Fifth Planet is a science fiction

Science fiction

Science fiction is a genre of fiction dealing with imaginary but more or less plausible content such as future settings, futuristic science and technology, space travel, aliens, and paranormal abilities...

 novel

Novel

A novel is a book of long narrative in literary prose. The genre has historical roots both in the fields of the medieval and early modern romance and in the tradition of the novella. The latter supplied the present generic term in the late 18th century....

 written by astrophysicist

Astrophysics

Astrophysics is the branch of astronomy that deals with the physics of the universe, including the physical properties of celestial objects, as well as their interactions and behavior...

 Sir Fred Hoyle

Fred Hoyle

Sir Fred Hoyle FRS was an English astronomer and mathematician noted primarily for his contribution to the theory of stellar nucleosynthesis and his often controversial stance on other cosmological and scientific matters—in particular his rejection of the "Big Bang" theory, a term originally...

 and his son Geoffrey Hoyle

Geoffrey Hoyle

Geoffrey Hoyle is an English science fiction writer, best known for the works which he co-authored with his father, the astronomer Sir Fred Hoyle. About half of Fred Hoyle's science fiction works were co-authored with his son....

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Plot summary

Another star is due to pass close to the sun, close enough for conventional spacecraft to reach it. The first planets observed are four gas-giants, but then an inner 'Fifth Planet' is found. It shows signs of life, and rival Russian and US expeditions are launched to visit it. (The world balance as it existed in 1963 is assumed to be still in place.)