Krypton (planet)

Krypton was a planet.

Suspecting the Seventh Doctor was an alien, Jack Bartlett asked him if he was from Mars or Krypton. (PROSE: Bad Therapy)

During the filming of Meet That Hero!, E.T. once a presented fellow alien-stranded-on-Earth, Supergirl, with a Kryptonian singing flower whose growth he'd accelerated using his special abilities. She was overjoyed, and called for the formation of an "alien club"; the Twelfth Doctor was enthusiastic about the idea and offered the help of his TARDIS. (WC: Supergirl Meets E.T.)

Behind the scenes
Krypton was the home planet of Superman in the DC comics. Before he was sent to Earth as a baby, the planet was destroyed. It is also the homeworld of Supergirl, hence why she is overjoyed in WC: Supergirl Meets E.T. when E.T. gives her a Kryptonian flower.