In the context of your question, the answer is no. There was never a time we had or knew of another planet and either forgot about or lost it.
Return to the Lost Planet was created in 1954.
The Lost Planet was created on 1953-06-04.
Lost Planet was first released for the X-Box 360, in Japan, on December 26th 2006. Lost Planet is the first of the Lost Planet series published by Capcom.
There is no such thing as a lost planet. Astronomers haven't seen another planet from somewhere else yet.
Pluto lost it's "planet" status and now is called a dwarf planet.
Lost Planet Airmen was created on 1951-07-25.
Lost Planet of the Gods was created on 1978-09-24.
nope but in lost planet 2 u can.
Lost Planet 2 was released on May 11 2010 Lost Planet: Extreme Condition was released on February 26 2008
Pluto
Pluto recently lost its status as a planet and is now a dwarf planet.
In our solar system; Pluto. It is now accepted as a "dwarf-planet".