Tau Ceti e

Tau Ceti e, renamed Adrian by Ryland Grace, is the third planet orbiting the star Tau Ceti and the central destination of the Hail Mary mission. It is the homeworld of Astrophage and the only planet in the local stellar cluster that remains uninfected by the Astrophage plague, making it the key to saving both Earth and Erid. Its unique biosphere, which includes a natural predator of Astrophage, provides the solution to the extinction-level threat facing two civilizations.

Discovery and Naming

Tau Ceti e was first identified by Earth astronomers as an exoplanet, but its true significance was revealed by Rocky, the sole surviving Eridian astronaut. Rocky had spent forty-six Earth years in the Tau Ceti system, cataloging six planets and their characteristics through astronomical observations from his ship, the Blip-A. He determined the planet's size, mass, position, orbital characteristics, and general atmospheric makeup. Grace, upon learning that Rocky had been in the system first, invoked an Earth-culture rule that the first discoverer gets to name what they find. Rocky chose to name the planet after his mate, and Grace assigned the English name "Adrian" to represent Rocky's spouse.

Physical Characteristics and Atmosphere

Adrian is a pale-green planet with wispy white clouds in its upper atmosphere, its surface completely obscured from space. It has 3.93 times Earth's mass and a radius of 10,318 kilometers, nearly double Earth's. Its surface gravity is 1.4 g. The planet's atmosphere is extremely thick, composed of 91 percent carbon dioxide, 7 percent methane, 1 percent argon, and trace gases. The presence of methane is significant because methane breaks down quickly in sunlight, indicating an active source—later confirmed to be biological. The Astrophage breeding zone exists at an altitude where the atmospheric pressure is 0.02 atmospheres, approximately 91.2 kilometers from the surface, with a width of less than 200 meters.

The Astrophage Homeworld and Its Predator

Adrian is not merely a planet infected by Astrophage; it is the Astrophage homeworld. The planet's biosphere contains not only Astrophage but a vast array of other microbial life, including a natural predator that Grace named Taumoeba. When Grace examined samples collected from the Petrova line around Adrian, he discovered that the population of Astrophage traveling toward Adrian was equal to the population traveling away from it, a discrepancy that Rocky immediately recognized as evidence of predation. Under a microscope, Grace observed Taumoeba, an amoeba-like organism, enveloping and consuming Astrophage cells, neutralizing their ability to absorb heat and light energy. This predator-prey relationship explained why Tau Ceti remained uninfected: the Taumoeba kept the Astrophage population in check, preventing the exponential growth that was devastating other star systems.

Role in the Mission and the Solution

Adrian was the primary destination of the Hail Mary mission, which sought to understand why Tau Ceti was immune to Astrophage. Grace and Rocky used the Hail Mary to orbit Adrian and deploy a ten-kilometer-long chain with a sampler to collect air from the Astrophage breeding zone. The sample contained Taumoeba, which Grace and Rocky then bred over many generations to create a nitrogen-resistant strain, Taumoeba-82.5, capable of surviving in the atmospheres of Venus and Threeworld. This engineered predator was the solution that would save both Earth and Erid. Grace later sent the Taumoeba-82.5 to Earth via the beetle probes, and years after his arrival on Erid, he received confirmation that Sol had returned to full brightness, indicating the Taumoeba had successfully eliminated the Astrophage infestation.