Threeworld

Threeworld is the third planet in the 40 Eridani system, a small world roughly the size of Earth's moon that serves as the Astrophage breeding ground for Rocky's home star. Its significance lies in its thin but persistent carbon-dioxide-rich atmosphere, which makes it the target for the nitrogen-resistant Taumoeba strain that Grace and Rocky develop to save Erid from Astrophage infestation.

Identity and Physical Characteristics

Threeworld is a tiny planet, only about the size of Earth's moon, with a surface gravity of just 0.2 g. Despite its low gravity, which should be insufficient to retain an atmosphere, Threeworld somehow manages to hold on to a thin gaseous envelope. According to Rocky's data, the atmosphere is 84 percent carbon dioxide, 8 percent nitrogen, 4 percent sulfur dioxide, and a mix of trace gases, with a surface pressure less than 1 percent of Earth's. The Eridians themselves have no ancient name for the planet; they only discovered it a few hundred years ago and refer to it by the designation "Planet Three." Grace, finding that label cumbersome, names it Threeworld.

Role in the Astrophage Life Cycle

Threeworld is the destination for Astrophage migrating from 40 Eridani, just as Venus is the breeding ground for Astrophage from Earth's sun. The Astrophage travel to Threeworld to obtain carbon dioxide, which they need to reproduce. The planet's atmosphere, though thin, provides the necessary CO2 at the pressure level where Astrophage can breed. Rocky's observations confirm that Threeworld's atmosphere at the 0.02-atmosphere pressure level—the same pressure at which Astrophage breed on Venus and Adrian—has a temperature of minus 82 degrees Celsius. This makes Threeworld the Eridian equivalent of Venus in the Astrophage infestation: the planet where the alien microbes go to multiply, and therefore the planet that must be seeded with a predator to stop the infestation.

The Taumoeba Solution

When Grace and Rocky discover that Taumoeba, a predator of Astrophage from the Tau Ceti system, cannot survive in atmospheres containing elemental nitrogen, they realize that both Venus and Threeworld have nitrogen in their air—3.5 percent for Venus, 8 percent for Threeworld. Grace's plan to breed a nitrogen-resistant strain of Taumoeba targets both planets simultaneously. He creates a simulated Threeworld atmosphere in his lab, using Rocky's data on the planet's composition and temperature at the breeding altitude. The Taumoeba-82.5 strain, which can survive 8.25 percent nitrogen at 0.02 atmospheres, is designed to work on both Venus and Threeworld. Grace tests the strain in a chamber that precisely replicates Threeworld's atmosphere—carbon dioxide, nitrogen, sulfur dioxide, and trace gases at minus 82 degrees Celsius and 0.02 atmospheres of pressure—and the Taumoeba thrive, eating the Astrophage and proving that the solution will work on Rocky's homeworld.

Narrative Significance

Threeworld represents the Eridian half of the dual-planet problem that Grace and Rocky must solve. While Grace's primary concern is Earth and Venus, Rocky's people face the same threat from Threeworld. The planet's existence underscores the parallel nature of the two civilizations' crises and makes the partnership between Grace and Rocky a matter of mutual survival. Threeworld's small size and improbable atmosphere also highlight the Eridians' different evolutionary path: they never discovered radiation because their planet's thick atmosphere and strong magnetic field block it entirely, yet they still managed to build an interstellar ship. The planet's name, chosen by Grace rather than Rocky, reflects the human tendency to name things and the cultural exchange between the two species. Ultimately, Threeworld is the destination for the Taumoeba that will save Erid, just as Venus is the destination for the Taumoeba that will save Earth, making the two planets symmetrical targets in the shared mission to end the Astrophage threat.