Blog, Science

Is The Moon Actually a Planet?

While looking out at the moon, you’ve probably asked yourself… how did it it get there?

Talking about the Moon is a little strange at first because in day to day life we often take the Moon for granted and can rarely give it a second look…

But the huge sphere sitting in our sky has a lot of questions to answer, the most obvious being; how did it get there?

The Moon is a mysterious feature of our planet and with the up & coming Solar Eclipse in North America, I thought it would be fun to write about the Moon and ask one of the big questions in planetary astronomy:

Is The Moon really a Moon?

At first glance, this may seem like a silly question. Of course the Moon is a moon! What else would it be?!

…Could the Moon actually be a Planet?

Let’s get into it.

First: How Language Defines Perspective

In Science, we often determine things by putting them into categories – labeling them with definitions.

We use categories in every aspect of language to help us make sense of the world around us.

If something is well defined then we can understand it better because we have the language to discuss it.

In the Sciences creating a category or definition automatically creates a boundary around a concept. If the object is one thing then it is therefore not something else. Cool.

Stay with me here, it’s important.

Because definitions are things Scientists can argue over and dispute. If the definition of something changes, it implies that we must have learned something substantial enough to change the way we comprehend a concept.

Take the example of Pluto

As we all know, Pluto used to be considered a planet… Until one day it wasn’t. What changed?

In Pluto’s case, we learned more about it. Modern satellite imagery helped increase our knowledge and understanding of Pluto. Enough to say that it wasn’t like the other planets that we know about.

The main reason for the definition change was mostly due to the fact that Pluto is small… even smaller than our Moon.

Another reason is that Pluto has orbiting moons that are around the same size as Pluto itself…

So we made a new category and called it a dwarf planet – whatever that means.

Note: Some scientists have now called for a new definition that simply puts all spherical objects in space (that aren’t starts) into the category of Planet. So Pluto could be considered a planet again… along with all the other round things up there… How utterly confusing, but forget all that!

Let’s Talk About What Makes Our Moon Different

If we examine all the other planets in our solar system, most of them have orbiting moons.

Jupiter has at least 67 known moons, 4 of them being substantially larger than the others. Mars has just 2 moons. Etc, etc.

But the interesting thing to note is that all of these moons are relatively small when compared to the mass of their host planets.

Jupiter is the largest planet in our solar system and as a result, it has some heavy moons but what’s interesting is that Jupiter’s largest moons are (roughly) the same size as Earth’s Moon.

That’s weird… Jupiter is more than 300X the mass of Earth, yet our orbiting moons are the around same size…

Logic would say that Jupiter’s moons should be much much bigger than ours… but they’re not.

What Gives?

I first found the answer in a fantastic book by my one of my all time favorite authors – Isaac Asimov. The book is called “On Time, Space and Other Things” and if you want to read it I found a great free.pdf on Google.

The book is a collection of lectures by Asimov and most of it actually covers history, specifically the complex history of our modern calendar system! I digress.

Asimov loves using established and existing information to craft entirely new stories. Re-framing the data to create new perspectives.

To this end, Asimov presents a list of facts about our solar system and about all the different moons that live here.

The interesting thing is that our Moon (Luna) is relatively huge next to the Earth!

In order to understand why this is, we must talk about where all these moons came from in the first place.

The Origin of Our Moon is Unique

Back in the early days of the solar system, there was a lot of free floating rubble and debris.

This rubble eventually started to fall into orbit around the young planets.

Rubble coagulated and melted together in orbit – forming small moons. Either this, or large bits of rock were captured into orbit by the gravity of the nearby planets.

Jupiter is the biggest planet so it could capture the largest bits of rubble. As a result, Jupiter has bigger moons than other gas giants like Saturn or Uranus.

But but a small planet such as Earth is simply not big enough to gravitationally capture a mass the size of our Moon.

Astrodynamics doesn’t adequately allow for it…

Note: that our Moon can still exist in orbit around Earth (like it currently does), but if a Lunar-Sized-Object was to just drift past an Earth-Sized-Object the dynamics won’t really allow for a gravity capture.

So… Where Did The Moon Come From?

Here’s a great little summary by NASA on lunar formation theories

The current understanding is that our Moon was formed during a collision event.

The Giant Impactor Theory says that in the early days of the solar system (when planets were still very, very molten) a Mars-Sized-Object collided with the early Earth, glancing off and sending a load of crust and debris into orbit.

When the debris from this collision settled and coagulated, Earth was left with a relatively large orbiting body that we now call The Moon…

This makes the relationship between the Earth and the Moon special.

Very special. In my option, the relationship is far too unique to simply thrown into the same category as the other bits of floating rock out there. As far as we know, none of the other planets experienced this kind of impact. This situation didn’t occur with the other moons

The Moon is a child of the Earth, born from a youthful planetary encounter.

From the book, On Time, Space & Other Things by Isaac Asimov:

We might look upon the Moon, then, as neither a true satellite of the Earth nor a captured one, but as a planet in its own right, moving about the Sun in careful step with the Earth.

To be sure, from within the Earth-Moon system, the simplest way of picturing the situation is to have the Moon revolve about the Earth; but if you were to draw a picture of the orbits of the Earth and Moon about the Sun exactly to scale, you would see that the Moon’s orbit is everywhere concave toward the Sun. It is always “falling” toward the Sun.

All the other satellites, without exception, “fall” away from the Sun through part of their orbits, caught as they are by the superior pull of their primary [planet] but not the Moon.