To Be or Not to Be: Pluto Loses Its Status as a Planet Again

As children we were all taught that Pluto is the ninth and outermost planet of our Solar System. Then as we grew up we were told it was no longer a planet. Recently, it was about to be re-declared a planet. And just yesterday it had officially been marked off the list of planets in our Solar System.

What is happening? Is Pluto’s size changing? Its atmosphere, geophysical structure, presence of water, its orbit path, its proximity or lack thereof to the Sun?

Definition of Planet

No, nothing even remotely close to that. Pluto is still the same size, shape, and structure. What keeps changing constantly, though, is the human definition of the word ‘planet’. Not everyone was happy with Pluto being cast out of the solar system due to two groundbreaking discoveries at that time – the new-found knowledge of trans-Neptunian objects and that there were worlds around other stars, too.

Thus in 2006 the International Astronomical Union (IAU) had to come up with a new definition of ‘planet’ which was that for any solid body in space to be considered a planet it :

  • had to be in hydrostatic equilibrium or have enough gravity to pull it into an ellipsoidal shape,
  • had to orbit the Sun and no other star or body, and
  • “it needs to clear its orbit of any planetesimals or planetary competitors.

Trouble with the Current Definition

Now, this definition is problematic because it does not define a planet with relation to any other star system than our own. Another thing to take into account is that it says “the planet must clear its orbit”, this statement is subjective and reliant on what else is there in the path of that particular orbit. For example, Mercury is placed farther away from the Sun than it is now which would not let Mercury clear its orbit, would Mercury stop being a planet then?

If this definition was applied to the exoplanetary system and the Sun replaced with “it’s parent star”, it would still not be a proper definition because there is no precise measurement of the distance of orbits of these exoplanets and therefore we cannot know whether they even clear their orbits or not.

Pluto and its History of Discovery

When Pluto was first discovered, it was accepted as a planet and questioning that it wasn’t one was illogical. There has always been flawed speculations and observations associated with the dwarf planet. To begin with, Pluto was thought to be larger and more massive than Earth itself and then there was the incorrect observation that Pluto deployed a significant tug on the gravitational field of Neptune, or the assumption that it would be larger than the inner, rocky worlds, but it’s barely even half the size of the Mercury. And then when the frozen worlds of Kuiper belt came into place, everyone began to question what was so special about Pluto.

The discovery of exoplanets was made around the time that the additional Kuiper belt objects were found.  The larger and more massive planets, revolving around their parent stars, were easier to find. But over time, technology and improved techniques have  played a great role in observing more of these worlds with a variety of planets and the different distances of the planets from their parent stars and their masses. Our Solar System is nothing special compared to the diversity found in these exoplanet Solar Systems.

Pluto should not be downplayed just because it is not defined as a planet. There are still many discoveries to be made and if this exoplanetary world does exist, there should be more efforts made in order to explore the parameters and mass of this system.

Via Forbes


  • Pluto is smaller than its moon. It’d be extremely weird to classify it as a planet and leave its moons out.

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