of when humans make contact in the 22nd century or later. Radio could be a native sense for them, since radiation belts are most apparent in the radio part of the spectrum. Near-UV, which Upsilon Andromedae peaks in, would be useful as well, for photosynthesis.
If some planets out there are a super-Earth, this one is a super-Thalassa: like Proxima Centauri b in my sci-fi universe but far more extreme, like it was filtered through a dreamscape.
As for the other two moons, they don’t get nearly as much radiation or tidal heating, so they’re less extreme than our front-and-center world. They might also have oxygen atmospheres though, a la Europa’s exosphere but far thicker (yes, Europa outputs oxygen, and the mechanism is the same: radiation splits water into hydrogen, which escapes, and oxygen, which escapes more slowly; it’s just extremely thin owing to the low mass and long distance from the sun).
The middle moon (our exotic radiation-bathed moon is the inner moon) might be about Mars mass and have a primarily oxygen atmosphere as well, perhaps a few bars worth, though the nitrogen component is more notable. There’s a human-habitable layer at a much more modest altitude. The outer moon has a thinner atmosphere that’s also oxygen-nitrogen and might be human-breathable at the surface; radiation exposure is also Callisto-esque, if we’re using Jupiter’s moons as analogues: far more modest.
Both of these moons would have similar climates: without nearly as much internal heating rising to the surface, and without nearly as much aurora, and without any greenhouse gases, these two outer moons experience the deep freeze (alert: this makes a dramatic contrast for stories!).
There is pronounced seasonal ice-albedo feedback, as near as I can tell. In summer it gets hot enough for carbon dioxide ice to sublimate, exposing older and darker water ice. With an albedo of 0.6 temperatures would be 216 kelvin at perihelion, -71 Fahrenheit. But when the system moves away from its sun and temperatures drop, carbon dioxide snows out en masse; even a few millibars a la Mars would be more than enough to drive global blizzards depositing fresh (and bright!) dry-ice snow on the surface (which would honestly be so awesome it makes me want to go on a starship now!).
With an albedo of even 0.8 at aphelion temperatures would fall to 137 kelvin or -213 Fahrenheit (!). With even higher albedo and in the polar regions at night (keep in mind these atmospheres are much thinner and don’t distribute heat nearly as well as the inner moon: these worlds are more Earth-like), temperatures could easily approach -300 Fahrenheit. Perhaps with oxygen and nitrogen starting to condense out of the atmosphere in the form of rain in the very deepest winter nights, but nothing too significant before it starts to warm up again. In these worlds such events would be like diamond dust is on our planet: an exotic curiosity of the polar regions.
The primary weather cycle is carbon dioxide snow and sublimation (as we see on Mars to some extent). This albedo feedback mechanism ensures that the seasons here in stark contrast to the inner moon are rather extreme, but it’s a stable climate regime. On the very hottest summer days in the tropics on these worlds there might be brief melting at the surface, but the ice sheets would be miles thick; perhaps at the equator internal heating combined with seasonal melt might just enable polynyas to open up briefly, especially on the middle moon.
So Upsilon Andromedae d could host no less than three moons with breathable air (albeit only at altitude), but with extreme and exotic and alien conditions. For bonus points Upsilon Andromedae d could have a massive inner icy set of rings near the Roche limit, perhaps the remains of what used to be a fourth major moon in the geological past.
The net effect? This would be one of the most spectacular worlds humans have ever encountered: at 44 light-years it’s distant for early interstellar expeditions but once laser sails to 95% of light speed are mastered or some such it becomes a doable destination early in. Thalassa will turn out to just be the beginning…