Planet Or Star?

The Universe And All That

Is the Moon A Planet Or A Star?

That question was posed in a video on our MAS Facebook page recently by a pair of nauseating astro-illiterates, “debating” on a ladies fashion TV show, the two nonsensical alternatives.

Her: “I believe it’s a star or something.”

Him: The Moon is a planet, Honey.

The status of the Moon should be understood by all but I guess they would probably both roll their eyes over my lack of basic ladies fashion knowledge. We should know from our own outreach events that while some people are clued up, many others in the population don’t have any idea about astronomy. It’s up to us to try and remedy that.

Of course we all know that the Moon is a natural satellite and is neither a star nor a planet. But I want to look at this in a bit more depth. So let’s rule out the star “theory” first.

Stars predominately consist of hydrogen and other light elements. They emit vast quantities of light and heat due to thermonuclear fusion reactions in the core. The Sun is a star. We feel its warmth but the Moon only radiates weakly reflected sunlight. Sometimes the Moon doesn’t shine at all. That should have been a big clue to the lady who was adamant that the Moon is a star.

Sometimes the Moon barely shines at all.
(Image © R. Powell 2010)

It’s not too hard to establish that the Moon is not a star but why isn’t it a planet like the guy said? It’s smaller than Earth but it’s still pretty big and it looks like a planet.

Defining The Solar System

In 2006, the International Astronomical Union (IAU) mandated three conditions for a Solar System body to be classified as a planet. Remember? The big loser was Pluto!

The IAU defined a planet as follows:

A “planet” is a celestial body that

  • (a) is in orbit around the Sun,
  • (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and
  • (c) has cleared the neighbourhood around its orbit.

Pluto ticked two boxes but failed (c) because it hasn’t cleared its orbit of debris.

The Moon cannot be defined as a planet, because it failed to tick box (a) (because it orbits a planet, not the Sun). It ticked box (b) and arguably ticked box (c). The Moon did its share of orbit clearing. Just look at the number of lunar craters!

Size Doesn’t Matter

It’s the ‘definition’ which calls all the shots – not size or appearance – and our fashion-loving friend, who believes that the Moon is a planet, would have been considered correct, had he lived in earlier times, when for centuries the geocentric model of the Solar System was both mainstream science and dominant religious dogma. This belief stated that the Sun and planets all orbit the Earth. The Moon was considered a planet.

Both Copernicus and the IAU changed the definitions of bodies in the Solar System.

When the Copernican heliocentric Solar System model was finally acknowledged, the Moon was “demoted” to satellite status, a similar fate to that which Pluto later suffered in 2006 when it was “demoted” to dwarf planet status.

Pluto Last Image Before Flyby
(credit: NASA New Horizons)

It’s interesting that Pluto is smaller than seven natural satellites, including the Moon, so the 2006 redefinition of the ex-planet was probably justified. Two of those satellites, Ganymede and Titan, have a diameter larger than the planet Mercury.

Notwithstanding the IAU official definition, which of course I totally accept, the Moon/Earth System has the largest diameter ratio of satellite to planet (0.27) of all, so in my mind the Earth-Moon system could very easily be described as a double planet system.

But that doesn’t make that fashion shop guy right.

References

IAU_definition_of_planet

List_of_Solar_System_objects_by_size and_radius

This article first appeared in:
Prime Focus Magazine, April 2020.

10 Comments

  1. For some reason I kind of like the idea that maybe, one day, there could be a strong case that the Moon and Earth are a double planet.

    I hope that the IAU’s definitions are healthily challenged as hypotheses are proposed as new information comes to light. Perhaps as more knowledge about exo-planets in the galaxy is gathered, astronomers will re-evaluate the definition of a planet.

    Your example of the fashion show is very illustrative. The layman’s view of astronomy is tremendously limited, I think no matter where you are in the world. For me it’s like not knowing how to read a map to understand where you are. Many are at a loss as to were they are in the solar system, galaxy, universe, and that is a shame.

    Liked by 1 person

    1. The IAU will eventually need to define exoplanets and exo-satellites. With over 4,100 known exoplanets, I have not seen anything about exo-moons but I may do some research on that.

      The question of double exoplanets is bound to come up eventually.

      When it does, I suspect that any definition they make might be designed not to conflict with the current definition of the Moon as a satellite. So a double planet might require, for example, the barycentre to be external to both objects, which is not the case with the Earth Moon system.

      Regardless, I reckon any aliens observing our solar system from afar would say, “look at that, a double planet!”.

      Liked by 1 person

      1. Interesting. If they do attempt to define it so as “not to conflict with the current definition of the Moon as a satellite,” that seems like a very Sol-centric bias. It may be our best starting point, but like you imply, we have, so far, little understanding of exo-moons. It will be fun to see if newer discoveries change our perspective on natural satellites.

        Liked by 1 person

  2. Oh yes, I’ve met quite a few who visit the observatory and ask me “so the Moon is a star right?”

    Another example,

    When I say “star cluster,” I met many who think that those are other galaxies… it’s the layman’s way of equating one definition with another object they heard about. It’s now at the point where I have to remind myself “many of these visitors don’t know the difference.”

    Liked by 1 person

    1. Everyone has their own area of expertise and we don’t expect everyone who comes to our telescopes to be experts about astronomy. Mostly they’re not – but no-one has ever suggested to me that the Moon is a star. If they did, I would be flabbergasted! Then I might ask them to explain crescents, craters and why it get’s dark at night, even when the Moon is up.

      Liked by 1 person

      1. Trust me, I definitely looked at her like “are you kidding me?” And it was literally her lack of understanding… nothing like the idiots who claim the Moon is not a rock but a ball of plasma.

        I’ve also met a couple who acted as if it was the first time they had been told the Sun was a star… Plus there was one time I showed the Sun (white light filter) to a dude that genuinely thought there were supposed to be craters on the Sun and asked me why he couldn’t see them.

        You and I probably have boat loads of stories of meeting astronomy illiterates at our telescopes.

        Liked by 1 person

  3. Interesting! I didn’t know there was a point at which the moon was “demoted” to its current satellite status. I learned the distinction between planet and satellite or moon fairly early, but it’s still nice to see the exact scientific parameters laid out.

    Like

    1. Not so much a “point” in time but a period, from the late sixteenth century onwards, when the heliocentric model became accepted and the geocentric model became extinct. Some religions resisted for centuries.

      Liked by 1 person

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