  Uranus and Neptune, and their diverse satellite and ring systems, represent
the least explored environments of our Solar System, and yet may provide the
archetype for the most common outcome of planetary formation throughout our
galaxy. Ice Giants will be the last remaining class of Solar System planet to
have a dedicated orbital explorer, and international efforts are under way to
realise such an ambitious mission in the coming decades. In 2019, the European
Space Agency released a call for scientific themes for its strategic science
planning process for the 2030s and 2040s, known as Voyage 2050. We used this
opportunity to review our present-day knowledge of the Uranus and Neptune
systems, producing a revised and updated set of scientific questions and
motivations for their exploration. This review article describes how such a
mission could explore their origins, ice-rich interiors, dynamic atmospheres,
unique magnetospheres, and myriad icy satellites, to address questions at the
heart of modern planetary science. These two worlds are superb examples of how
planets with shared origins can exhibit remarkably different evolutionary
paths: Neptune as the archetype for Ice Giants, whereas Uranus may be atypical.
Exploring Uranus' natural satellites and Neptune's captured moon Triton could
reveal how Ocean Worlds form and remain active, redefining the extent of the
habitable zone in our Solar System. For these reasons and more, we advocate
that an Ice Giant System explorer should become a strategic cornerstone mission
within ESA's Voyage 2050 programme, in partnership with international
collaborators, and targeting launch opportunities in the early 2030s.
