## Sunday, March 19, 2017

### Do you really think the Moon is a planet, Kirby?

Phys.org informs us about lots of legitimate news but sometimes it loves to spread hype about some absolute nonsense. When it switches to the nonsense mode, it usually promotes the craziest articles to the "featured" category. On Friday, they posted a crazy article about a topic that everyone should be able to understand,
Scientists make the case to restore Pluto's planet status
Pluto is a hero of the title but this very fact is ludicrous. Some people feel sad about the downgraded status of a piece of rock they have never seen with their eyes. But there's something else that the title doesn't convey: The people who want to redefine a "planet" again intend to make sure that there are over 100 planets in the Solar System so that the list would include the Earth's Moon – where some TRF readers have been – among many others.

Two Plutos, taken from the article about a Daesh astronomer who wants to rename Pluto to the Moon of Mohammed LOL. See also ISIS plans to carry attacks on Pluto.

The main proponent of the new definition is Mr Kirby Runyon (and "Mr" should be understood in the same way as when Dr Gablehauser talks to Mr Howard Wolowitz), a graduate student at John Hopkins, a Christian, and an owner of a cat. Quite some credentials.

We learn that there is "nothing non-planety" about Pluto and Jupiter's Europa and the Earth's Moon should be planets, too, along with 100+ other objects. Their new definition should be all about the "intrinsic qualities" of the celestial bodies.

Holy cow, what a pile of rubbish.

What I find amazing is that the journalist focuses on Pluto – whether it's a dwarf planet, like it is today (since a 2006 decision, see some older TRF texts here), or whether it would be turned into a planet again. But the fact that the Moon would become a planet as well seems like an unimportant detail to the journalist. Please, you can't be serious! Whether the Moon is a planet is much more important than whether Pluto is a planet. The Moon is a body we see almost every night and some of us have been there. We haven't considered it a planet for some 500 years so if someone forced us to say that it is a planet, it would be a pretty radical change.

The history of the term "planet" is long and people tried to be rather scientific from the beginning. But they didn't quite understand the laws of the Universe – even who orbits whom – and they didn't have a sufficient technology to observationally answer all the questions they wanted to be answered. That's why it was unavoidable that scientifically, the lists of planets and other things made "almost complete sense" but not quite.

So one of these anomalies was fixed in 2006 and we got a fully scientific definition of a planet – even though there may still exist marginal or disputable cases. A planet is something that is
1. rounded (by self-gravity)
2. not yet a star (no fusion)
3. dominant in its orbit (has swept the garbage in its neighborhood)
It's so crisp and meaningful. We have only had it for 11 years and the change was controversial, like every change, but this makes so much sense. Imagine some rational young people who have already learned that there are 8 planets and those things are consistent with the new definition of the planet. Do you think that they will find it logical to return to some outdated definitions or invent completely weird revolutions whose main purpose seems to be to celebrate the glory of a rock named Pluto – while paying no attention what the change does to everything else?

The current definition isn't the only possible definition that would "make sense" but it is surely one that makes a lot of sense. One eliminates the stars – where the thermonuclear fusion is taking place because they're massive enough. It eliminates rocks that fly somewhere but are so light that they haven't bothered to round their shape by the self-gravity. And it eliminates some Moons and groups of Moons and round bodies that still have lots of rocks around them. It simply has to be a dominant "sweeper" somewhere. When reasonably applied, the list of such planets almost coincides with the informal list of planets that people had before they bothered to work with accurate enough definitions. It's a good efficiency.

Note that the condition of "dominance in its orbit" is an "external" condition that depends on the relationship of the would-be planet with its environment. Kirby and pals want to use intrinsic properties of the objects only. Why? It makes absolutely no sense. After all, the term planet arose from the Ancient Greek
πλάνητες ἀστέρες (planētes asteres, "wandering stars") or simply πλανῆται (planētai, "wanderers").
So the very word "planet" is linguistically linked to "wandering". When you are "wandering", you must be "wandering somewhere", so where you are wandering clearly matters. "Wandering" means "moving relatively to the rest and from some point of view". It is not an intrinsic adjective. The reason is the same as in the case of the word "tourist", among many others. A tourist isn't defined by his having muscles or vegetables in the stomach etc. A tourist is defined by his location relatively to his home – they are different – and the purpose of his location's being different from his home – fun or recreation. Similarly, a planet is "wandering" which means that it probably orbits something else (a star) but does so actively enough – so it's not just one of many rocks that do so etc.

