Pluto should be our ninth planet. A planetologist explains why

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There are topics that elicit strong opinions from people. East Die hard a Christmas movie? (Yes.) Is there a place for pineapple on pizza? (Also yes.) Is Pluto a planet?

I say it is. But this is not a universally shared opinion.

Those of us born at the end of the last century grew up learning that there are nine planets – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto, at an increasing distance from the Sun – and we have had smart mnemonics to help you.

But in 2006, the International Astronomical Union, the body responsible (among other things) for deciding what to call things in space, held a vote that overnight reduced the number of planets in the solar system by nine. at eight.

From its discovery in 1930 until the 1990s, Pluto was the largest known object in a remote region of the solar system that has been called the Kuiper Belt. Shortly after the detection of Pluto, astronomers speculated that other objects of similar size could be lurking in the far reaches of our planetary system.

But the first confirmed detection of another Kuiper Belt object did not occur until 1992, with the discovery of a body ultimately called 15760 Albion. Since then, more than 2,000 bodies have been identified in this part of space, with the actual number of worlds over 100 km in diameter possibly numbering in the hundreds of thousands.

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And this is where the crux of the problem with classifying Pluto as a planet lies: If Pluto were to retain its planet status, then so would everything we find in the Kuiper Belt – and before we know it. , we would have hundreds of planets! Crazy, right?

(It doesn’t matter that we have hundreds of countries, thousands of languages ​​and 8.7 million known animal species.)

So, to avoid cluttering the walls of children’s rooms with unreasonably huge posters, or something like that, IAU held a vote at its General Assembly in Prague in August 2006, during which it held a vote. been decreed that a planet must meet three criteria:

  1. that it is in orbit around the Sun (sorry, planets ejected from the solar system or in orbit of other stars, you are outside);
  2. that is, it has reached hydrostatic equilibrium (that is, its gravity has dragged it into a spherical shape, or quite close); and
  3. that he “cleaned up his neighborhood”.

Whereupon Pluto ceased to be a “classical planet” and began to be a “dwarf planet”.

This last criterion states that a planet must be the gravitationally dominant object in the area of ​​space in which it orbits. This rule makes sense for a place like, say, Earth, which is much more massive than the Moon and anything else along its orbital path. But in the Kuiper Belt, where neighboring bodies are much, much further apart than in the inner solar system, Earth wouldn’t necessarily be able to clean up its neighborhood.

In fact, if we were able to somehow teleport Earth past Neptune, our world could become gravitationally dominated by this ice giant and thus lose its own planet.

It is this idea of ​​”neighborhood clean-up” that is at the heart of the problem regarding the definition of the IAU of a planet. And this is because it is a dynamic criterion: it is a function of the place where a body is located in space, and does not take into account the character of a body beyond being big enough to be a bullet. This criterion does not allow geology of a world to consider.

This argument predates the July 2015 flight over Pluto from NASA’s New Horizons spacecraft, but the images returned by that spacecraft really help justify the case: Pluto is an enigmatic world with towering mountains of ice, vast glaciers of nitrogen ice, a thin atmosphere, a thick icy outer shell and an ocean of probably liquid water below, all atop a huge rocky interior. By any geological measurement – including the fact that there are surface processes acting on Pluto today – Pluto is a planet.

But it’s the International Astronomical the Union, not the International Geophysics Union. And the people who voted on the new classification of the planets were overwhelmingly astronomers, although some (most?) Were planetary astronomers.

The properties that matter to some scientists – where an object is located, how massive it is, even what its orbit looks like – might be much less important to others. And for those of us who study the surfaces and interiors of bodies across the solar system, cleaning up quarters just isn’t important in how we view them.

A photo taken by NASA’s New Horizons spacecraft, showing the mountains and icy plains of Pluto © NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute

To be fair, this is not a simple question. Notwithstanding that the IAU’s definition at present does not allow any planets outside the solar system (the IAU has said it plans to adjust the definition of “planet” to include exoplanets, but 15 years after The Vote hasn’t done it yet), nature tends to be a lot more complex than simple categories allow. If we recognize Pluto as a planet, then should we consider the Moon as a planet? Or Ganymede, Jupiter’s biggest moon, bigger (but less massive) than Mercury?

As a geologist, my point of view is “yes, why not? Personally, I don’t see any use in breaking things down into individual categories because the universe tends not to work that way. (There’s also the fact that Pluto has a lot more in common with Mars than Mars does with Saturn, say, yet the latter two are unmistakably planets.)

Several hundred planets? Great! Children (I) would recite their names with the ease with which they (I) list dinosaurs (of which about 700 species have been documented).

In short, there is more value in being a lumper than a divider.

There is another aspect to this whole issue that concerns me. And this is the optical of what happened to Pluto. Supporters of the IAU vote argue that Pluto is still a certain type of planet. Yet the IAU definition explicitly excludes Pluto from the list of “classical” planets to which it once belonged.

In other words, the “dwarf” in “dwarf planet” is not the same as “terrestrial” in “terrestrial planet” (eg, Venus) or “giant” in “giant planet” (eg, Uranus ). In all respects, even though IAU did not use the term, Pluto was demoted.

And even a quick internet search brings up this word. Demoted is what the public perceives to be what happened to Pluto; shortly after the IAU vote, the American Dialect Society chose “plutoed” as the word of the year, writing: “To Pluto is to demote or downgrade someone or something, as happened to the ancient planet Pluto when the General Assembly of the International Astronomical Union decided that Pluto no longer met its definition of a planet.

Whenever I give a public speech and mention Pluto, the first The question I’m asked is whether Pluto is still a planet – not, say, why its surface looks so weird. Fifteen years later, Pluto’s retrograde attracts more attention than the interesting things we’ve learned about it, and it’s a big failure in science communication.

A planet can be whatever we want, and there’s no reason we can’t have hundreds of them in the solar system. And from this geologist’s point of view, Pluto’s IAU reclassification was wrong.

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