Comstock Images/Stockbyte/Getty Images
When we talk about how long a planet takes to complete one full orbit around the Sun, we’re referring to its “orbital period.” While the term “year” is relative to Earth’s orbit, we can express any planet’s period in Earth‑years for easy comparison. By examining both the orbital period and the distance each planet travels, we can identify which planet moves the slowest along its path.
Pluto once held the record with an orbital period of 248 Earth‑years. In 2003 the International Astronomical Union (IAU) redefined the word “planet,” and Pluto was reclassified as a dwarf planet, or “plutoid.” Consequently, the title of longest orbital period now goes to Neptune, whose period is about 165 Earth‑years.
Orbital speed is found by dividing the distance a planet travels in one full orbit by its orbital period. If Pluto were still considered a planet, it would have the slowest speed at roughly 10,438 miles per hour (16,780 kilometers per hour). Neptune, however, has the lowest speed among the eight recognized planets at about 12,148 miles per hour (19,500 kilometers per hour). For comparison, Earth orbits the Sun at an average speed of 66,621 miles per hour (107,000 kilometers per hour).
These figures illustrate how the farther a planet is from the Sun, the longer its journey and the slower its motion—principles that are a direct consequence of Kepler’s laws of planetary motion.
