Study suggests we reclassify the moon as a planet—reopening a centuries-old debate

February 27, 2017 by Stephen Pumfrey, The Conversation
Will the moon move from servant to equal? Credit: Gregory H. Revera/wikipedia, CC BY-SA

Every now and then a scientific paper makes a real splash. We had one recently, to judge from recent headlines. "Moon rises to claim its place as a planet" said The Sunday Times on February 19, while the Mail Online asked "Is this lunarcy?". The articles were among many responding to the humble paper: "A Geophysical Planet Definition", which suggested that the criteria for determining what constitutes a planet need an overhaul. It argued that the moon, Pluto and several other bodies in the solar system should be upgraded to planets.

The paper, published in Planetary and Lunar Science, was written by a team including Alan Stern. Stern is famous for NASA's New Horizons mission, which made its spectacular flyby of Pluto in July 2015. The paper is a bit technical, but it basically argues that the geophysics of a body should determine whether it is a planet – not just whether it orbits the sun.

Of course, Stern has an axe to grind. He remains furious that, in 2006, the International Astronomical Union [IAU] deemed that Pluto was not a planet. By the time his probe reached its destination, Pluto was a mere "plutoid", a "trans-Uranian dwarf planet". In the article he strikes back. He is fed up with people asking "why did you send New Horizons to Pluto if it's not a planet anymore?"

Lessons from the past

We are so used to thinking of the Earth's satellite as a moon that the idea that it could be a planet is truly shocking. But ancient Greek and medieval astronomers all assumed that the moon was indeed a planet.

Ancient observers knew that the stars maintain their relative positions night after night: they saw constellations such as Leo or Gemini just as we do. But they also saw seven heavenly bodies slowly change their positions, wandering from west to east through the sky. The most important was the sun. The 12 signs of the Zodiac it passed through marked out the circle astronomers call the ecliptic (see figure below). The sun (we would say the Earth, of course) orbited in one year, while Saturn wandered through this plane every 30 years, Jupiter every 12 years and Mars every two years. Planet Moon did so in 1/12 year – one month. In fact, the word for planet comes from the Greek πλανήτης (Latin planeta) meaning "wanderer".

Study suggests we reclassify the moon as a planet –  reopening a centuries-old debate
Ecliptic with earth and sun animation. Credit: Tfr000 /wikipedia, CC BY-SA

The moon was of special interest. Its proximity made it the only "planet" with visible features – "the man in the moon". Aristotle (384-322 BCE) asked several questions about the physics of the moon – including why we always see the same face, and never the far side? It's a good question, and astronomers now explain it as the result of gravitational forces between and large moons, and they call it "tidal locking".

Aristotle drew a different conclusion. He thought it proved that the moon had no innate ability to rotate or move. He assumed the same was true of all planets. They only move, he said, because they are carried in a circle. This was the origin of elaborate Medieval cosmology in which the planets and stars are rotated by a nest of celestial spheres. Had our moon not been tidally locked, astronomy might have taken a different path.

Did our predecessors have good reason to include the moon with the other planets? I think so, but mainly because of a strange astronomical coincidence. Almost all large moons orbit in, or very close to, the equatorial plane of their parent planet, but our moon does not – it inclines by as much as 28 degrees. However, Earth's equatorial plane is tilted with respect to the ecliptic by angle of 23.5. The combination of these two unusual circumstances means that the moon does appear to move in the plane of the ecliptic – and never more than 5 degrees above or below it. Without it, ancient astronomers might not have treated the moon as a typical planet.

An illustration of the Ptolemaic geocentric system by Portuguese cosmographer and cartographer Bartolomeu Velho, 1568. Credit: wikipedia

Lingering ambivalence?

With Copernicus's heliocentric astronomy, published in 1543, the moon ceased to be a typical planet. Uniquely, as Copernicus's critics pointed out, its orbit was centred on the Earth, not the sun. It was now Earth's "satelles", meaning servant, from which our word satellite derives. And there was more loss of status in store. When Galileo trained his telescope on Jupiter in 1610, he discovered four satellites. Lovely news for Copernicans, but not for Luna. It was no longer THE moon, but one of five, a number which rose rapidly towards the 182 moons we know today.

Seemingly, there is nothing new under the sun. In Galileo's time the moon was the subject of an argument between the new cosmologists, who saw it as Earth-like with seas and lands, and the old astronomers who insisted that it was a proper, perfect heavenly body.

With his new definition of a planet, Alan Stern has renewed that battle. According to his paper, astronomers "may find the IAU definition perfectly useful" but "our geophysical definition is more useful for planetary geoscience practitioners, educators and students." Or, as Stern put it bluntly in 2015: "Why would you listen to astronomers about a planet [instead of] planetary scientists that know something about this subject". And they know, or think they know, that the should become a planet again. Whether that will actually happen is completely down to the International Astronomical Union, which would have to make the decision.

