Eridians originate from the planet Erid, orbiting the star 40 Eridani (18.7 light-years from Earth). Their world is hot, dense, dark, and crushing — conditions that shaped every aspect of their biology. Life there runs on ammonia, heat, and pressure that would kill a human instantly.
Two complementary anatomy diagrams: the top-down plan view, showing five limbs in perfect pentagonal symmetry and the heat-rejection cap with all five in-vents and out-vents; and the front elevation, showing Rocky as a visitor would see him face-on, with full internal systems labelled and height dimensions marked. Together they define the complete three-dimensional form of an Eridian.
All five limbs are identical and interchangeable. Any limb can serve as an arm or a leg at any time. When walking on all five, an Eridian is virtually impossible to knock over on uneven terrain. When using three as legs, they move fast. At rest: three down, two up as hands. Eridians have no concept of "facing" a direction.
Eridians have two completely separate, unconnected bloodstreams — both running liquid mercury. The cold ACS (Ambient Circulatory System) carries worker cells throughout the body at ~210°C. The hot HCS (Hot Circulatory System) runs at 305°C and powers every muscle. Worker cells cannot survive in the HCS. Five hearts — one per limb joint — pump 85.6 kg of mercury against 2.09g gravity.
Every muscle movement is powered by water boiling into steam. HCS vessels deliver heat to tens of thousands of tiny water-filled vesicles in each muscle. The water (boiling point 230.2°C at 28atm) flashes to steam, expands the vesicle, and the piston fires. ACS vessels cool it back to liquid. Each Eridian is, mechanically, a collection of millions of tiny steam engines.
The pentagonal outcropping on top of the carapace is the most critical survival system. It must radiate 286 watts continuously. Five in-vents draw cool ammonia air in through the perimeter. A diaphragm below a sponge-like capillary lattice pulls the air across the hot capillaries, then pushes heated air out through five top-facing out-vents. If this fails, Rocky has 31 minutes before death.
Here is the single strangest fact about Eridian biology: the brain is made of crystal, and its signals are pulses of light — not electricity. Fiber-optic crystal tendrils carry nerve impulses throughout the body. The brain and its worker cells live together in a central chamber called "the Colony" — sealed in Babbitt alloy, like a beehive that runs the ship. Less than 1 kg of organic matter controls 167 kg of metal body.
Eridians have no eyes. They perceive their world entirely through sound and magnetic fields. The carapace is studded with hundreds of thousands of auricles — each tuned to a precise frequency — converting soundwaves to piezoelectric flashes that feed directly into the fiber-optic brain. From the resulting data, the Eridian constructs a complete real-time 3D model of its environment — better than human sight.
Five gas bladder sets inside the body. Five apertures. Five simultaneous voices. Eridians speak in chords — not just pitches. They have perfect pitch (C-sharp and C-flat are as distinct to them as red and yellow are to us). Their language carries 6× more information per second than any human language. Rocky's speech sounds like whale song.
Eridians eat once per week. They don't chew — their hands are their teeth. The mouth is on the bottom (ventral), heals shut between meals, and reopens by cracking along a pre-weakened spot in the carapace. Food is sterilised at HCS temperature (305°C) for days, then worker cells extract nutrients. Each meal is 32 human meals worth of energy, lasting ~86 hours. After eating, they sleep it off.
Eridians evolved in an environment with almost no radiation — their planet's enormous magnetic field and thick atmosphere filtered everything out. As a result, they have zero evolved radiation resistance. Their mercury blood makes it worse: it becomes radioactive itself. Rocky's crew died of cosmic ray exposure on the way to Tau Ceti. Rocky survived only because his engine compartment was shielded.
Their mathematics is base-6 — two hands, three fingers each. They are ahead of humans in math and materials science, behind in computing, and have no concept of relativity whatsoever. Their "day" is 5.1 Earth hours. Writing is textured and tactile. Their walls are hyper-soundproofed (they can hear through walls). And they evolved communal sleeping-watch out of necessity.
Every quantified fact from Andy Weir's canon biology document, in one place. All measurements were calculated by Weir from first principles — atmospheric physics, thermodynamics, materials science. This is real engineering applied to a fictional species.