At the beginning of the 1990s, astrophysicists thought that, owing to the advancement of technology since the time of Otto Struve and owing to some luck, they would be able to identify exoplanets around nearby stars. In addition, as we have said, that was precisely what happened with the discoveries of Wolszczan and then those of Mayor and Queloz. Today, more than 5000 planets have been discovered, with distances between 4.2 and 27,700 light years. Some are less massive than the Moon, whereas others have masses close to that of a brown dwarf and radii in the range of 2000–200,000 km. The variety of exoplanets discovered is so vast that we cannot use the classification used for our solar system, and at the same time, they are not representative samples of those existing in our galaxy. Our interest is fundamentally in the habitability of these planets. Therefore, we will determine whether there are habitable planets in the groups into which they can be divided.