OTHER CONCEPTS | EMPIRE OF A THOUSAND SUNS & MULTI-STAR SYSTEMS

'HOW TO BUILD 1000 STARS'

You may have watched Artifexian's video about How to Build 1000 stars, if not, go watch it now.

Done?

Well, so why am I bringing this up?

Realistic night skies, actually, this is something I wanted to do for a long time - actually, tried once with a 3D environment, plotting the positions of stars in space, in order to get the sky map from the Ran system (eEridani). It didn't worked out well since my hard drive fried - by experience I will warn you that this is extremely wearing work, for little pay out other than a well thought environment that is rather cool to look at.

TL;DR:

Milkyway-wise, it can be about 900 Main-Sequence, 96 Degenerates, and 4 Giant stars.

This can be further expanded as containing:

O 1

B 1

A 5

F 30

G 70

K 100

M 700

WD 100

III&IV 4

And maybe 1 Neutron star and 1 Black Hole - we will not talk about them over here though.

Remember that Main Sequence stars have masses between 0,08Msol and 50Msol.

Stellar masses roughly distributed as:

O +16

B 2,1~16

A 1,4~2,1

F 1,04~1,4

G 0,8~1,04

K 0,45~0,8

M 0,08~0,45

Humans, and life as we know it, could only appear, and survive around K, G, and F-type stars.

You can define your stars using the automatic STAR&PLANET calculator in our [CALCULATORS and SPREADSHEETS] section later on.

INTRODUCING MULTI-STAR SYSTEMS

One thing Edgar didn't touch, is about the multiplicity of stars, ~1/3rd of all stars have at least 1 companion, and at least 5% of wide binaries and 50% of close binaries do have the stability conditions for habitable planets.

Even though only 30-ish percent of binaries have planets, we are still talking about (100*0,3*0,3*0,5= 4,5) ~4,5% of all stars with suitable conditions to habitable worlds, compared with the figures of 2,8% for single stars like the Sun, for that we can take that at least half of our habitable planets are around double stars.

So, let's take a look at how those multi-sun systems work:

You can pretty much calculate the percentage of multiplicity for a star of a given mass using:

Mindex% = 75 * (-0.92^Msol) + 100

For example, by entering 0,94Msol, we get that ~30,6% of stars with this mass have at least 1 companion.

As a rule of thumb for double stars:

• If your component B is as bright as component A, it is more likely that the two are from the same spectral class.

• If B is considerably dimmer than A, the component B is more likely to be hotter

How cold? You can use this, derived from MSC graph data:

TB = T A - (0,6 * (1,00149 ^ TA))

Temperatures given in Kelvin, for example; If component A's temperature is around 4750K (K1V), then is more likely that component B's temperature would be about 707K cooler, or around 4043K (K6V).

The temperature difference gets larger as the primary get's hotter, a sun-like star at 5800K would have an M5V red dwarf as companion by this trend.

The lower the mass of the primary star, the more similar the secondary will be to the primary.

This formula also will yield a brown-dwarf compatible temperature for primaries between 5900~6200K, above 6200K the correlation states no companion is possible, you will point out there is binaries hotter than 6200K, - although I believe that is due some star systems are not formed within the same molecular cloud, at least, not so close they would interfere with the other's formation, or within a certain mass limit, the correlation becomes less clear above 5000K, but is a good overview so far.

ORBITAL STABILITY AROUND BINARIES

You will see that there are certain limits for stable planetary orbits around double stars, Edgar's Forbidden Zone.

I would like to point out that it's more of a rule of thumb than actual physics - it works, but works the same way it's easier to hit a basketball hoop from across the Atlantic ocean if you use the Moon as a basket ball.

In Build your own Tatooine, the Makkel system is formed by two stars, one F-type 1,3Msol, and one G-type 0,9Msol, separated by 0,3AU.

Now, he calculates the inner and outer edges using 1/3*min Sep and 3*max Sep, what yields 0,05AU and 1,32AU.

Using an actual study on planet stability around binaries, we see that the actual edge - the minimum distance a planet can be from the close-binary pair, is about ~1,09AU.

For reference, if Edgar wanted Makkel to be an S-type system around both stars, he would have to put the stars 21AU apart.

• Is worth noting that circumbinary planets (P-type) found to the date of the paper seem to form closer to the inner limit the wider the binary is, and also form not much further than 10AU.

