Pluto is a dwarf planet located at the edge of the solar system. Its great distance results in an extraordinarily long orbital timeline. Its journey around the Sun is a slow, methodical march, characterized by its composition of ice and rock and its highly eccentric path.
Pluto’s Orbital Period: The 248-Year Journey
Pluto’s sidereal orbital period—the time it takes to complete one full revolution around the Sun relative to the background stars—is approximately 247.9 Earth years. One “year” on Pluto lasts nearly two-and-a-half centuries of Earth time. This vast difference is a direct consequence of Pluto’s enormous average distance from the Sun, which is about 39.5 astronomical units (AU).
At such a remote location, the Sun’s gravitational pull is significantly weaker than it is on the inner planets, resulting in a much slower orbital velocity for Pluto. For comparison, Earth travels at about 30 kilometers per second, while Pluto moves at roughly 4.7 kilometers per second. This slow speed over a massive circumference is the primary reason for its extremely long year.
The First Full Orbit Since Discovery
Pluto’s current orbital milestone began when it was discovered by astronomer Clyde Tombaugh on February 18, 1930, at the Lowell Observatory. Since its orbital period is just under 248 Earth years, Pluto has yet to complete a single circuit around the Sun. For nearly a century, we have only watched a fraction of its cosmic journey unfold.
By adding the orbital period to the discovery date, astronomers project when this first complete orbit will be finished. Pluto is expected to cross the point in space where it was first spotted in 1930 on March 23, 2178. This date signifies the completion of the first “Plutonian year” observed entirely by humans.
The Mechanics of an Unusual Orbit
Pluto’s orbit stands out from the nearly circular paths of the major planets due to its high eccentricity. Eccentricity measures how much an orbit deviates from a perfect circle. Pluto’s value of about 0.25 is far greater than Earth’s, making its path noticeably elliptical. This elongated shape means Pluto’s distance from the Sun varies widely, from a closest point (perihelion) of about 29.7 AU to a farthest point (aphelion) of around 49.3 AU.
For about 20 years during each orbit, this high eccentricity brings Pluto closer to the Sun than Neptune, whose average distance is about 30 AU. Pluto’s orbit is also highly inclined, meaning it is tilted by about 17 degrees relative to the ecliptic, the plane where most other planets orbit. This combination of an elliptical shape and a significant tilt creates a three-dimensional separation that prevents it from colliding with Neptune, even when their paths appear to cross.
The 3:2 Resonance with Neptune
The stability of Pluto’s unusual path is maintained by a precise gravitational relationship with Neptune, known as the 3:2 mean-motion resonance. This resonance dictates that for every two orbits Pluto completes around the Sun, Neptune completes exactly three orbits. This synchronized gravitational influence locks the two bodies into a stable, repeating pattern.
This rhythmic interaction ensures Pluto and Neptune never come close enough to destabilize each other’s paths. When Pluto is at its closest point to the Sun, Neptune is always situated far away, ensuring a minimum separation of about 16 AU. The 3:2 resonance has protected Pluto’s orbit from major gravitational perturbations for billions of years, despite the apparent intersection of their paths.