Why is Pluto no longer a planet?

For a celestial body to qualify as a planet, it must be in orbit around the sun, be large enough that it takes on a nearly round shape and must have cleared its orbit of other objects. Pluto fails the last requirement.

Pluto has been demoted ― it is no longer a planet. At a meeting of the International Astronomical Union on Aug. 24, astronomers from all over the world revised the universal definition for what qualifies a celestial object as a planet and confirmed that the solar system has eight planets, not nine as previously known. Today we are going to learn why Pluto was stripped of its status as a planet.
If we start our spaceship journey of the solar system from the sun, we would first see four terrestrial planets in a row: Mercury, Venus, Earth and Mars. Then, we would pass through an asteroid belt where asteroids are concentrated and are in orbit, including the largest, Ceres, forming a “belt” between Mars and Jupiter. As the spaceship continues it would pass the gas giants Jupiter, Saturn, Uranus and Neptune, which do not have solid matter and consist mostly of hydrogen, helium and water.
If we go further forward, we would see Pluto and 2003 UB313, nicknamed Xena, and thousands of pieces of ice debris. The orbits of some celestial objects are linked to and begin within the orbit of Neptune. Other objects are not linked with Neptune itself as they revolve around the sun beyond Neptune in what is referred to as the outer solar system. Thus, the area is often called the “trans-Neptunian region” or “Kuiper belt,” named after a Dutch-American astronomer, Gerard Kuiper, who theorized the existence of icy planetesimals ― small, asteroid-like bodies that are formed from the collision of particles in space ― just beyond the orbit of Pluto.
A large object orbiting the sun was traditionally considered to be a planet. There were nine planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. The days of the week were named after the sun, the moon, and five planets ― Mercury, Venus, Mars, Neptune and Saturn ― which can be seen with the unaided eyes. But for the last 20 years, more than 1,000 celestial objects were discovered in the Kuiper belt and hundreds of new objects have been found every year since. One of the objects in the belt, and the largest, is Pluto.
When 2003 UB313, an object larger than Pluto, was discovered in October 2003, debates began on whether 2003 UB313 qualifies as a planet. In addition, it became necessary to establish the definition of a planet in case another similar object was found later. Eventually, nearly 2,500 astronomers from 75 countries gathered last month and redefined the term, demoting Pluto from planet status.
According to the new definition, the solar system consists of three groups ― planets, dwarf planets and small bodies. For a celestial body to qualify as a planet, it must be in orbit around the sun, be large enough that it takes on a nearly round shape and it must have cleared its orbit of other objects.
Let’s assume that we put a large bumpy rock in space. If it stays as is, it would not become rounder. But if we put another rock onto the rock, the object would have its own gravity and gradually turn into a rounder shape at some point. Such state of balance is called hydrostatic equilibrium. In this state, atmospheres might be formed and geological movements could start inside the body.
When a planet is just beginning to form, other small celestial objects exist around it. The planet tends to draw the objects towards itself with its own gravity and after the process, no other objects are left around the planet. When this happens, we say the planet has cleared its orbit of other objects.
In the solar system, nine formerly defined planets, as well as Ceres and 2003 UB313, meet the first two conditions to qualify as planets: orbiting around the sun and maintaining a hydrostatic equilibrium. But Ceres has not yet cleared millions of small objects around it; Pluto and 2003 UB313 coexist with other objects in the Kuiper belt. Therefore, only eight planets maintained their planet status under the newly established definition.

Choi Young-joon is a post-doctorate researcher at the Jet Propulsion Laboratory of the U.S. National Aeronautics and Space Administration. The article was translated by JoongAng Daily staff.

by Choi Young-joon