• And when the binaries are separated further than 10AU, then is way more probable the planets in the system are actually circumstellar (S-type).

Both figures for A~1,0Msol and B~0,5Msol.

PLANETS AROUND BINARIES

FORMATION

P-type planets are somewhat rare, current findings put a low-estimate of 2,8~3%, while simulations put the frequency at ~10%.

This may be limited by our current detection technology, which works fine for planets around single stars because they orbit close enough to cause visible perturbations in the system.

Planet formation around binaries is also narrower than around single stars, the planetary disk's inner edge tend to be wider, and the outer edge tighter in close binaries.

Three items should be met in order for planets to form in stable and habitable configurations.

1. The planetary disk must exist either inside the S-limit for wide binaries, or beyond the P-limit for close binaries.
2. The habitable zone must exists either inside the S-limit for wide, and beyond the P-limit for close binaries, on top of the planetary disk. 
3. The planet should exist within the habitable zone, but not closer than 1/3rd the minimum separation between the stars for S-type orbits.

The close binary pair on top can host a habitable planet, while the wide binary at the bottom can only host a habitable planet on it's dimmer component

FREQUENCY

Current technology lay a frequency of planets of: ~5Re at 10~15%, 6~9Re are as low as 5%, and planets larger than 10 Earth radii at ~1%

In this perspective, Jupiter and Saturn-sized planet frequency is <0,5%.

Most systems are also flat, ~4º in inclination, but the larger the planet, the more inclined it's orbit gets, planets with 10Re are found to be ~40º from the system's plane.

Speaking of large planets, hot-jupiters are more common in binaries wider than 100AU, at 10~15%, compared to <5% in binaries wider than 1AU and closer than 50AU - there is a zero frequency gap between 50AU and 100AU.

Stars that are 10AU apart are the more varied in planet abundance, having from as little as 12% and up to 80% as much planets as single stars, and average at 20%.

For pairs which combined masses are around 1,5Msol, the most common orbital distance is between 0,25~2,5AU, the second most common is between 2,5~3,8AU.

The most common orbital periods are in decreasing order: 5, 6, 4, 8, and 7 days.

The remaining ~30% of binary stars have orbital periods of: 3, 2, 9, 1, 10, and less than 1 day.

See [CALCULATORS and SPREADSHEETS] for all the essential calculations about binary stars.

HOW MANY COMPANIONS?

This paper, cites the proportion of multi-star systems as (470 :108 : 27 : 5 : 2), for 3, 4, 5, 6, and 7 stars systems, respectively - although "is not necessarily representative of the true proportion [...]".

But assuming that's is somewhat similar to the true proportion, a regression of y~ab^x gives us a sample of ~16.078 stars, we then have 1302 binaries, and the previously stated numbers for multiple systems, which unfortunately gives us only ~11,3% of stars as multiples.

If we maintain those proportions but raise the numbers by 3x, we should get 33,4% of stars as multiples - however that may be unnecessary, one of the authors of this paper, recently in 2008 published another study regarding that matter.

Where we find this table:

We end up with multiplicities of: 59.62%, 31.52%, 6.25%, 1.88%, 0.44%, 0.24%, and 0.04%.

Table 6, from page 8, A catalogue of multiplicity among bright stellar systems (P.P. Eggleton, A.A. Tokovinin), colored for visual clarity

Applying that to our 1000-sun empire, we have the following amount of stars available:

This chart above shows the available components for each formation, for example, the 26 double G-type stars means that, there are about 26 stars of type G available to be part of a binary system - regardless of what the other components are

This way, you are free to choose if you'll have 24 GG binaries, 1 GK, and 1 GM binaries - or pair each of the 26 G

You could group an FG-KM quadruple system, and pile nested hierarchical systems with doubles and triples.

Systems with 100+ stars are better defined as dynamical open clusters - it would make sense to have your most massive stars as part of a 

THOUSAND SUN EMPIRE AS AN OPEN CLUSTER

THE HOT SINGLES IN YOUR AREA

Place your high-mass stars at least 50~100Ly away from your main habitable systems.

Supernovae up to 1000Ly distance can have still noticeable effects on a planet like Earth, with global warming effects of 3~4ºC.

