How did a science fiction novel name a branch of biology?

Its author simply invented the word. In his 1954 science fiction novel The Star Beast, the American writer Robert Heinlein coined "xenobiology," and the term survived: it is now used in a specialized sense, meaning biology based on foreign chemistry, whether extraterrestrial or made on Earth, typically synthetically. It is one of several related names, alongside "exobiology," coined by molecular biologist Joshua Lederberg, and "astrobiology" itself, proposed in 1953 by the Russian astronomer Gavriil Tikhov, for the same underlying question: could life exist, or have existed, somewhere other than Earth?

Astrobiology studies the origins, evolution, distribution and future of life in the universe, built on the simple premise that life might exist beyond our planet. In practice, the field splits into three main lines of work: hunting for environments elsewhere that could support life, searching for biosignatures, chemical or fossil traces of past or present life, and studying how life first arose and evolved here on Earth, to understand the conditions life needs to form on other planets.

The modern field grew directly out of the Space Age. The 1957 launch of Sputnik 1 marked the start of the Space Age and boosted research on life on other planets, and NASA moved quickly: funding its first exobiology project in 1959, founding a dedicated Exobiology Program in 1960, and later funding Project Cyclops in 1971 to scan radio frequencies for any signal from intelligent life beyond the Solar System. NASA's Viking program, the first US mission to land on Mars, searched directly for metabolic signs of life there in the 1970s, though its results were inconclusive. A different kind of breakthrough came in the 1980s and 1990s, not from space but from Earth's own deep sea: the discovery of microbes thriving around hydrothermal vents, helped show that life could be feasible in harsh conditions elsewhere too.