Pluto is still not an actual planet in our solar system, according to the International Astronomical Union, the institution in charge of identifying all celestial bodies and determining their classification. I absolutely disagree with this judgment, but the IAU has decided not to overturn Pluto's demotion.


Pluto was named a planet soon after its discovery in 1930, becoming the ninth in our solar system. After Pluto was found, many astronomers assumed it was to blame for the changes in Neptune's orbit that they had detected. These disturbances were what sparked the quest for a planet beyond it. However, subsequent observations revealed that it was far smaller than previously supposed. In addition, once American astronomer James Christy found Pluto's biggest moon, Charon, in 1978, scientists were able to calculate Pluto's mass and determined that it was a lightweight with no gravitational impact strong enough to cause the observed perturbations.

Is Pluto a Planet

Pluto was discovered to be smaller and more massive than the other planets. Furthermore, its orbit is 17 degrees inclined relative to the ecliptic, the plane defined by Earth's orbit around the Sun. The inclinations of the other planetary orbits are less.

Astronomers discovered worlds of comparable size beyond Pluto in the early twenty-first century, including Sedna, Eris, Makemake, and others. These findings raised the question of whether the IAU should grant planetary status to all of these additional planets. The IAU had its triennial summit in Prague in August 2006. They decided toward the close of this conference to adopt Resolution 5A: "Definition of 'planet." A body must meet three criteria to be declared a planet under this newly approved definition. A body must first achieve a stable orbit around the Sun. This condition is met by thousands of bodies. Second, a body must have evolved into a spheroidal form.

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Gravity will shape a body into a spheroid if it is huge and massive enough. Pluto meets this requirement. Finally, the body must have cleansed its debris field. It must be sufficiently big to include all nearby items. Pluto fails this test because its orbit travels near to or even within the Kuiper Belt, a region where short-period comets form. The IAU, in one fell sweep, demoted Pluto, solidly confirmed the other eight planets as planets, and rejected all things beyond Pluto.
 
Although recent studies by the New Horizons spacecraft have revealed that Pluto is bigger, more geologically active, and has a thicker atmosphere than previously thought, it still fails to meet the third criteria of Resolution 5A. To restore Pluto's planetary status, the IAU will need to establish a new definition of planet.

Of course, others insist that Pluto is still a planet and that no resolution will persuade us otherwise.

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