Historically, about 20 supernovae happened within 1000Ly from Earth in the last 11Myr, and 1 supernova happens within <30Ly every 240 Myr.

The problem with supernovae is that the gamma-ray bursts they emit can destroy a considerable part of the ozone layer, exposing the planet's surface to dangerous UV light and cosmic rays, damaging life on the surface, reefs, and phytoplankton communities.

In worst case scenarios, a type-II supernova happening less than 30Ly could happen as rarely as 1 every 2Gyr, up to 10 per Gyr, within less than 26Ly and half of your planet's ozone layer (the side facing the star) is destroyed.

Only stars with more than 8Msol go supernova, which means that we have at least 1~2 stars which can go supernova, however, if we consider the other 4 giant stars which can easily be put into the supernovae mass requirements, we can have as much as 6 stars ready to explode in our empire.

We shouldn't expect to ever be within safe distance from any star. Given the dimensions of our empire (63x63x63), the core worlds would be at high risk (30-ish Ly from the outer reaches), the external colonies would be at moderate risk (30~60Ly from other stars), it would be safer to put the supernovae candidates in the surrounding

CLUSTERING MODEL: PLEYADES

For instance, the Pleyades Cluster nucleus is about 16Ly wide, and the cluster itself is about 86Ly wide. While the Pleyades can easily weigh out 800Msol, our empire could be a relatively small cluster with mass topping at 550~600 Msol.

If the galactic average distance between stars is little more than 1pc (3~5Ly), at the center of our cluster the distance between stars could be of ~1Ly, and increase towards the edges to the galactic average.

Closer than ~1Ly and the gravitational interactions would disrupt the entire system, and lower the planetary stability in systems to as low as 100Myr - that's why globular clusters aren't a good place to search for planets - plus, the sheer proximity between stars, comparable to that of the size of the solar system, makes planetary accretion nearly impossible, and even if some did managed to have a permanent planetary system, it would be exposed to dangerous levels of radiation and solar winds, and night wouldn't be darker than Earths twilight, because there is a thousand nearby suns in the sky. The centers of globular clusters are also believed to host black holes, which carry all this flock of stars around with them, again, emitting more radiation.

If we divide our cluster in 6 concentric regions, each 5,25Ly thick, we increase the average distance in half a light-year per level, as much as 28% of the cluster population, lies just in the first level, in the second level, we have encompassed about 54% of the stars, and 74% in the third level, we easily have ~3/4th of our total population of stars half-way through the empire, and the other 25% spread over a volume 7x greater than on the inner half, where counter-intuitively, the true spacing between stars end up being less than half that of the galactic average (

From hundreds of light-years away, the cluster may look like this:

number of stars is heavily exaggerated for visual clarity

Still taking the Pleyades as an example, we could also include ~2% more mass by adding several brown dwarfs, if every brown dwarf we add weighs about 0,05Msol, then we have added between 11~12 thousand other places in between stars, and even companions/sub-planetary systems.

CLUSTERING MODEL: HYADES

Looking for something more solid to base our empire on, we can use Open Clusters instead.

For a typical cluster with 1,000 stars with a 0.5 parsec half-mass radius, on average a star will have an encounter with another member every 10 million years. The rate is even higher in denser clusters. These encounters can have a significant impact on the extended circumstellar disks of material that surround many young stars. Tidal perturbations of large disks may result in the formation of massive planets and brown dwarfs, producing companions at distances of 100 AU or more from the host star.- Wiki

This means that we can have a typical cluster, where half of it's mass is located within 1,63Ly from the center. However, this default cluster is too tightly packed for us, as it would imply a core density of 50 stars/Ly³.

Looking at a more sparse cluster, the Hyades, we can have an idea of what we're looking for although the group have less stars than we have (between 100~300 stars):

The core radius is 2.7 parsecs (8.8 light-years, a little more than the distance between the Sun and Sirius), while the half-mass radius, within which half the cluster's mass is contained, is 5.7 parsecs (19 light-years). The tidal radius of ten parsecs (33 light-years) represents the Hyades' average outer limit, beyond which a star is unlikely to remain gravitationally bound to the cluster core.

Multiplicity in those types of clusters seems to be concentrated in the core region due mass segregation - while typical galactic standards for binary K and A type stars are about 25% and 36%, on the Hyades, it goes up to 26% and 87%, respectively - binaries in the Hyades also show an average separation of less than 50AU.

