On Earth, life is of great abundance. For roughly 3.5 billion years, it has manifested itself in countless forms, from simple, protoplasmic masses to towering giants of flesh and bone. Only recently, however, has life on our planet developed to the point where it has grown sufficiently aware of itself and its surroundings. For generations, humankind has wondered what lies beyond its line of sight, be it the island across the sea or the forest on the other side of the mountains. Today, that curiosity manifests itself in a unique way: For millennia humankind has postulated the existence of beings hailing from worlds beyond our own, from the flying deities of the Ramayana to the angels of the Old Testament. In the 21st Century, our aspirations to find extraterrestrial life has shifted from sacred sanctification to scientific speculation. Using the laws of physics, chemistry, and biology, how might life develop among the stars?
In our first example, let’s travel to the hypothetical world of Holda. Orbiting a small K-class star, whose rank puts it just under the Sun in terms of size, mass, and luminosity, this sub-Earth planet has a rotational axis of 42º, an atmosphere roughly twice the barometric pressure of our own, and only two-thirds of the gravity, averaging at around 6.7 m/s2. Its sun, which is slightly fainter than our own, is less radiant -- and therefore doesn’t emit as much radiation as the star our planet orbits. As a result, its habitable zone -- that is, the distance from a star at which a planet can support liquid water -- is a bit closer than what we’re used to on Earth. Holda orbits at roughly 0.85 astronomical units, or 85% of the distance between Earth and the Sun. While Holda’s orbit and atmospheric pressure allow it to support liquid water on its surface, life has evolved quite differently to our own:
As a result of the lower mass, Holdan life has evolved without as intense a gravitational constraint, allowing for much larger organisms to develop. Circulating water and other nutrients throughout a living system would require less energy input, so despite the fact that there would, of course, be limits to the size of an organism -- they could, in theory, dwarf our largest terrestrial organisms. Here, however, is where we run into a conundrum: To maintain homeostasis, life on Holda would need to regulate its internal temperature. To do so, it must discharge thermal energy as a result of cellular respiration -- meaning that larger organisms on Holda need to strike a balance between size and heat.
To better analyze how this works, see the hypothetical organism Psittacotherium rhinus:
The species has an average height of 3.7 meters -- 50% larger than a typical African elephant. While its body may seem heavier than physically possible, its bone-like skeleton is filled with spongy, vascular tissue that helps to support its massive torso and head. To balance itself, it and the rest of its evolutionary clade have developed six limbs for walking, grasping, and/or attacking. In the case of P. rhinus, the hind legs partner with a counterweight-like hindquarters to balance the beast. Because it is so large, having an average mass of 5,600 kg (almost that of an African elephant), its metabolism is greatly reduced, meaning that most of its torso and organs are surrounded by blubbery, adipose tissue saturated with fatty acids and sugars.
Because of this, P. rhinus can wander the open plains of Holda’s equatorial mountains without needing to feed multiple times a day. In fact, it often over-eats in one sitting to build up fat and store fuel for its next expedition, moving slowly and gently as to expend less energy at one time. Because it is so large, it spends most of its energy on its limbs and auditory membranes, seen just behind its bony skull above. In fact, it is these limbs that provide safety in the toughest of storms. Because of Holda’s axial tilt and atmospheric pressure, weather conditions on the planet are far more severe than on Earth -- controlled primarily by latitude, air pressure, and humidity. Vast swaths of wind seep in from the southern oceans of Holda, which combine with the equatorial humidity and dense atmosphere to produce cyclones more deadly than anything possible on Earth. As a result, P. rhinus has evolved a method of survival: During a cyclone, the giant lowers itself to the ground, focusing its center of gravity on the lowest possible point. Using its hoof-like feet to dig into the ground, P. rhinus anchors itself in the ground, using its fat reserves to sustain itself until the storm passes. While this may seem like a clever feat, it is, sadly, the limit of P. rhinus’ intellect.
Tragic as it is, P. rhinus is a titan among little green men. As a result, it cannot dedicate energy or metabolites to a complex brain. On Earth, humanity’s ancestors gave up brawn and strong muscles for brainpower. Because life on Earth requires a lot of energy to maintain itself, and because brain and nerve tissue needs even more energy to relay complex signals along a cellular network, an organism can either be large and dull or small and crafty. Due to the amount of tissue in our brains, we need a lot of surface area to fuel the entirety of the brain with the oxygen we’ve absorbed into our bloodstream. If we had chimp-like bodies, our brains would atrophy to account for all our extra muscular tissue. If we dedicated all our energy to nerve tissue, we’d have no body left to support. Thus, the relationship between brain and brawn comes together, and as such P. rhinus is too massive to develop human-like intelligence. At best, it may be able to think to the extent of a common horse or canine -- provided its atmosphere has enough oxygen to support its massive structure, but not enough to inflict oxygen toxicity on larger life-forms.
