Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant:  Human: Tell me why Pluto is no longer a planet.

Assistant: 
There are a few reasons:

1. In the early 2000s, astronomers began discovering other objects in the outer solar system that were similar in size to Pluto. These discoveries challenged the notion that Pluto was unique and highlighted the lack of a formal definition of what constitutes a planet.

2. In 2005, the IAU formed a committee to officially define what constitutes a planet. After much debate, they proposed a definition in 2006 that a planet must directly orbit the sun, be large enough to be nearly round in shape due to its gravity, and must have cleared the area of its orbit of other objects.

3. Pluto did not meet the third criterion, as its mass is much smaller than the other planets and its orbit is filled with many other objects in its size range (called the Kuiper Belt). So in a controversial vote, the IAU demoted Pluto from planet to "dwarf planet" status in 2006. The vote was controversial because it was a narrow majority and there was ongoing disagreement about the criteria.

4. However, the new definition is now widely accepted. Classifying Pluto as a dwarf planet recognizes that it is different from the other major planets in our solar system—it is much smaller, has an unusual elliptical orbit, and shares its orbital space with many other objects. So while it was an adjustment, the new classification is more scientifically accurate and helps clarify what officially constitutes a true dominant planet vs. other objects in the solar system. There are three criteria according to the International Astronomical Union (IAU) to be classified as a full-sized planet in our system:
- It is in orbit around the Sun.
- It has sufficient mass to assume hydrostatic equilibrium (a nearly round shape).
- It has “cleared the neighborhood” around its orbit.
Pluto satisfied the first two, but has not cleared its neighboring region of other objects, therefore it is not a planet. Pluto was reclassified as a "dwarf planet" by the International Astronomical Union in 2006 because it didn't meet their new definition of a planet, which requires a celestial object to orbit the Sun, be spherical, and clear its orbit of other debris. Despite controversy, this definition is widely recognized by the scientific community.