About 90% of all open clusters dissipate early on on the first billion years, and very few survive to the age of the solar system, bear in mind we want the latter.

Mass segregation also mean that our high-mass stars will be located closer to the center, while our K and M type stars will be on the surrounding Frontier (yes that's a Titanfall reference).

Of what I could find, the Hyades are a rather unusual cluster, as they lack extreme mass stars such as K, M, and brown-dwarfs on the light side, and any stars heavier than 3Msol.

As we have defined early on this post, our empire's mass is about 60~70% contained in low-mass stars, while the remaining 40~30% in only 37~41 stars, so unless you wanna flip that over, we won't follow much of this population distribution.

Using the Hyades as model for what's a cluster dimensions would be, I've also created a star cluster calculator you can use.

See [CALCULATORS and SPREADSHEETS] section for BINARY STAR and STAR CLUSTER calculators.

- M.O. Valent, 31/07/2020

A Critical Analysis on Wayne D. Barlowe's [Expedition]

THE 'EXPEDITION' IS A GREAT WORK...

...OF ART

This is a follow up post to A Critical Analysis on Discovery's [Alien Planet], this time, tackling into the original book Expedition, which inspired the Discovery's docufiction Alien Planet.

We are introduced to a dying-Earth scenario, and interstellar travel is a quite recent trope brought up by a star-faring civilization that contacts Earth - the Yma, by the 22nd ~ 23rd century. Mankind is no longer in control of planet Earth, at least, what is left of it from our mishandling of things.

The Yma then organize a cooperative expedition to a not so distant binary system named Darwin, and humanity can both fill itself with hope and re-educate itself from this mission. The narrator is an artist brought there to more naturally depict the wildlife of the fourth planet from the star pair, Darwin IV.

THE PLANET

Aside from other characters and other details on the mission structure, we are then presented with a couple of data about the planet Darwin IV:

Darwin IV (presented)

Distance to Earth: 6,5 light-years

Diameter: 6.563km (0,515x Earth's)

Moons: Two small moons of unspecified size or distance from the planet, however, as they are shown to be nearly perfectly round in the sky, they must be larger than 600km, they are also implied to be smaller than Earth's moon.

Rotational Period: 26,7 hours (1 day, 2 hours and 42 minutes)

Orbital Period: said to ~2 years

Atmosphere Composition: Not specified, implied to be rich in Oxygen and Hydrogen.

Predominant Color: Dusky Ochre, with redder regions.

Host Star: F-class binary star, there is no further specification on class, the secondary pair is implied to be considerably smaller, and the two orbit close enough to appear as one single star in Darwin IV's sky.

Distance to Host Star: about 2 AU

Surface Gravity: 0,6G (5,88 m/s²)

NOTES:

Planet Mass: ~0,1588 Earth masses, calculated from given Diameter and Surface Gravity.

Other: No oceans, except for a small sea on the north which is stuffed with microorganisms, and scattered ponds and lakes; Vegetation covers about 5% of the planet's surface. Polar caps behave somewhat similar to Mars, as the planet's seasonal changes are more affected to it's distance to the stars than it's inclination.

The physical depiction of the planet so far is somewhat similar to ancient Mars, and it is even specified that the planet once was warmer and richer in both biodiversity and water, but for some reason (so far), it is no more there is quite a while.

Darwin IV (calculated corrections and it's implications)

F-type stars have masses between 1,0~1,4Msol, and given half binary stars of this weight class have at least 1 companion, we can say this is accurate so far - we are also informed that the secondary star is rather small. The fourth drawing on Page 14 depicts the smaller star being 1/5th as wide as the bigger pair, so we will consider that the mass ratio of the system is 1:5.
This puts the secondary star mass range into 0,2~0,28Msol, between an M5V and M3V red dwarf star. Assuming an intermediate mass of 0,24Msol (M5V) for the secondary pair, the primary is then 1,2Msol, settling as F7V class.

Binary System: ~1,44Msol ; 1,643 Lsol

Darwin A ~ 1,2Msol F7V type star (1,62 Lsol)
Darwin B ~ 0,24Msol M5V type star (0,023 Lsol)

Actual Orbital Period at 2AU: ~ 2 years, 131 days

Received Light:  41% that of Earth's 

Surface Temperature:  -47°C 

Climatic Index:  0,382 - Martian, cold enough to freeze atmospheric CO²

Needless to say, Darwin IV's climate stats is very messed up at this point.