For our next simulated journey, we travel to the harsh world of Harðgreipr -- a super-Earth world orbiting a G3-class star – much like our own in terms of heat, mass, radius, and luminosity. With a gravity of 14.06 m/s2, Harðgreipr would be hard for humans to survive on -- though not impossible. Due to this intense force, we would need to expend more energy to circulate blood, water, and other fluids throughout our bodies, not to mention passing substances and structures throughout individual cells or small tissues. It is for this reason that Harðgreiprian organisms have evolved a failsafe:
Meet the stocky Stegoderma microtherium, the dominant organism of the arid Harðgreiprian surface. A feat of evolutionary prowess, S. microtherium has adapted near-perfectly to the environment of its species. Possessing a thin keratin shell, the organism uses fingernail-like plates to trap moisture through a thin layer of skin that just covers the creature’s shell. Within its front segment is a small, worm-like head whose protruding mouth is used to capture unsuspecting desert-dwellers -- though it doesn’t get this chance very often. Because of its minuscule size, S. microtherium is perfect for the high-gravity environment of Harðgreipr. Using a lower proportion of energy to body size, the organism requires less food and water to maintain itself, and often improves upon this system by laying dormant for long periods of time.
Using its stocky legs to dig into the Harðgreiprian regolith, S. microtherium burrows underground, feeding off of moisture and micronutrients by way of its antennae-like tongues. Then, when it senses movement aboveground -- perhaps by a prey item or even a fellow S. microtherium -- it expends nearly all of its energy on subduing and consuming its prey, using its toothy, jawless mouth to pierce the armor of its victims. Once sated, S. microtherium either returns to its burrow or moves along in search of arable soil, where it can lay in wait for a chance to resurface. Sadly, though, this small creature falls to larger predators:
The apex predator of the Harðgreiprian sky, Pteroascellus aberrus picks off S. microtherium one by one. Flying at almost six meters in length, this terrifying creature picks up thermals in the wind across its severe clime. In order to both maintain its body and resist the pull of gravity, P. aberrus flies in a peculiar path, dipping very close to the ground and swooping upwards to gain back some potential energy. Occasionally flapping its wings, most of the beast’s energy goes towards movement, gathering water from the upper troposphere as it flies. As such, its body is light and thin, having roughly the same mass-to-volume ratio as a typical sparrow -- leaving only a sparse bit of energy to direct towards its peripheral eyes and underdeveloped brain. Its only motives are to swoop down, collect unsuspecting prey items, and soar back up, using some of its remaining energy to regain momentum. Truly, life is harsh on Harðgreipr, as it would be in any ecosystem on any planet. And yet, among these truly alien creatures there is an even more bizarre possibility for life -- alternate biochemistry.
For decades, scientists have explored the laws of molecular biology and organic chemistry to divulge from the cosmos how it functions. In doing so, they have uncovered a variety of substances, solvents, and elements by which life may arise. A common answer to this is the silicon atom, which like carbon possesses four valence electrons and can thus form four covalent bonds. What people forget, however, is that while silicon can form complex molecules, it is not electronegative enough to attract the same kinds of bonds as carbon, and so it is too weak to substitute for carbon in biochemistry. To model an alternate biochemical pathway, we will travel to the third and final fictional world on our tour -- a small, sub-Earth planet called Baldr V.
Existing within a binary star system roughly 10.47 light-years from Earth, Baldr V orbits the larger of two stars -- a K0V-class star by the name of Baldr. This star is 80% the mass of our sun, 75% the solar radius, and only 32% the luminosity of the sun. Normally, this would be a problem for Earth-like planets and ecosystems, whose water-based biochemistry would need a narrow habitable zone to maintain itself for at least a few hundred million years or so. However, Baldr V functions on a different set of chemical laws. With a perihelion of 609.3 million km and an aphelion of 609.9 million km, Baldr V orbits at an average of 4.075 astronomical units -- over four times the distance of Earth to our sun. Because of this, it’s unsuitable for liquid water -- but ammonia is a much different story. Because hydrogen and nitrogen are so abundant in the known universe, it’s not outlandish to assume that it’s quite common to find ammonia on other worlds, and even in the interstellar medium. As such, Baldr V has been crafted to suit liquid ammonia oceans.
With an atmosphere over 60% denser than our own and a core with over 2% more U-238 and Th-235, Baldr V maintains an average temperature of around -3º C, due to both its nitrogen-rich atmosphere and radionuclide-rich core, which keeps its mantle and lithosphere warm over the many billions of years that Baldr V has left to exist. With a cohesive, simple solvent of NH3, Baldrian life can function in a thermally stable medium, whose specific heat is just above that of H2O. With this water-like liquid, metabolites such as gaseous N2O (a viable substitute for oxygen, a known oxidizer and metabolite) and mevalonic acid (capable of both chemolysis and polymerization) can combine to yield energy molecules reminiscent of ATP, all with the aid of protein-like structures derived from acidic monomers, which in of themselves are derived from sulfamic acid H3NSO3. These “sulfamoid acids,” encoded by genetic material based around pthalic acid and its derivatives, are the basis of life on Baldr V. With this basic model, alien life can take a variety of forms beyond our wildest dreams -- though to describe it would take pages and pages of dissertative work. To hear more about Baldr V and its theoretical biochemistry, contact Billy Bernfeld at [email protected].