Fifth Planet — Fred Hoyle

"A planetary system, consisting of a star and five planets, is travelling through our galaxy and will pass close to Earth. The four largest planets are gaseous, but the fifth, named Achilles, appears capable of supporting life. Two rival expeditions - one Anglo-American and one Russian - set out to land on Achilles and explore its mysteries. But almost from the moment of their arrival things begin to go terribly wrong ... and when they return to Earth, something not human will be coming back with them.
One of the most exciting novels by world-renowned scientist Sir Fred Hoyle (1915-2001) and Geoffrey Hoyle, ""Fifth Planet"" (1963) is a thrilling science fiction tale that ranks alongside Hoyle's masterpiece ""The Black Cloud.""
'An exciting book for all science fiction enthusiasts ... will leave the reader wondering about our future life on earth.' - ""Evening Journal""
'Fred Hoyle, a celebrated astronomer, is a man of vast imagination ... the story is sharp, clear and full of suspense.' - ""Washington (D.C.) Evening Star""
'A mature performance ... the best so far from the Hoyle stable.' - ""Sunday Times"""