LIFE FORMS AND THEIR PARTICULARITIES

It's said that most of the native fauna falls into 5 classes, which are said to be vertebrates, and thus, have some kind of internal skeleton.

• Floaters & Flyers
• Monopedes
• Bipeds
• Tripeds
• Quadrupeds

Due unspecified larger quantities of Oxygen and lower gravity than Earth, Darwinian fauna evolved larger than on Earth, and the gigantic creatures of the planet evolved to use a hollow thin-walled bone-like structure like Earth's birds.

This hollow structure (and implied abundance of oxygen for energy supply) allowed Darwin IV's large predators and other animals to top at speeds of 50~90km/h.

Darwin IV's animals evolved to be liquovores, piercing and drinking their prey, trees or carcasses with their tongues, after inserting a sort paralyzing digestive substance into it.

Darwinian flora, is said to be less diverse than the Earth of our time (to the narrator, a distant past), is described to be composed of:

• The dominant class of plant life are similar to Earth's Succulents
• Plains are dominated by a blue, tube-like succulent, equivalent to grass.
• Plains are also inhabited by round and rolling succulents that can be carried away by the breeze ; And an onion-shaped, translucent succulent.
• Mountain-wise, succulents give space to more dynamic plant life, some of which explodes to spread it's seeds around, and others that form an impenetrable coat over the soil, and some which resemble Earth's whisper plants.
• Plaque-barks, a tree-like organisms which is limited to pocket forests scattered across the planet, and seemingly, the only type of tree around.
• Towards the poles, lichen-like plants and low-dwelling flowering plants coat the soil.
• The atmosphere is then stuffed with airborne algae, called aerophytes - which sometimes can cast a shadow on the surface, due the sheer number of them in a particular area.

It's said that the closest to an ocean that Darwin IV is able to offer, are it's succulent rich savannas, which by themselves, hold onto as much water - they are a ready-to-consume source of both food and water for herbivores and omnivores.

Given the plants are adapted to a more dry environment, and thus accumulate water for the long run, yet, it's clearly said once the planet were a warmer and moist planet, it's then implied the environmental change didn't happen a long time ago - geologically speaking, because if that were the case, there wouldn't be trees still around. And animals followed the trend to feed on those plants, both on land, and airborne filter-feeders.


WHY EYES? WE CAN HAVE SONAR AND PIT ORGANS!

One remarkable characteristic of Darwinian fauna, is it's lack of true eyes - it's said that optical sensory have, through eons of evolutionary selection, been supplanted by sonar and infrared faculties.

It turns out that, the animals of Darwin IV perceive their environment through their skin - an array of subcutaneous pressure sensors to deal with sonar, and pit organs for infrared detection, are distributed along the creatures sides. Secondary to that, creatures also posses bioluminescence, which is a visual side-effect of the infrared-emitting spots on the creatures, which aid in both mating, communication, and ally/enemy identification. It's said that mating behavior often involves one "flaring" (changing it's color and brightness) it's biolights to attract and impress potential mates, and the flaring can attract mates from as far as 10 km.

Fur is an absent trait in Darwinian fauna, as their pit organs and pressure sensors must remain uncovered to work, temperature regulation is done by metabolic management, dictated by how much heat the creature is detecting as ambient-source - actually, the only way the creatures have to know if it's night or day, is knowing whether it's hot or cold outside.

As for the sonar workings, all animals have a large cavity in the skull which is filled with a dense fluid, the structure have the same function as dolphin's melon - to modulate and focus sound waves, so I'll be referring to it as such. The animal's vocalization is produced by a larynx-like organ, and because the sonar that is effective to use in this environment is high-frequency, ie, ultrasound, which has an effectively short range -  the creatures of Darwin IV have to constantly ping their way around wherever they go.


THE MIST THEORY

The narrator says that the Expedition crew has a theory (a guess actually) that, Darwin IV was covered in some kind of mist in it's early days, and that the creatures which beared these faculties were way better at surviving, and over time, they totally suppressed sighted creatures.

I have one objection to the Mist Theory however.
While I was reading and pondering about the lack of eyes, I've actually stumbled upon a similar theory to explain why eyes haven't developed, and if they did, why they haven't turned into such a common trope as it did on Earth.