When I mention etymology, it's good to know that a satellite comes from Latin or Middle French where it simply meant a "companion". It makes perfect sense. A satellite is something that orbits someone else, his boss. Note that the Russian word Sputnik is just a direct translation to Russian (the analogous Czech word would be "souputník"). Sputnik is literally a "fellow traveler". The letter "s" or "sou" in Czech is "fellow" (or "co-") while "putn*" are Slavic roots related to traveling or wandering or journeys.

The word "Moon" is derived from Old Germanic languages where it meant both the Moon and a month. Those two are related because it takes approximately a month for the Moon to complete the orbit around the Earth. Luna is an alternative Latin name that is more mythological and alchemists and others were also linking it to light and silver, possibly assuming that our satellite is made of silver.

The jihad defending the pride of Pluto is utterly irrational. It's driven by nothing else than some stubborn defense of protoscientific factoids that one was taught as a kid. These days, Pluto is simply considered a dwarf planet, along with Ceres, Haumea, Makemakea, and Eris. There are probably hundreds of dwarf planets in the Solar System that haven't been seen or understood properly yet. But it's simply a fact that at least these four other objects are highly comparable to Pluto in all the characteristics you can invent. For example, Eris is three times heavier than Pluto.

It may be confusing that "dwarf planets" aren't a subset of "planets" – "dwarf" seems to be just an adjective that refines the noun "planet". But it's normal for similar terms not to define subsets. For example, a shadow minister isn't a minister and junk science isn't science. You simply cannot disentangle the term "dwarf planet" to small pieces – this term, like others, should be understood as if it were a single new word. The adjective doesn't "refine" the noun; in this case and in some others, it modifies it.

In the past, the Solar System was presented as the Sun, nine planets, and other things which are less important. So Pluto made it into the top ten, above a big gap. Whatever your precise conditions are, as long as they are scientific – and not just some cultural habits – this high status of Pluto was indefensible. If one does science, Pluto's status simply had to be downgraded. I find the word "planet" so elementary that it's a good idea to try to make it possible to remember the full list of planets in the Solar System. And that's how it was done: the list of planets according to the 2006 definitions includes the "objects previously known as planets" minus Pluto.

The subtraction of Pluto from the list of planets was scientifically meaningful. But it wasn't even contradicting centuries of cultural traditions. Why? Because Pluto was only discovered in 1930. It was only called "a planet" for some 76 years because it wasn't known prior to 1930 and it wasn't a planet after 2006. Those 11 years since 2006 are already about 1/7 of the period when Pluto "was" a planet. It's becoming increasingly reasonable to consider the era in which "Pluto was a planet" a temporary anomaly.

Note that Neptune was predicted mathematically and observed in 1846. So between 1846 and 1930, for some 84 years – long than the 76 years above – the list of planets in the Solar System contained exactly the same 8 objects as the list since 2006. That's a reason to say that the claim that "Pluto should be kept in the list of planet" is unjustifiable even historically. It just wants to revert the terminology to a period that was neither the most glorious or accurate one nor the longest one. It just makes no sense.

But to promote the Moon to a planet would contradict centuries of culture and history. Yes, I must admit, not thousands of years. The ancient Greeks' list of planets was: the Sun, the Moon, Mercury, Venus, Mars, Jupiter, and Saturn. (Uranus was only discovered in 1781.) They're all the things that are moving relatively to the stars which seem static. But since the acceptance of the heliocentric model, the Sun and the Moon were removed from the list of planet – for analogous but opposite reasons. The Sun is more important than the planets because the planets orbit it. The Moon is less important than the planets because unlike the planets, it orbits the Earth which is substantially heavier.

To reincorporate the Moon to the list of planet would be basically denying a fundamental lore of the whole astronomy since Copernicus. I can't imagine that something like that would be approved by a majority of astronomers or astrophysicists anywhere. I can't understand how someone would be willing to fight for such a brutal change of the meaning of the words just because of the pride of some dwarf planet.