  Ariel has been selected as ESA's M4 mission for launch in 2028 and is
designed for the characterisation of a large and diverse population of
exoplanetary atmospheres to provide insights into planetary formation and
evolution within our Galaxy. Here we present a study of Ariel's capability to
observe currently-known exoplanets and predicted TESS discoveries. We use the
Ariel Radiometric model (ArielRad) to simulate the instrument performance and
find that ~2000 of these planets have atmospheric signals which could be
characterised by Ariel. This list of potential planets contains a diverse range
of planetary and stellar parameters. From these we select an example Mission
Reference Sample (MRS), comprised of 1000 diverse planets to be completed
within the primary mission life, which is consistent with previous studies. We
also explore the mission capability to perform an in-depth survey into the
atmospheres of smaller planets, which may be enriched or secondary. Earth-sized
planets and Super-Earths with atmospheres heavier than H/He will be more
challenging to observe spectroscopically. However, by studying the time
required to observe ~110 Earth-sized/Super-Earths, we find that Ariel could
have substantial capability for providing in-depth observations of smaller
planets. Trade-offs between the number and type of planets observed will form a
key part of the selection process and this list of planets will continually
evolve with new exoplanet discoveries replacing predicted detections. The Ariel
target list will be constantly updated and the MRS re-selected to ensure
maximum diversity in the population of planets studied during the primary
mission life.