First, let me explain why the Mist Theory doesn't fully explain the absence of proper optical sensory.

It isn't so much detailed on what that said mist could have been, but if the narrator is referring to mist, as the common mist we often think of - what seems to be the case for an ancient warm and wet Darwin IV we are told of, then it's rather questionable.

According to this paper on infrared imagery in both rain and fog:

Fog is a visible aggregate of minute water droplets suspended in the atmosphere at or near the surface of the earth. When air is almost saturated with water vapor, this means that the relative humidity is close to 100%, and that fog can form in the presence of a sufficient number of condensation nuclei, which can be smoke or dust particles.

So we can say that for a good chunk of it's early history, Darwin IV used to be a warm, heavily humid world with dense forests and oceans and lakes covered by fog - somewhat similar to what Venus was previously thought to be.

According to the International Civil Aviation Organization (ICAO), fog can be classified in 4 categories:

Category I: visual range 1220 meters
Category II: visual range 610 meters 
Category IIIa: visual range 305 meters
Category IIIc: visual range 92 meters

What makes fog, well, fog, is it's ability to scatter light, Mie Scattering properly said, giving the environment this cloudy appearance, and depending on the substance and size of the suspension it can behave differently depending on the light wavelength that passes through.

Category I fog has an infrared window between 3~5 microns and another one between 8 and 12 microns - most infrared cameras could well see in this fog much further than the naked eye, still, it isn't really any advantage, at all, because you can barely make out people and other animals 500m away, what to say a whole km away, at least for short term purposes.

Going into Category II, only the 8~12 micron band is superior to visible light optical sensory.

And even further into Category III, there is substantially no difference between optical and thermal imagery.

Pit organs sensitivity is between 5~30 micrometer infrared, some would argue that beyond 15 micrometers up to 30 micrometers the scattering of infrared isn't as strong in any of the paper's tests - however, the wavelengths of body heat you are looking for occurs along the lines of 10 microns only - SO, unless the average internal body temperature of the creatures in Darwin IV is around -80ºC or colder, you won't be picking up any 15~30 micrometer infrared emissions.

That, not to speak of the 50~150ms lag of pit organ sensory compared to typical human brain-eye 40ms of delay.

The theory makes sense on the aspect that indeed creatures with proper eyes may have once existed, given fog doesn't affect the water visibility by much, so at least, both blind and sighted creatures would have developed to some point - that, if actually, sighted creatures haven't somehow suppressed the blind ones before life got out of the sea to land..

So, scientifically speaking, there is no real reason why having pit organs would allow a group of creatures to thrive over  other creatures with proper eyesight - or putting in other words, at least both infrared and optical sensory are equally likely to evolve parallel to each other.


HOW EFFECTIVE IS ULTRASOUND SONAR, THEN?

As of what ultrasound advantages, there is actually no real reason why wouldn't sighted creatures develop it too - after all they did develop here on Earth even with non-misty conditions. Ultrasound used by animals, such as bats, fits the range between 100~200kHz, this paper on ultrasound absorption in air, shows that at a frequency of 100kHz, temperatures of 57°C, the absorption efficient is 6,3dB/m, of course, Darwin IV's temperature isn't that high, looking at a lower temperature on the image 2b, at about 20°C the absorption coefficient is along the lines of 2dB/m.

For perspective, listening from 1m away, elephant calls may be as powerful as 112db - if Darwinian fauna, with their bodies being as large as elephants, it wouldn't turn up much of a surprise if they had similar capabilities, this give the strongest ping they can produce a range of 56m, however, to have proper working sonar they need to get some audible feedback, that cuts our distance of 56m in half, what makes their sonar, actually less than 28m in effective range.

In order to keep that 50-ish meter range, the creature would have to ping a 200dB sound, which is by our standards - at least 4x louder than the launching of a rocket, and sounds above 120~130dB can cause extreme pain and hearing damage to humans, and other eared animals, in a couple seconds - if that extreme case with the 50m range were true, then humans would be affected the moment they walk outside, even with spacesuits, unless they had sound-proof panels built-in.

That either doesn't sound to be the case, so it is probable that the animals aren't using noises nearly powerful than around 120dB.

