‘Planetary sustainability’, as developed in this article, is a transitory term, marking the conceptional change from perceiving the Earth as a globe to recognising it rather as a planet. Although the traditional Brundtland sustainability definition comprises ecological, economic and social dimensions to perpetuate the fulfilment of humankind’s needs for the next generations, the planetary aspect of sustainability leads to the acknowledgement that there will be an end to human civilisation if humankind does not move into space sooner or later. Concerning space mining, this move might happen in our age. Given this fact, and the contemporary situation concerning space debris, ‘ethics of planetary sustainability’ as well asspace environment ethics are needed. It would also make sense to add an 18th Sustainable Development Goal called ‘Space Environment’ to the current 17 Global Goals, as a sort of a political demand to complete what then could better be called the ‘Planetary Plan’. CONTRIBUTION: The increasing economic use of outer space makes it necessary to include this domain in the sustainability discussion. The article calls for an 18th United Nations Sustainable Development Goal “Space Environment”, providing theological and philosophical reasoning for the need of an ethics of planetary sustainability.