11.   The excuse was that it could not afford a mission to the only planet not yet visited by a space probe.

12.   These are the only planets we know of outside the nine in our own solar system.

13.   Uranus had been believed to be the only planet not to have such moons.

14.   What some astronomers believed to be the only planet ever directly observed outside the solar system has turned out to be an ordinary dim star, NASA announced Thursday.