If that is truly the case as it seems to be, such a creature as the Arrowtongue cannot see further than 2x it's body-length away (~30m), and the sonar sensory of the Skewer is totally useless.
Plus, in category IIIc fog, optical sensory actually turns out to be more than 3x as effective than sonar sensory.

No, the denser atmosphere wouldn't affect the speed of sound at all, as it is affected by it's temperature, so if the planet temperature is somewhat similar to what you would find here on Earth, you would have no significant changes in it's speed to increase or decrease the effective radius of the sonar ping. 

It would make sense however for small animals to use sonar and optical sensory - as it turns out to be the case on Earth, when the animal's scale are on the order of a dozen centimeters, even if a ping as loud as 80dB can render the animal with a range of ~20m, it's about 20~40x their body-length, that sheer change of scale can be very useful - see bats for instance.

OTHER ANIMAL IMPROBABILITIES

The Emperor Sea Strider is by far the largest improbability of the Darwinian fauna, in all senses - not only because the sonar pings are useless at this huge scale, but also because the mouth isn't located on tube-like tongue alike on other species, but there are actually two oral tubes at the sole of it's feet, so the animal actually feed through it's feet as it walks by the amoebic sea.

While it's probable that the feet of the ESS aren't true feet, but actually feeding tubes the animal also happens to use for locomotion, give it's immense adult size renders it unable to fly, it's a great unknown the why such creature would have two feeding tubes in the first place - it's a great breakdown of the already established rules for other animals.

It's also a rather questionable it's place among the other fauna of Darwin IV, it is described as being rather adapted to the life, walking over the amoebic sea, however, given how recent the story of the amoebic sea is in geological time scales, it's rather unlikely, that such radical adaptations would happen to any existing clade at the time. It's difficult to conceive from what did the ancestors of the ESS ever fed on back the old foggy days of Darwin IV, as there wouldn't be anything similar to the amoebic sea at the time.

Along the same lines are the monopedes, which have two limbs like the bipedes, but they are fused after the first joint / knee, giving the animal one strong leg with two sockets at the pelvis, for rather obvious reasons, I'm not going deep into the issue here, as why animals would evolve such a trait, if it makes moving more efficient, why would bipeds still exist in the first place and in more diverse species than the monopedes?
Animals on Darwin IV seem to have bilateral symmetry to start with, so why and how would evolution have given birth to tripeds and monopedes is sketchy, if not questionable, at all.

Limbs are a so basal trait in Earth's fauna, it can be traced all the back to ancestral fish 450~500Mya, when the first jaws appeared, and also, limbs such as fins to help steering animals through water. So every tetrapod today is just a tetrapod because it's a basal trait inherited from long ago, like eyes are too. No matter if the limbs are wings, or being lost (like the rear limbs of whales), there can be and will be traces of this tetrapod ancestry for that long.

That would imply that rather than evolving from early tetrapod ancestors and then having their limbs fused for some absurd reason whatever it is, it would be more feasible to have it starting from a completely separate branch on the tree of life, from a radially symmetrical animal with 3 lobes, and from there walk it's way to land, even tackling on some similar adaptations for the land environment to the other tetrapod animals while being virtually unrelated for at least some 500 to 600 million years. That have actually happened once on Earth, a clade of sea cucumbers which are animals with radial symmetry actually evolved bilateral symmetry and are motile, these are known as sea pigs (Scotoplanes) - so the later isn't far fetched, as some would think at first.

You could fit both tripeds and monopedes as being in the same basal clade, one which evolved to use it's 3 lobes as feet, and one that preferred to use only one lobe as a saltatorial limb - what would be way more clever and elegant than the fused limb argument.

Tripeds and monopedes could actually be a rather new introduction to environment, following the Recent Catastrophe Theory argument, it would be expected that, with most of pre-existing life forms that would otherwise predate or compete fiercely with the more fragile tripeds and monopedes - now extinct, and the receding seas, those would emerge from the water to colonize the land, giving birth to newer exotic species among the sole survivors on land.

From the flying Skewer and beyond, every other animal suffers from the already explored problems, like the apparent lack of cladistic connections - what, of course, could be solved if we had access to skeletal structure of the animals, because many seemingly radical differences can be of soft tissue rather than the actual basal structure of the animals.




CONCLUSION

Amazing art, not so good at science, nor it's own rules.

- M.O. Valent, 18/07/2020