Thursday 5 November 2020

The Many Mysteries of Darwin IV - Part 1

Fig. 1: The classic cover of the first edition, soon to be replaced by the reprint.

Ever since watching E.T. the Extraterrestrial, aliens were one of the biggest fears I had as a child. This was because most aliens I had seen since then were those weirdly humanoid ones, which are intentionally designed to evoke the uncanny valley. This fear of being abducted by greys or Spielberg’s roasted gremlin subsided when I saw two things: Star Wars, with its light-hearted and imaginative alien life, and a 2005 documentary on Discovery Channel called Alien Planet. Especially the latter made me see the idea of alien life in a new, positive light and gave me hope that we might discover some of it one day. What young me did not realize at the time (because who reads credits anyway?) is that Alien Planet was based off a science-fiction book by painter Wayne Douglas Barlowe. In Expedition a wildlife illustrator goes on a journey to a mysterious and wild planet called Darwin IV and paints the local creatures in all their beauty while detailing his travels. This book was first published in 1990, which means that this year marks its thirtieth anniversary! To commemorate the occasion, Barlowe is now selling a reprint of the book on Echo Point Books. If you never had the chance to read it before, now is your time! Now how do I want to commemorate one of the most monumental works in speculative evolution? By playing armchair-scientist of course!

One of the greatest strengths of Expedition is the great aura of mystery it not only creates but leaves behind about Darwin IV. After the first (and so far only) expedition we leave with more questions than we arrived with. This is what makes it feel almost real, as an actual first encounter with alien life would probably be just as if not more confusing. What I want to do in this post and its sequels is to seriously examine Darwin IV and the life on it in a semi-scientific way and maybe work out a few answers to its many mysteries. While I am doing this in part to put the knowledge I gained in studying Earth System Sciences to good use, my intention is also that we both learn something about real-life star systems, planets, geology and lifeforms in the process. This first post mainly concerns the orbital and geophysical characteristics of Darwin IV.

Fig. 2: The cover of the more well-known Alien Planet documentary, more precisely of the German DVD I owned since childhood.

Before we start, I just want to clarify that I will be very biased and apologetic about some of the criticisms made against the book by people in the spec-evo community. I just love Darwin IV so much that something deep inside me wants something like it to be real. When it comes to questions of plausibility, I will therefore play devil’s advocate (which is fitting since Barlowe has also extensively painted hell).

What does any of this have to do with dinosaurs or paleontology, you ask? You will see in time.

The Darwin System

Darwin IV is part of the Darwin star system. It is a binary star system 6.5 lightyears away, consisting of at least one F-type-main-sequence star and six planets (Barlowe 1990, p. 11-12). If both stars in this system are F-type stars is not exactly made clear, though they are said to be very different in size (p. 12). The real-life binary F-type star systems we know of that contain exoplanets are Upsilon Andromedae, Tau Boötis and HD 142 and in all three the second star is a red dwarf that circles the main F-type star. It is therefore safe to assume that the main Darwin star (let us call it Darwin A) is an F-type that is accompanied by a red dwarf (Darwin B). Now the first obvious question some of you may have is if a planet with large animal life could even exist in such a system. Our own sun is a G-type-main-sequence star. F-type stars can be up to 1.4 times heavier than our own star and are considerably hotter. Binary star systems can also lead to planets with unstable climates, depending on how they rotate which star. That said, Darwin IV’s stated position in the system actually makes its habitability quite plausible. There can be two types of planets in a binary star system: Non-circumbinary (S-Type) planets, which orbit only one star in the binary system, and Circumbinary (P-type) which orbit both stars at once if those stars are close enough to each other. Both types of planets can be habitable given the conditions, but P-types are more likely to be as they have more stable orbits. Darwin A and B are said to orbit so closely to each other that they look like a single star from one of their planet’s surface and Darwin IV orbits both of them at once, making it a P-type planet. Darwin IV being such a circumbinary planet may sound contrived at first, but there are multiple real-life examples of such circumbinary planets. Kepler-47c is one such example and even circles its binary system in its habitable zone. Speaking of which, Darwin IV is said to orbit Darwin A&B at a distance of two AUs or Astronomical Units (p. 12). One astronomical unit is the distance between Earth and our sun, in other words meaning Darwin IV is twice as far away from its sun(s) than we are from ours, which in our solar system would be about the position of the asteroid belt between Mars and Jupiter. Would that not mean the planet should be completely frozen over? No. Remember that F-type stars are larger and hotter than our sun. A 2014 paper determined that the habitable zone of an F0-type star (hottest in the F-class) extends from 2 to 3.7 AUs, while that of an F8-type star (coldest in the F-class) extends from 1.1 to 2.2 AUs (Sato et al. 2014). It is interesting to note that both these ranges are larger than our own sun’s estimated habitable zone which ranges from approximately 0.8 to 1.5 AUs. Darwin IV being 2 AUs away places it firmly well in the habitable zone of all F-type stars. Mind you that Wayne Barlowe accurately predicted this habitable zone 24 years before the study I cited was made. Because we do not know where in the F-class Darwin A is, it becomes difficult to say whether Darwin IV is comfortably in the middle or the rim of the habitable zone. We also are not informed (at least in the book) about the age of the system, which is also significant as the luminosity of a star changes throughout its lifetime. Depending on these two factors, Darwin IV might be (distance-wise) Darwin’s equivalent to Earth, but could also be its Venus, something to keep in mind. The same paper that estimated F-type stars’ habitable zones also determined that lifeforms around these stars are under stronger exposure to damaging UV-radiation, but this can be countered if the planet in question has an ozone-layer (something Darwin IV definitely has) and/or a decent magnetosphere (which it probably has).

