An ionosphere is a collisional plasma embedded in the neutral envelope of gas around Solar System bodies (e.g., planet, moon, comet). Though ionospheric plasma represents less than 1% of the atmospheric number density, this natural plasma plays a critical role throughout the Solar System. As a highly conductive layer, it allows the closure of current systems around magnetised planets, represents an obstacle to the solar wind at Venus and comets, and renders the detection of sub-surface ocean more difficult at the icy moons. As a player in space weather, the ionosphere perturbs communication and contributes to power grid outage at Earth. As an organic factory, it is responsible for the formation of complex aerosols at Titan with prebiotic implication at the surface. Usually cold (<1eV), an ionosphere is produced through the ionisation of atmospheric neutrals by solar Extreme Ultraviolet radiation and energetic particles originating from the space environment of the body (e.g., magnetospheric plasma and solar wind). These particles not only ionise the atmospheric species, but are also responsible for the auroral emissions. Hence aurora can be used as remote sensing of the ionosphere. I will provide a few examples to illustrate key roles played by the ionospheric plasma in the Solar System under auroral skies.