Gharusa Prime was a planet.
Once, Jack Harkness send a transmission from there. A long time after this, while travelling with the Ninth Doctor and Rose Tyler, Jack saw the transmission on the TARDIS's scanner, but claimed that he wasn't able to remember sending it.
The Doctor recognised Gharusa Prime because of a skyscraper on the background of the video, the Delamar Solar-Needle. Therefore, they went to Gharusa Prime to investigate the origins of the message and discover why Jack couldn't remember sending it. (COMIC: Doctormania)