Fig. 3: Darwin IV would be a P-type planet.

All that said, even if Darwin IV were not ideally or not at all in its star’s habitable zone, it could still be habitable for its lifeforms as we see. What many people forget or maybe just do not know when talking about habitable zones is that it just describes a region around a star in which without any modification or protection temperatures are in theory capable of maintaining liquid water and are tolerable for life as we know it. In other words, it is a zone determined by simply putting a blank thermometer into empty space and seeing where an astrophysicist would feel comfortable in his spacesuit. It does not take into account things like: Albedo caused by icecaps or clouds, greenhouse effects, densitiy and make-up of atmospheres and nuclear or tidal heating. As a consequence, some scientists consider the classic concept of habitable zones totally useless (Cohen 2002, p. 10) and I must frankly agree. The most poignant demonstration of this is Earth itself, as without its greenhouse atmosphere our temperatures would drop to minus 18 degrees Celsius on average, rendering most or all our surface water into solid ice. Temperature drops close to this have in fact happened billions of years ago, possibly suppressing the development of multicellular life for many millions of years. We are therefore, arguably, outside our solar system’s strictest habitable zone. More examples abound, such as Mars, which had plenty of liquid water on its surface at a time when the sun was at only 70% of its current luminosity, and the moons of the gas giants, far away from the sun but filled with liquid water thanks to the tidal forces enacted by their parent planets. According to calculations such as those by David Stevenson, even rogue planets which do not orbit any star at all but just wander through the eternal darkness of deep space could have liquid water on their surface thanks to a combination of an insulating atmosphere and geothermal heat produced by the decay of radioactive elements (Cohen 2002, p. 8). Even the other extreme, a planet way too close to its star, could have habitable conditions. Imagine for a moment if Mercury were tidally locked to our sun, (something we once used to think but then found out it rotated three times around its axis every two revolutions around the sun). One side would be infernally hot and the other permanently engulfed in cold darkness. Between these two unchanging extremes a gradient would span and inevitably at one point of this gradient a permanent ring would form around the planet on which, with enough air pressure, liquid water and life as we know it could exist… while at the same time farther away Venus would still be inhospitable (Cohen 2002, p. 131). So much for goldilocks zones…

At this point we might discuss what sources we should consider, as there is of course not just the book but also the documentary and its accompanying material. Bizarrely one of the most informative sources I could find about Darwin IV was of all things the brochure to my DVD of Alien Planet. I scanned a part of it here:

Now the first thing to note is that the orbital image here directly contradicts what is stated in the book, as Darwin IV is depicted as an S-type planet orbiting only one of the two stars and Darwin B is shown as similarly sized to Darwin A while also orbiting it at a significant distance. This makes little sense in the context of both the book and the documentary. The length of a Darwin IV-year is also given as 1.6 times that of an Earth-year, while in the book it is stated as being 2 Earth-years long (p. 12). The information given here should therefore not be taken as seriously as our main sources… which is unfortunate as the brochure also gives us information about three things which are not stated in either the book or documentary: The planet’s age (around 2 billion years old), density of the atmosphere (2 times that of Earth) and solar energy received (79% that of Earth). Especially the age and atmospheric density are pretty important, which makes it frustrating that they only come from an unreliable source. I will therefore treat these as estimates, but not necessarily canon. Also interesting is that we get a glimpse of planets Darwin I-III, all apparently larger than Darwin IV. Darwin V and VI are either outside the picture or do not exist in the documentary’s canon. Perhaps they got Pluto-ed.

