Is Pluto a Planet?

Is Pluto a Planet
Pluto is a dwarf planet rather than a planet mainly because it has not cleared it orbit of debris.

Is Pluto a planet? The answer is no, according to the International Astronomical Union (IAU). It’s a dwarf planet. Here’s a look at why Pluto is a dwarf planet rather than a planet as well as arguments for considering Pluto as a comet or as a planet once again.

What Is a Dwarf Planet?

According to the International Astronomical Union (IAU), a celestial body is a dwarf planet if it:

  1. Orbits the sun.
  2. Has sufficient mass for its self-gravity to give it hydrostatic equilibrium (a nearly round shape).
  3. Has not cleared its neighboring region of other objects.
  4. Is not a satellite. (or else Titan and some other moons would be planets)

This definition differentiates a dwarf planet from a regular planet. Pluto, Eris, Ceres, and several other bodies are dwarf planets because of the third criterion. While a planet has cleared its orbit of other objects, a dwarf planet has not.

Reasons Why Pluto Is a Dwarf Planet

Pluto orbits the Sun and meets the second criterion, which is a nearly round shape. However, it has difficulty with the other criteria for planethood.

It resides in the Kuiper Belt, a region of small icy bodies, signifying that it has not cleared its neighborhood of other objects. But, clearing an orbit is about mass and not just about location. Pluto is about two-thirds the size of Earth’s moon, but only one-sixth of its mass. It contains a larger proportion of ice to rock than a typical planet and just doesn’t pull in the debris it encounters on its journey around the Sun.

The other issues concern Pluto’s moon, Charon. Pluto has at least five moons, but Charon is special because it’s about half the size of Pluto. It is so massive that the center of mass (barycenter) of the Pluto-Charon system doesn’t actually reside within Pluto, but in the space between the two bodies. Due to this unique characteristic, some argue that Pluto and Charon are a binary (dwarf) planet system. In other words, Pluto and Charon are essentially satellites of each other. However, the IAU has yet to officially recognize binary dwarf planets as a separate category.

Other Reclassified Bodies

When the IAU proposed the definition of a dwarf planet, Pluto was not the only body with a status change. For example, Ceres and Eris also experienced reclassification. Discovered in 1801, Ceres was initially considered a planet. It was reclassified as an asteroid about half a century later due to the discovery of many similar bodies in its vicinity, forming the asteroid belt. With the introduction of the dwarf planet category in 2006, Ceres once again changed status and is now considered a dwarf planet.

Eris, discovered in 2005, is larger than Pluto. It was initially hailed as the solar system’s tenth planet. However, the introduction of the new definition of a planet in 2006 led to its classification as a dwarf planet, like Pluto.

Why Did Pluto’s Status Change?

Pluto was discovered in 1930 by Clyde Tombaugh, an American astronomer. For over seven decades, it enjoyed its status as the ninth planet of our solar system. The need to define dwarf planets arose from a wave of new celestial discoveries in our solar system in the late 20th and early 21st centuries.

The discovery of Eris and several other large objects in the Kuiper Belt led to a dilemma. Should these objects be classified as new planets, leading to a solar system with dozens or even hundreds of planets? Or should the definition of “planet” be revised?

In 2006, the IAU redefined the term “planet” in a way that excluded Pluto and its newly discovered kin. By creating the category of dwarf planets, astronomers could acknowledge the planet-like characteristics of these objects – such as their size and shape – while maintaining a more manageable number of objects in the official list of planets. This decision also reflects the growing understanding of the solar system’s structure and diversity.

However, the redefinition was not without controversy, and debates about the classification of Pluto and other dwarf planets continue to this day.

Is Pluto a Comet?

Pluto actually shares several characteristics with comets, which leads some scientists to argue that Pluto might be better described as a giant comet. Let’s delve into these shared characteristics:

1. Composition: Both Pluto and comets consist primarily of rock and ice. They contain water ice, as well as ices of nitrogen, methane, and carbon monoxide. When the New Horizons spacecraft flew by Pluto in 2015, it detected mountains of water ice and plains of nitrogen ice on Pluto’s surface, similar to materials found in comets.

2. Origin: Pluto resides in the Kuiper Belt, a region beyond Neptune populated with millions of icy bodies, many of which are comets. Pluto and other Kuiper Belt objects likely formed from the same primordial material that gave birth to comets.

3. Highly Eccentric Orbit: Like comets, Pluto has a highly elliptical orbit, which differs significantly from the more circular orbits of the planets. Pluto’s orbit is also more inclined to the ecliptic plane (the plane in which Earth and most other planets orbit the Sun) than the eight planets.

However, it’s important to note that these shared characteristics do not necessarily mean that Pluto is a comet. For instance:

Size and Complexity: Comets are usually small, often just a few kilometers across. Pluto, however, has a diameter of about 2,377 kilometers, making it much larger than any known comet. Pluto is also geologically complex, with mountains, valleys, and plains, which contrasts with the relatively simple structures of comets.

Activity: A defining characteristic of comets is their activity. When comets come close to the Sun, their ices vaporize, creating a glowing coma around their nucleus and often forming two tails (one of dust, one of ionized gas) that always point away from the Sun. Pluto, however, does not display this type of activity.

Orbit: While both Pluto and comets have elliptical orbits, Pluto’s orbit, although more eccentric than the planets, is still less extreme than typical comet orbits. Also, many comets have highly inclined or even retrograde orbits, while Pluto’s orbit is less inclined and prograde.

In conclusion, although an argument can be made that Pluto is a giant comet based on certain shared characteristics, there are significant differences that uphold Pluto’s classification as a dwarf planet rather than a comet.

The Argument for Saying Pluto Is a Planet

There have been and continue to be debates about whether Pluto should be classified as a planet rather than a dwarf planet. Many of these arguments center on the interpretation and validity of the International Astronomical Union’s (IAU) definition of a planet, which requires a body to have “cleared its orbit” of other debris. Those arguing in favor of Pluto’s planethood often focus on the following points:

1. Ambiguity in “Clearing the Orbit”: The criterion that a planet must “clear its orbit” is somewhat ambiguous. There is no precise, quantitative measure of what it means to clear an orbit, which leads to differing interpretations of this criterion. Some argue that this definition is flawed or too restrictive.

2. Geophysical Definition: Some scientists prefer a geophysical definition of a planet, rather than the dynamic one used by the IAU. Under a geophysical definition, whether a body is a planet depends on its intrinsic properties rather than its external orbital characteristics. For instance, planetary scientist Alan Stern, the principal investigator of the New Horizons mission to Pluto, argues that a planet is any body in space that’s large enough to be rounded by its own gravity. By this definition, Pluto qualifies as a planet.

3. Complexity and Activeness: Proponents of Pluto’s planethood also point to the complexity and geological activity of the dwarf planet. Pluto has a multi-layered atmosphere, weather, five known moons, a diverse surface with mountains and valleys, and evidence of geological changes. These characteristics are typical of planets rather than the simpler structures usually categorized as dwarf planets.

4. Historical Precedence: Pluto was a planet for over 75 years, from its discovery in 1930 until the IAU redefined the term in 2006. Some scientists and members of the public argue for the restoration of Pluto’s status as a planet based on this historical precedence and its symbolic significance.

While these arguments for classifying Pluto as a planet are compelling to many, the IAU’s current definition remains the internationally accepted standard. Regardless, this ongoing debate underscores the continually evolving nature of scientific understanding and classification.


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