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.
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".
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.