prompter: 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.

prompter: Name some other dwarf planets and where they are

assistant: Dwarf Planets:

Pluto - It is located in the Kuiper Belt, beyond the orbit of Neptune
Eris - It is located in the scattered disk region beyond the orbit of Neptune
Haumea - It is located in the Kuiper Belt, beyond the orbit of Neptune
Makemake - It is located in the Kuiper Belt, beyond the orbit of Neptune
Ceres - It is located in the asteroid belt between Mars and Jupiter
Orcus - It is located in the Kuiper Belt, beyond the orbit of Neptune
Quaoar - It is located in the Kuiper Belt, beyond the orbit of Neptune
Sedna - It is located in the scattered disk region beyond the orbit of Neptune
Varuna - It is located in the Kuiper Belt, beyond the orbit of Neptune
2007 OR10 - It is located in the Kuiper Belt, beyond the orbit of Neptune
Salacia - It is located in the Kuiper Belt, beyond the orbit of Neptune

Dwarf Stars:

Proxima Centauri - It is located in the Alpha Centauri star system, which is 4.24 light years away from the Sun
Barnard's Star - It is located in the Ophiuchus constellation, approximately 5.96 light years away from the Sun
TRAPPIST-1 - It is located in the constellation Aquarius, approximately 40 light years away from the Sun
Wolf 359 - It is located in the Leo constellation, approximately 7.78 light years away from the Sun
Lalande 21185 - It is located in the Ursa Major constellation, approximately 8.29 light years away from the Sun
Sirius B - It is located in the Canis Major constellation, approximately 8.6 light years away from the Sun.

assistant: In our Solar System, Eris, Haumea, Makemake, Gonggong, Quaoar, Sedna, Ceres, and Orcus are generally agreed upon to be dwarf planets. Of these, Ceres is the closest, residing in the asteroid belt less than 3 AU from the Sun, while the rest are either in the far-off Kuiper belt, the scattered disk, or (in the case of Sedna) detached from the rest of the Solar System.

assistant: As the authority on the naming and classification of celestial objects, the International Astronomical Union officially recognizes five dwarf planets in the solar system:

Pluto
Eris
Ceres
Makemake
Haumea
Several dozens more are being considered for the category, and scientists estimate that hundreds or even thousands of dwarf planets may exist in the solar system.

assistant: 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.

prompter: What means for a planet that has "cleared the neighborhood"?

assistant: "Clearing the neighbourhood" around a celestial body's orbit describes the body becoming gravitationally dominant such that there are no other bodies of comparable size other than its natural satellites or those otherwise under its gravitational influence.

assistant: In astronomy, a planet is said to have "cleared its neighborhood" when it has become the dominant gravitational force in its orbit and has removed or accumulated most of the smaller objects, such as asteroids and debris, that were previously present. This means that the planet has gravitationally cleared its orbit of most other bodies and has become the dominant body in that region of space.

A planet that has cleared its neighborhood is considered to be a "planet" in the strict definition used by astronomers, as opposed to a dwarf planet or a small body that has not cleared its orbit. Planets that have cleared their neighborhoods include Earth, Jupiter, Saturn, Uranus, and Neptune.

In our solar system, the process of clearing a neighborhood is a gradual one that occurs over billions of years, as a planet grows in size and attracts nearby objects. The time it takes for a planet to clear its orbit depends on a variety of factors, including its size, mass, and location within the solar system.

assistant: It means that it has removed all other objects from around its orbit through gravity.

assistant: 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.