How does it happen that Pluto is not the fa rthest planet of the solar system now?

Pluto is not always the farthest planet from the Sun due to its substantial orbital eccentricity of approximately 0.25 [t2][t4]. While most planets have nearly circular orbits, Pluto's path is highly elongated and inclined at 17.2° to the ecliptic [t2][t6]. This high eccentricity causes Pluto's distance from the Sun to vary significantly, ranging from 29.7 AU at perihelion to 49.3 AU at aphelion [t2]. Consequently, Pluto's orbit 'cuts' or crosses inside the orbit of Neptune for a period of about 20 years during its 248-year revolution [t2][t4]. For instance, between 1979 and 1999, Pluto was closer to the Sun than Neptune, making Neptune the farthest planet during that interval [t2][t3]. Although they appear to cross, a 3:2 orbital resonance and orbital inclination prevent the two bodies from colliding [t1][t2].

Sources

1. [1] https://www.britannica.com/place/Pluto-dwarf-planet
2. [2] https://www.sciencedirect.com/topics/earth-and-planetary-sciences/pluto-planet