The diagram above shows how a constitutional elective monarchy works. The idea is that the people will elect a parliament once in a while (say, every four or five years), and in turn parliament will elect the monarch. However, unlike the MPs the monarch will serve for life – unless removed from office.
All kinds of variations of the basic scheme are possible. Parliament can be unicameral, bicameral or even tricameral (I am working on story which features a tricameral parliament), or the method of election might vary. Also this basic plan does not tells what powers are vested into the monarch.
Of course, this basic model is simplistic and different types of elective monarchy can be proposed, such as a monarch elected by and from lower (hereditary) rulers or by a committee whose members are appointed by the current monarch. Nevertheless the model presented above is the one which is most compatible with democratic ideals, and hence most suitable for future space countries.