Category Archives: planetary science

planetary science, planets

Planets: A New Way to Define them

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What makes a world a planet?
What makes a world a planet? Does the definition start at birth? What processes are involved? Planetary scientists are proposing a geophysical definition to replace earlier definitions.

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.

Earth science, life, planetary science

Life, Flammable Ice, and Exoplanets

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“It certainly changes how I think about things. Providing they have ice and a little heat, all those frigid cold planets at the edge of every planetary system could host tiny microhabitats with microbes building their own ‘death stars’ and making their own tiny little atmospheres and ecosystems, just as we discovered here.“

Is this a quote from a futuristic sci-fi character about a mad-scientist-type discovery? Not quite. It’s about a discovery made about something called “flammable ice”, and it raises questions about the search for life elsewhere in the solar system.

A ice/gas hydrate found off the coast of Oregon. In the Sea of Japan, microbes appear to have subsisted inside such a hydrate. Courtesy Wusel007, CC-by-SA 3.0

Deep underwater in the sea of Japan, microbes live happily munching salt water and oil. And, probably other things. The oil comes from deep wells, and probably some natural seeps. The microbes somehow get caught up in the flammable ice, where they create little bubbles of space to live in. As they digest (or break down) their fuel, they leave behind residues. So, far from being a weird alien civilization on another world, they’ve carved out their own living spaces right here on Earth. Granted, they live in the deep, chilly ocean, inside ice crystals, but they’re life.

That is why the quote above is so apropos. It’s from Dr. Glen T. Snyder, a scientist who found the residues. He engaged a team of other researchers to work on figuring out what was going on. His point is that if those microbes do it here on Earth, they do it on other worlds. That’s because the ices he was studying exist on other objects orbiting the Sun.

What’s Flammable Ice?

A burning hydrate, courtesy of Office of Naval Research.

To understand why this is so interesting in the search for life elsewhere, it helps to know a bit about flammable ice. It’s the non-technical term for “methane clathrate”. That’s a lattice of ice crystals surrounding trapped pockets of methane gas. It exists in the cold regions of the outer solar system, where water ice is fairly common. I’ve written here before about methane at other worlds and on comets. It’s possible to burn the methane even when it’s encased in an icy lattice.


Finding Flammable Ice

Methane gas and in clathrates on other worlds sounds alien, but they’re not. They can be fairly common on comets, ice moons, and even the giant planets. On Earth, they’re usually in areas where permafrost exists or in ocean sediments. They’re often in shallow deposits on Earth’s surface, which includes the seafloor. Some freshwater lakes also have methane clathrate layers, and in some places may also lay atop gaseous methane deposits. Methane is familiar to many of us who use natural gas for heating, and it’s a product of gas and oil exploration and extraction.

Methane clathrates are also known as “flammable ice” because they can burn under certain conditions. It comprises the world’s largest supply of methane, which is a greenhouse gas complicit in climate change. T here is some fear that if methane clathrates melt, they release the gas into the oceans and atmosphere. The evidence for that is still being studied. In a 2017 study, the United States Geological Survey found that methane often stays in ocean bottom sediments. It dissolves in seawater or is converted to carbon dioxide by microbes. (Of course, the oceans do sequester a lot of CO2, but there is also concern that it’s changing their acidity due to that. So, the release of methane may have unexpected effects. But, that’s the subject of another article, someday.)

Microhabitats In Flammable Ice

So, back to the quote above. The story behind it is pretty interesting. Snyder and other scientists studying samples of this “flammable ice” found in the Sea of Japan first found strange deposits as they were melting some of the material. The deposits contained tiny spheroids with dark cores. After much study, Snyder and his team determined that they were the leftovers from microbes munching on oil and seawater. Without getting too technical, what they found was the evidence for life inside a frozen shell of water ice, nestled inside the methane gas. Essentially, these microbes adapted to the conditions and created habitats inside the ice. In reporting their find, Dr. Snyder made the statement above. The team’s work is opening up a whole new way of understanding how life could survive on the frozen worlds of the outer solar system.

Life on Exoplanets and other Strange Places

Of course, the search for life elsewhere isn’t limited to our own solar system. As astronomers study the many worlds discovered by such missions as TESS and Kepler, and by ground-based observers, the question continues to arise: is there life out there? Not all worlds are like Earth, which we have always considered the epitome of a life-bearing planet. But, since life can exist in some pretty hostile environments, there’s no reason to assume life isn’t out there. Microbes don’t just subsist on eating flammable ice here on Earth. They also exist in some pretty hot environments, too. And, look at the clusters of life forms that swarm around deep-sea volcanic vents! Those hydrothermal vents are chimneys from undersea volcanic activity. They send out jets of superheated, mineral-rich water. And, guess what? There are life forms that think that’s a pretty good diet.

A colony of tubeworms and other sea life clusters around a vent in the Galapagos Rift. Courtesy NOAA.

It turns out that several worlds in our solar system very likely have deep-ocean warm regions, so if life could exist there. And, if similar conditions exist on worlds around other stars, it’s likely to be there, too. Of course, we have NO proof of any life beyond our planet. Yet. But, it’s coming. And, with studies like the one that Dr. Snyder and colleagues did, scientists will have a better handle on what to look for when they do further work searching out life in other places.

If you want to get into the nitty-gritty of the finding, check out Snyder, et al’s paper, Evidence in the Japan Sea of microdolomite mineralization within gas hydrate microbiomes. I first found the http://astrobiology.com/2020/02/methane-hydrate-discovery-offers-clues-to-life-on-other-planets.htmlstory in a blog entry at the Astrobiology Web page.