One dense motherf...

Let us come to the orbital details and geophysics of Darwin IV itself.  As mentioned, going by the book’s information, a full rotation of the planet around its two stars takes about 2 earth-years, so about 730 earth-days. A single day on Darwin IV lasts 26 hours and 42 minutes (p. 12). A Darwinian year therefore lasts about 656 Darwin-days. This is comparable to Mars, whose days last 24h39min and whose year is about 1.8 times as long as that of Earth.

Fig. 4: Physical characteristics of our inner rocky planets. Darwin IV has similar dimensions as Mars, but might internally be structured more like Mercury.

A further similarity between Mars and Darwin IV is that they are small compared to Earth. Earth has an equatorial diameter of about 12’756 kilometers, Mars one of 6792 km and Darwin IV one of 6563km (p.11). This translates to respective volumes of 1.08321 x 1012 km3, 1.6318 x 1011 km3 and 1.48015 x 1011 km3. In other words, Darwin IV has about 51% of Earth’s diameter and only 13.6% of its volume, while sharing almost 97% of Mars’ diameter and 90.7% of its volume. From that we would assume that Darwin IV has a similar mass and density to Mars and therefore also a similar gravity. Gravity is usually measured in gravitational acceleration, which on Earth measures about 9.8 m/s2 (1 g) and on Mars 3.72 m/s2 (0.38g or 38% of Earth-gravity). Now, directly from the text, the gravity of Darwin IV is described as being 60% that of Earth (p. 12), which would mean it has a gravitational acceleration of about 5.88 m/s2. Here we meet our first major mystery. How can Darwin IV be smaller than Mars yet have considerably higher gravity? This must mean it has a higher mass and, due to its size, a way higher density. We can determine the mass of Darwin IV if we multiply its gravity acceleration (5.88) with the radius squared (32815002) and divide this by the gravitational constant (6.673 x 10-11), giving us a value of 9.48858 x 1023 kg. That is about 15.9% the mass of Earth but over 146% the mass of Mars! If we divide this mass through the volume of the planet (1.48015 x 1020 m3) we get a density of 6410.55 kg/m3 or 6.41 g/cm3. That is higher than the density of the Earth, which measures only 5.51 g/cm3! Coincidentally Earth happens to be the densest object in our solar system, meaning Darwin IV is denser than any of our planets. The closest analogy we could compare it to is the planet Mercury. Despite being a lot smaller than Mars, Mercury has a density close to that of Earth with 5.43 g/cm3, making it the second-densest object in the solar system (if we correct Earth’s value from gravitational compaction, Mercury would actually be number 1). The hypothesized reason for that is that Mercury’s inner iron-rich core must make up around 55% of its volume, compared to Earth’s which only makes up 17%. The safest assumption is therefore that Darwin IV’s core makes up a similar proportion of its volume. Determining the exact radius of this core and the rest of the planet’s internal structure is however difficult without having seismological data as we have for Earth. A more pressing question is perhaps how Darwin IV ended up with such a proportionally large core. Three proposed explanations exist for Mercury’s situation:

  • Mercury used to be larger in the past but a giant impact early in its history stripped off much of the crust and outer mantle, leaving only the planetary core and little else behind.
  • Due to its proximity to the sun the lighter elements of Mercury’s outer layers essentially vaporized and then were stripped off the planet by solar wind.
  • During the formation of the solar system, the solar wind of the proto-sun pushed away low-density material from the innermost orbits, leaving Mercury with only high-density material to form out of.

