The perpetual difficulty of picking what is and isn’t a planet
The ancient Greeks originally described planets as asteres planetai, or “wandering stars,” thanks to the way they moved more than other specks of light in the sky. Today, we can observe a swath of differences between planets and stars, but that original definition does point out how names can be outgrown as we get more information about the universe we live in. While we still hang on to the word “planet,” its definition has changed a lot, even explicitly ruling out the idea of wandering, much less being stars. It’s also far from a settled question, and that’s not just because people feel their memories have been violated thanks to Pluto being classified as a dwarf planet.
Past planetary rosters
For a long time, the debate was focused on the original count of five or seven planets. The obvious objects in the sky included Mercury, Venus, Mars, Jupiter and Saturn, as well as the Moon and the Sun. Depending on who you asked, the Sun and Moon were lumped in the category of planets, since they were all things that, in a geocentric model of the solar system, orbited the Earth. By the Middle Ages, western astronomers were coming to realize that the Sun and Moon did not behave like planets exactly, and they were referred to more frequently as something distinct from those heavenly bodies. The next big step in breaking down these definitions was the revelation of a heliocentric model of the solar system, which then forced the issue that the Earth itself is indeed a planet.
Details of the definition
Fast-forwarding through centuries of astronomical discoveries and refinements, the International Astronomical Union (IAU) currently defines a planet according to three criteria, none of which seem totally satisfying at this point. The first rule is that a planet has to orbit not just a star, but our Sun specifically. This of course bars any of the growing number of exoplanets we’ve discovered in the past few years from being planets, as well as otherwise planet-like objects that may be truly wandering the galaxy outside a star’s orbit. The second rule is that the object is big enough for its gravity to have crushed itself into a nearly round shape (or point of hydrostatic equilibrium.) Basically, smaller objects like lumpy, lopsided asteroids are too small to count, although people have questioned what the exact, and necessarily arbitrary, threshold is for “nearly round.” The final rule is that a planet’s “neighborhood” is cleared of other, potentially intersecting objects. So a planet can have moons that travel the same route, but Pluto is disqualified because of it’s proximity to other Kuiper Belt objects like Eris, plus the fact that its solar orbit is so tied to Neptune’s gravity.
Fortunately, the ongoing discussions with what constitutes a planet are trying to take into account all the new data we have about objects in our solar system and beyond. When we discovered Eris and other Kuiper Belt objects, Pluto seemed more closely related to them than larger objects like Jupiter. However, the practicality of the “cleared neighborhood” rule is actually proving to be difficult to work with. Written with freshly-discovered Kuiper Belt objects in mind, the rule doesn’t hold up well with different parameters— for instance, even an Earth-sized planet in Pluto’s position would be entangled with Uranus’ orbit, which seems to go against the sort of filtering people were aiming for. Proposals for new definitions have been offered, mainly to strip the final rule from the current definition. This would then reclassify Pluto, Eris as well as every other dwarf planet and moon in our solar system. We’d go from eight (maybe nine) planets to 102, which is such a dramatic shift that you might wonder what about the point of such a definition the first place?
The primary job of a good label for planet would be to help us sort and understand these objects as clearly as possible. However, the word ‘planet’ now also carries some baggage with it, including a sense of gravitas that people inside and especially outside the scientific community value greatly. When Pluto was reclassified as a “dwarf planet” in 2006, people referred to it as a “demotion,” and even protested the change. Even if scientists are more concerned with accuracy, the public’s take on the importance of being a planet can’t be overlooked. Securing funding and interest in sending a probe to a planet is still much easier than even a dwarf planet, much less a “small bodied object.” It seems that we all think that it’s cool to be a planet, even if we’re not completely in agreement about what that means.
Source: Should Pluto Be a Planet After All? Experts Weigh In by Mike Wall, Space.com