Esto

Esto was a planet located in Lagoon Nebula. (WC: Number 1, Gallows Gate Road) It was home to a species of psychic parasites (WC: Number 1, Gallows Gate Road) and a species of telepathic plants that screeched when anyone stood between them, thus disrupting their communication. The Doctor and Susan had visited Esto. (DW: The Sensorites)


 * Presumably the two mentions are of the same planet.