Definitions

An exoplanet can be defined simply as a planet outside the Solar System, but... that’s a bit short-sighted. We need to specify what a planet is, then what differentiates an exoplanet: is it its mass? Its brightness? The way it was formed? Its place in the Universe? So many questions, so many definitions.
Furthermore, in the world of celestial objects, the domain of exoplanets has very blurred boundaries. It is surrounded by the immense territory of stars, the neighbouring region of brown dwarfs ^? , the distant lands of red dwarfs ^? , the countries of exotic white dwarfs ^? and fascinating black holes.