Current surface-data obtained by satellites does not support the first two hypotheses, leaving the last one as the likeliest. Darwin IV is however too far away from its suns for hypotheses II and III to work, unless maybe the planet used to be a lot closer to its parent-stars in the past but then wandered farther out. Such behaviour has been proposed for gas giants, but does not seem likely for rocky planets, so we may assume that Darwin IV formed at roughly its current position. The possible explanations we are therefore left with are either a giant impact or the possibility that planet-formation around F-type stars simply does deal with denser materials than we are used to in our G-type system. I unfortunately could not find any sources about the latter topic, though the higher energy output of F-type stars might result in denser material at farther distances than is normal for our system, meaning a modification of hypothesis III could still work. All that said, the best guess for our subject so far is the impact-hypothesis. The best evidence we might have for it are Darwin IV’s two moons. Not much is said about them, but we can see both of them in the Darwinian sky on plate XXVI (p. 136.) and on the original edition’s cover. Unlike the two moons of Mars, Phobos and Deimos, the Darwinian moons (let us call them Wallace and Lamarck because why not) appear perfectly circular in shape like our own moon. This means they both have reached hydrostatic equilibrium or in other words have enough mass that gravity squishes them into spheroids. The smallest known object with hydrostatic equilibrium is the dwarf planet Ceres, so the moons must be at least that massive. However, since Ceres is mostly made of ice and the Darwinian moons are (probably) mostly made of rocks, it means they are likely considerably heavier. That is a suspiciously high amount of mass in Darwin IV’s orbit. It is not impossible that these moons used to be dwarf- or proto-planets that Darwin IV captured with its surprisingly strong gravity, but you probably guessed what I am getting at here. It seems just as plausible that during its formation a younger, larger Darwin IV was hit by another proto planet, just like Earth was. This impact would have however been considerably more violent than ours, stripping away much of the outer layers of the planet and scattering them into orbit where the debris could develop into fully fletched moons. This impact, perhaps in combination with a higher density of the impactor, fusing of the two cores and/or a generally high density of the Darwin-proto-planetary-disk, led to the formation of the current small and dense Darwin IV.

Fig. 5: The two moons of Darwin IV as seen from its northern tundras.

With these assumptions we will then move on to part 2 where we will examine the geography, geology and atmosphere of the planet. Thank you so much for reading and see you until then.

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Related Posts:

Literary Sources:

  • Barlowe, Wayne: Expedition. Being an account in word and artwork of the 2358 A.D. voyage toDarwin IV, New York 1990.
  • Barlowe, Wayne: The Alien Life of Wayne Barlowe, Beverly Hills 1995.
  • Cohen, Jack/Stewart, Ian: Evolving the Alien. The Science of Extraterrestrial Life, London 2002.
  • Costard, François/Forget, François/Lognonné, Philippe : Planet Mars. Story of Another World, Paris 2006.
  • Grinspoon, David Harry: Venus Revealed. A New Look below the Clouds of our Mysterious TwinPlanet, Cambridge 1997.


Online sources/Further reading:

Image Sources:

  • Fig. 1: Barlowe 1990, cover.
  • Fig. 3: Wikimedia
  • Fig. 5: Costard 2006, p. 24.
  • Fig. 6: Barlowe 1990, p. 136.


  1. Just stumbling in here, but I feel like I should point out that habitable zone definitions do actually account for greenhouse effects. In particular, the general standard for defining the habitable zone today was set back in 1993 ( (though with some modification since then here ( and in a few later papers) and assumes a world broadly similar to Earth with CO2 and water vapor as the dominant greenhouse gasses and the carbonate-silicate cycle controlling CO2 levels to produce broadly earthlike temperatures, giving anywhere from under 10 K to over 100 K of greenhouse heating. The limits of the HZ are set where the process breaks down; at the inner limit, even with no CO2 the planet will experience a runaway greenhouse effect due to water evaporating into the upper atmosphere; and at the outer limit, CO2 levels are so high that CO2 clouds form and reflect away more heat than they trap, cooling the planet and eventually leading to atmospheric collapse.

    Still, it is fair to say that this definition doesn't fully encompass the possible range of habitable conditions. Further modelling has suggested that very dry planets with thin atmospheres could remain at least marginally habitable within the orbit of Mercury (, and that thick hydrogen atmospheres could do the same beyond the orbit of Saturn (or perhaps indefinitely far out for a world with substantial geothermal heating) (, and of course we could speculate at length on the possibilities of exotic alien biochemistries requiring different temperature ranges for survival. But still, there is a bit more thought put into the idea of a habitable zone than you've suggested.

    1. I should have specified more that I was talking about the classic idea of a habitable zone (the simplistic one laypeople commonly think of when they hear the term). The definition at large has of course been further developed by astrobiologists due to the conceptual problems mentioned.