The Janus conjunction

"The planets Janus Prime and Menda are diametrically opposed in orbit round a vast Red Giant star. But while Menda is rich and fertile in the light of the sun, Janus Prime endures everlasting night, its moon causing a permanent solar eclipse. When the Doctor and Sam arrive on Janus Prime, they find themselves in the middle of a war between rival humans colonizing the area. The planet is littered with ancient ruins, and the Mendans are using a mysterious hyperspatial link left behind by the planet's former inhabitants. But what is its true purpose? The Doctor and Sam must piece together a centuries-old puzzle. How can Janus Prime's moon weight billions of tons more than it should? Why is the planet riddled with deadly radiation? As the violence escalates around them, will the time-travelers survive to discover the answers?"--Page 4 of cover.