Final answer:
Pluto was considered a planet after its discovery due to initial assumptions about its mass and orbital characteristics, which were later corrected, leading to its reclassification as a dwarf planet by the IAU.
Step-by-step explanation:
People thought that Pluto was a planet for several decades after its discovery because it appeared to fulfill the criteria for planethood known at the time. Initially, assumptions about its mass led to the classification of Pluto as similar to Earth, making it seem like a fifth terrestrial planet misplaced in our solar system. This view persisted until the discovery of its moon Charon in 1978, which allowed astronomers to measure Pluto's actual mass, revealing it to be significantly smaller than Earth.
Pluto's orbit was also notably more eccentric and inclined than those of known planets, further distinguishing it. In the 1990s, with the discovery of other small objects in the outer solar system, such as Eris and Makemake, it became evident that Pluto was not unique, leading to the reconsideration of its status. The eventual reclassification of Pluto as a dwarf planet by the International Astronomical Union (IAU) reflected its unique characteristics and the presence of similar small bodies in its vicinity.
The IAU's criteria for classifying a world as a planet includes clearing the neighborhood around its orbit, which Pluto does not do, contributing to its reclassification. The decision in 2006 was met with mixed reactions from astronomers and the public, with som