  Exoplanets number in their thousands, and the number is ever increasing with
the advent of new surveys and improved instrumentation. One of the most
surprising things we have learnt from these discoveries is not that small-rocky
planets in their stars habitable zones are likely common, but that the most
typical size of exoplanet is that not seen in our solar system - radii between
that of Neptune and the Earth dubbed mini-Neptunes and super-Earths. In fact, a
transiting exoplanet is four times as likely to be in this size regime than
that of any giant planet in our solar system. Investigations into the
atmospheres of giant hydrogen/helium dominated exoplanets has pushed down to
Neptune and mini-Neptune sized worlds revealing molecular absorption from
water, scattering and opacity from clouds, and measurements of atmospheric
abundances. However, unlike measurements of Jupiter, or even Saturn sized
worlds, the smaller giants lack a ground truth on what to expect or interpret
from their measurements. How did these sized worlds form and evolve and was it
different from their larger counterparts? What is their internal composition
and how does that impact their atmosphere? What informs the energy budget of
these distant worlds? In this we discuss what characteristics we can measure
for exoplanets, and why a mission to the ice giants in our solar system is the
logical next step for understanding exoplanets.