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dogbert
3.7 / 5 (6) Feb 27, 2017
Isaac Asimov noted years ago that the earth/moon system was more properly viewed as a double planet system since the moon moved always toward the sun in its orbit and never away from it. Moons generally move toward and away from the sun in their orbits about their planet.
big_hairy_jimbo
4.3 / 5 (6) Feb 27, 2017
Personally (ie my opinion) I prefer the term Moon. For me a planet is spherical and orbits the sun. A moon orbits a planet. Good enough for me. Why make that harder?
If they are going to change things, then have they looked at the "moons" of Mars? Seem a little like asteroids to me, so do we still call THOSE Moons, or are they trapped asteroids or BOTH? See the term MOON to me, means it just orbits a planet. Mind you I haven't read the paper referring to Geophysical Planet Definition.
ulao
5 / 5 (1) Feb 27, 2017
The Sun too was a planet in those days.
Jonseer
5 / 5 (1) Feb 28, 2017
I find it odd that they perceive the term dwarf planet as some sort of demotion.

It's still a planet, just a very small one.

It also makes it a bit unique.

The fact that he crafting a scientific explanation to get Pluto it's full planet status, won't make it any bigger. It still amounts to a small fraction of our own Moon's mass with a diameter of just over 1/2 of the moon.

And comparing Pluto to Mercury, makes it clear that Pluto is very much a dwarf. In terms of size, mass, volume and most measures Pluto relatively speaking measures up to the Moon, about the same way our moon measures up to Mercury.

If I had my way, planets would also be categorized by type. The first 4 would be rocky planets, ad Ceres (and maybe Eris) would be rocky dwarf planets. Jupiter, Saturn, the gas giants. Uranus and Neptune the ice giants with Pluto Etc. being ice dwarfs.
Ojorf
5 / 5 (5) Feb 28, 2017
For me a planet is spherical and orbits the sun. A moon orbits a planet. Good enough for me. Why make that harder?


The moon is exceptionally large in relation to the earth compared to other 'planets' and 'moons' we have studied.
Also the earth moon system orbits the barycenter (https://en.wikipe...ycenter) which is actually quite far, about 4670km, from the center of the earth (radius about 6380).

It doesn't really matter one way or the other, as long as they settle on a consistent and logical definition that makes the distinction clear.
MrVibrating
5 / 5 (2) Feb 28, 2017
@Ojorf - yes Earth / Luna a binary system, ie. two planets..
NoStrings
1 / 5 (1) Mar 01, 2017
Creating a controversy where is none? Monkey brain problem: getting lost in abstractions we ourselves create, or giving abstractions other created a reality of existence.

As to what is a correct definition - in case like this, where it doesn't matter for any practical purposes, it is whatever you decide it to be. I always called it Moon, and I don't care to argue with anyone who wants to call it differently.
BrettC
1 / 5 (1) Mar 01, 2017
The Moon was first heavenly body to be named such. Therefore it is a moon. All other, heavenly bodies previously labeled moons, must there fore be renamed if they do not conform to our moons definition.
maholmes1
5 / 5 (1) Mar 01, 2017
Well, geophysically the Moon is a planet, but it orbits another planet. Terminology like "planet-type moon" would acknowledge both these facts.
someone11235813
5 / 5 (1) Mar 01, 2017
I'm all for including the Moon as a planet such that the Earth/Moon system is then a double planetary system. Firstly it would have looked more like a planet if any one was around to view it a few billion years ago when it was very much closer. What I like most about classifying it as a planet is that we have a new (and more accurate) definition of 'Earth like planet'. Earth like should then be taken to be mean a 'double planetary system'. As far as I'm aware we have found no other double planetary systems consisting of two Earth sized rocky planets in the so called habitable zone.

Which supports my contention until proven otherwise that life and especially intelligent life is exceedingly rare in the Galaxy, maybe the Universe. Maybe life only happens once per galaxy or once per Universe or maybe only one in a hundred universes.
someone11235813
5 / 5 (1) Mar 01, 2017
Personally (ie my opinion) I prefer the term Moon. For me a planet is spherical and orbits the sun. A moon orbits a planet. Good enough for me. Why make that harder?


Currently the centre of mass of the Earth/Moon system is inside the Earth so it appears that the Moon orbits the Earth, but when it was first formed the centre of mass was between the two bodies albeit closer to the Earth, therefore you could not then really say that the Moon orbited the Earth, they both orbited their common centre of mass. So you must accept that it was a planet and then became a moon as it drifted away. One day it will be a planet again when it eventually escapes from the Earth and continues to orbit the Sun, but that will be close to the time that the Sun engulfs the Solar System anyway.
nikola_milovic_378
Mar 02, 2017
This comment has been removed by a moderator.
maholmes1
not rated yet Mar 04, 2017
The Sun too was a planet in those days.


We know it's not now. We're still trying to get other stuff about what orbits it settled. That's how science works.
SiaoX
not rated yet Mar 05, 2017
The renaming Pluto to asteroid and Moon to planet degrades the Earth in my eyes and makes me feel useless and insignificant.
SiaoX
not rated yet Mar 19, 2017
The Orbit of the Moon around the Sun is Convex! So it's a planet for sure... BTW How Moon perceives its role in solar system.

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