Jacqueline’s Space: Pluto is a planet, and nothing will convince me otherwise

Last Tuesday, Pluto-lovers and most children who went to elementary school before 2006 grieved on the 15th anniversary of the greatest tragedy in astronomical history: the demotion of Pluto from a planet to a dwarf planet.

The International Astronomical Union (IAU) voted on Aug. 24, 2006, to accept a definition of “planet” that had three criteria:  A planet must orbit the Sun, have enough mass to form a nearly spherical shape and clear all other objects from its zone around the sun.

Pluto, orbiting the Sun 3 billion miles from Earth in the Kuiper Belt, a ring of asteroids beyond the orbit of Neptune, passed the first two criteria but failed the third. In the eyes of the IAU, this tiny but intriguing body that Clyde Tombaugh discovered in 1930 would now be a part of a new, separate category of “dwarf planets.”

Yes, they voted Pluto out, but I still hold that our icy, purple plum still is a planet. 

And there is more than a childhood grudge and pop culture’s (righteous) anger to support the cause. 

First, what must be made clear is that the IAU is not the ultimate authority in defining planets. The IAU is a group of astronomers, non-experts in planetary science. 

Their decision method was not scientific, even though their definition is seen as scientific fact. 500 members of the IAU sat in a room in Prague and held up yellow cards to make a scientific decision by voting, a process usually successful in politics but not the proper structure of the scientific method. The widespread acceptance of this method turned science into a display of arbitrary politics.  

Second, the definition itself is innately flawed. Since the definition’s inception, a group of planetary scientists, experts who actually study planets, have been fighting for Pluto’s planethood, with Alan Stern, the principal investigator for NASA’s New Horizons mission to Pluto, leading the charge. 

In his book “Chasing New Horizons” and his many talks, Stern scientifically tears down the IAU’s definition. 

The scientific fault in the IAU’s planet definition, according to Stern, lies in its emphasis on location. Pluto and similar objects, like Ceres and Eris, were not the only ones excluded from planethood. Under the IAU’s first criteria that planets must orbit the Sun, the over 4,000 known bodies that orbit stars in other solar systems were also demoted into a separate category of “exoplanets.” The logic fails when you consider that it would be beyond question to require a star to be a part of the Milky Way, yet it is accepted that a planet must be in our solar system.

The overemphasis on location is also obvious in the third criteria, an ambiguous standard that a planet must have “cleared its neighborhood.” Pluto does not pass this test but, according to Stern, neither do Venus, Mercury, Mars nor Earth. Asteroids constantly enter the orbital area of these planets making it impossible for any object to truly clear its neighborhood. 

In response to the IAU’s definition, Stern and many other planetary scientists proposed the geophysical definition of a planet, which states that a planet must have enough mass to form a nearly spherical shape and has never undergone nuclear fusion. 

This is a much better definition because it resembles the use of the term planet for years – to describe these geologically active and diverse bodies in our solar system. It takes into consideration the actual make-up of the body, not simply its location in the universe. Under this definition, not only is Pluto a planet, but so are hundreds of other similar objects, and many of our solar system’s largest moons. Just as suns can orbit one another, be part of a solar system or float alone in a galaxy, so too does this planetary definition accept the wide variety we find in our universe. 

Ultimately, our categorization of an object cannot change its innate qualities. In a vote, the IAU’s definition threatened to change our perception of Pluto and yet, 3 billion miles away, nothing changed. Pluto is the same pinprick of light Clyde Tombaugh discovered in his telescope and the same mountainous and diverse land the New Horizons probe came within 5,000 miles of in 2015. 

That is the beauty of it. We can learn about, explore and categorize the universe, but we must do so from a position of awe and humility because we cannot change it.

The universe is still teeming with planets, hundreds around our star and innumerably more around others. Pluto is a planet, but whether you agree or not, it is still the same, small yet fascinating world it has always been. 

Jacqueline Hale is the Editor-in-Chief. Find her on Twitter at @HaleJacquelineR.

2 comments

  • Great article!

  • If planetary scientists said there’s only eight stars in our galaxy and voted on that to be the official definition because having more stars would be too difficult for school kids to memorize, the stellar astronomers would be very upset. The same is true in the other direction, the planetary scientists have every right to be upset.

    Our solar system has hundreds of planets and that’s an exciting discovery! Fantastic article!

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