An exoplanet is a planet outside the Solar System that orbits another star.
Direct observation is difficult because the planet is seen close to its host star and contributes far less light.
Brightness contrast
The host star is much brighter than the light reflected by the planet. The planet's weak signal can therefore be overwhelmed by the star's light.
Angular separation
The apparent angular separation between the star and planet can be extremely small compared with the resolving ability of a telescope, making them difficult to distinguish as separate objects.
For these reasons, exoplanets are commonly detected indirectly by looking for effects that the planet produces on the observed light from its host star.
Exam Tip: If asked why direct detection is difficult, give both distinct reasons: the star greatly outshines the planet, and the angular separation can be too small for the telescope to resolve them separately.