A few weeks ago, I read a pre-print of a paper that discusses the evolving definition of “planet” and outlines a proposed geophysical planetary definition of the term. That paper was just released today and it’s definitely bringing a new POV of planets to the world. You can read it here. It’s by Philip T. Metzger, et al, and the title is “Moons are Planets: Scientific Usefulness Versus Cultural Teleology in the Taxonomy of Planetary Science.”

“Planet” as Nexus

That’s a mouthful of a title. However, it hints at a major paradigm shift in our understanding of planets: that they are more than the simple definition that describes them as rounded bodies circling stars. Planetary scientists and geophysicists see them in a deeper context. For them, planets lie at the nexus of geological, chemical, biological, and—quite possibly—civilizational complexity in the cosmos.

Think about that: planets are geological powerhouses. They undergo complex chemical processes in order to exist at all. As far as we know, planets are the places where biochemical and biological processes work to form life. And, that life, if it evolves far enough, becomes the basis for civilizations. Each one of those aspects is a story of its own. And planets bring them all together. Not just in our own solar system, but for the millions of other worlds throughout our galaxy, and beyond. This nexus of processes requires a re-look at the taxonomy and language we use to define planets. It requires that we undergo a paradigm shift in our own thinking about them.

What’s a Planet to You?

You live on a planet, and when you look out at the night sky this month, you can see three of them not long after sunset. They’re Venus, Jupiter, and Saturn and they’re gorgeous. But, what are they? Well, you say, they’re planets. Or, if you want to get technical, Venus is a rocky world, while Jupiter and Saturn are gas giants. Does this tell you why they’re planets? Not necessarily.

We all think we know what planets are since we grew up learning about them in school and observing them in the sky. But, do we really know? And, does the current definition of “planet” really describe what these other worlds really are in scientific terms?

Defining “Planet”

Certainly, people have tried to define “planet” over the centuries. Those definitions generally rely on cultural ideas of distant places. The Greeks coined the word planetes to describe these objects they observed that seemed to move against the fixed backdrop of stars. The term means “wanderer”, and that’s an apt observational moniker. But, it doesn’t give us an intrinsic understanding of such an object. For centuries, all we had on the “wanderers” were their orbital motions because those could be tracked. And, thanks to Galileo Galilei, they could be observed more closely. Also thanks to him, we got the first observational sense of planets as other worlds.

Yet, try as he might, Galileo’s revelations about planets didn’t extend beyond the ones he could observe with his small telescope. Today, we know of other worlds in the solar system. What are they? Planets? Asteroids? Dwarf planets? It’s clear our taxonomy needs some work. What we call them semantically doesn’t necessarily explain exactly what a planet is or how we classify it.

Exploring Planets

The invention of the telescope, and later on, the use of space probes, provided a scientific understanding of other worlds in our solar system. Today, we explore Earth, Mars, Jupiter, and the Kuiper Belt. We’ve sent spacecraft to every realm of the solar system except the Oort Cloud (that’ll happen). We’ve seen enough of the solar system to classify it into several realms: inner (Sun, Mercury, Venus, Earth, Mars), middle (Asteroid Belt, Jupiter, Saturn, Uranus, Neptune), Kuiper Belt (Pluto, Arrokoth, etc.), and Oort Cloud. I suspect that, as time goes by, those boundaries may shift or subdivide as we learn more about other worlds and environmental conditions in each section.

Our understanding of the worlds themselves has also changed over time. That’s particularly true for the Kuiper Belt objects. This is, in fact, the way science works. Each new discovery leads to greater understanding. (For a good example, just look at what we’ve learned about planets around other stars in the past several decades. They began as theoretical ideas and now we know of thousands that have been directly observed. There are millions more to be discovered.)

So, with all the advances in our understanding, it surely seems to me that we need a re-examination of what planets are and how we define them, both scientifically and semantically. Not just in our own solar system, but throughout the cosmos. And, that’s what the authors of the paper above are trying to do: bring a scientific sensibility to planetary definitions and taxonomy.

Toward the Planet Paradigm Shift

For centuries, we’ve let myths, legends, social practices, tradition, and pseudo-science shape how we classify planets. The whole planetary definition thing came to a head with Pluto and its supposed “reclassification” based on old, outmoded ideas. The exploration of Pluto and Charon showed us new worlds and a way out of the old definition.

Now, it’s time to apply physics and science to planetary taxonomy. Today, we see the discovery of thousands of exoplanets in our own “near” neighborhood of the galaxy. Should we use our outmoded cultural taxonomies (ways of classifying worlds) to those worlds, too?

Nope. It seems to me that we need a fresh look at planets and how to define them. The Metzger, et al. paper is an opening salvo in a discussion about the scientific way of redefining and understanding just what makes a world a planet. Ultimately, the definition of the term may not be so limited as it is today and tomorrow’s explorers will be visiting worlds in our own solar system (and beyond) that deserve the moniker “planet” as much as Earth and others do today.