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President Trump In Office for a Month -- Pluto May Be Reinstated as a Planet
Posted by Guy Pirro on 2/24/2017 1:59 PM

The New Horizons spacecraft helped us see Pluto and its largest moon Charon more clearly than ever before. Some scientists (including some prominent NASA scientists) are proposing a new definition for "Planet" that would reinstate Pluto as a planet. In keeping with emphasizing the intrinsic properties of planets, the new geophysical definition would be directly based on the physics of the world itself rather than the physics of its interactions with external objects. (Image Credit: NASA)


First of all, I must apologize for the deliberately provocative and over-hyped title for this News Item. I got all wrapped-up in the sensationalism surrounding any mention of our new President Trump and I just couldn't resist. FACT #1 - President Trump has been in office for about a month. FACT #2 - Some scientists (including some prominent NASA scientists) are proposing a new definition for "Planet" that would reinstate Pluto as a planet. What I deliberately left out of the title is FACT #3 - The two statements are totally unrelated to each other. But face it -- You are reading this because you fell for a cheap trick referred to as a "Post-Hoc Fallacy," where you as a reader infer a cause and effect for two facts simply because they occurred about the same time and are presented together. But enough of this -- Back to the News Item. If this new definition is accepted by the IAU, then Pluto may once again be a planet.

In the mind of the public, the word "planet" carries a significance that is lacking in other words used to describe planetary bodies. In the decade following the so-called "demotion" of Pluto by the International Astronomical Union (IAU), many members of the public have assumed that alleged "non-planets" cease to be interesting enough to warrant scientific exploration, according to a paper submitted to the IAU. In response to this, the group, including some prominent NASA scientists, is proposing a new a geophysically-based definition of "planet" that emphasizes a body's intrinsic physical properties over its extrinsic orbital properties.

According to the group, the planet definition adopted by the IAU in 2006 is technically flawed for several reasons. First, it recognizes as planets only those objects orbiting our Sun, not those orbiting other stars or orbiting freely in the galaxy as "rogue planets." This was clearly an oversight. Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs (Near Earth Objects). Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth sized object in the Kuiper Belt would not clear its zone, and by definition, would not be a planet. Does that make sense?

The eight planets recognized by the IAU are often
modified by the adjectives "terrestrial, giant,
and ice giant," yet no one would state that a giant planet is not a planet. Yet, the IAU does not consider dwarf planets to be planets. The group eschews this inconsistency.

Thus, in the new proposed definition, dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth's Moon would be "fullfledged" planets. This seems especially true in light of these planets complex geology and geophysics. While the degree of internal differentiation of a given world is geologically interesting, we do not use it as a criterion for planethood, in the spirit of having an expansive rather than a narrow definition.

The new proposed definition goes as follows:

A planet is a sub-stellar mass body that
has never undergone nuclear fusion and
that has sufficient self-gravitation to assume
a spheroidal shape adequately described
by a triaxial ellipsoid regardless of its orbital parameters.

A simple paraphrase the new definition -- especially suitable for elementary school students -- could be, "Planets are round objects in space that are smaller than stars." The definition clearly excludes stars or stellar objects such as white dwarfs, neutron stars, and black holes and the whole issue surrounding brown dwarfs (Are they stars or planets?) That can be decided at a later date. In keeping with emphasizing the intrinsic properties of planets, the new geophysical definition is directly based on the physics of the world itself rather than the physics of its interactions with external objects.

Astronomers with research interests in dynamics
may find the IAU definition perfectly useful. However, many planetary scientists are closely aligned with the geosciences. Accordingly, our geophysical definition is more useful for planetary geoscience practitioners, educators, and students.

With the above definition of a planet, there would be at least 110 known planets in our Solar System. This number continues to grow as astronomers discover more planets in the Kuiper Belt. Certainly 110 planets is more than students should be expected to memorize, and indeed they ought not. Instead, students should learn and focus on only a dozen or so planets of interest.

For an analogy, there are 88 official constellations and about 94 naturally occurring elements, yet most people are content to learn only a few. So it should be with planets. Understanding the natural organization of the Solar System is much more informative than rote memorization.

Teaching the zones of the Solar System from the
Sun outward and the types of planets and small bodies in each is perhaps the best approach -- The zone closest to the Sun consists of rocky planets; the middle zone consists of gaseous, rocky, and icy planets; and the third zone consists of icy planets. All zones also have
small, non-round, asteroidal/cometary bodies.

Implicitly using the geophysical planet definition in this context is easy. Teachers may introduce new moon-planets to their students with phrases such as, "In the 2020s, NASA will send a spacecraft to study the planet Europa, which orbits around Jupiter as one of its many

The group finds that the public resonates happily with the newly proposed geophysical
definition, especially since it is a definition that reflects a body's intrinsic physical properties, not its location. And it is a definition that leverages their intuition.

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