Gliese 581d

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Gliese 581d, as per Wikipedia

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Gliese 581d
Type:	Colony
Appearance:	Smile

Gliese 581 D was one of Earth's first colonies settled after Earth's population had been evacuated. The planet that supported the colony was 20 light-years from Earth.

When Bill asked the Twelfth Doctor travel to the future with her, he took Bill to Gliese. (TV: Smile)