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black holes merging and creating gravitational waves. — Artist’s concept of the two black holes merging that created the 22-millisecond gravitational wave signal received by the LIGO detectors on September 14, 2015. Courtesy LIGO.

You’ve probably heard of gravitational waves. They get generated by the collisions of really dense objects, such as black holes or neutron stars. Scientists have speculated about gravitational waves ever since Albert Einstein first predicted their existence. But, he also suspected they would be hard to detect because those waves are quite small. Still, scientists like a challenge. So, they set out to build ultra-sensitive equipment to sort out gravitational waves from the “noise” produced by more mundane things, like earthquakes or passing dump trucks. And, six years after the first detection, we know about 90 of these events!

The first grav-wave discovery was announced in 2015. That’s when the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the detection of waves from the collision of two black holes. One was a 29 solar-mass object and the other had 36 solar masses. The collision actually happened 1.3 billion years ago, but the waves generated by the event took that long to reach Earth (and the detectors).

Since 2015, there have been 90 detections of these gravitational waves. And, there are more than just the two LIGO installations waiting to detect them. Three facilities have banded together to form the LIGO-Virgo-KAGRA Collaboration. LIGO is located in the U.S. in Louisiana and Washington State. Virgo is located at the European Gravitational Observatory in Italy, and KAGRA (Kamioka Gravitational wave detector), is located in Japan.

Each provides data to collaborations of scientists from countries around the world, interested in studying gravitational waves. And, they have banded together in both the detection of waves and the analysis of the data provided by their instruments. The results are starting to give scientists some amazing insights about those objects and the universe itself.

Making Gravitational Waves

Recently the LIGO-Virgo-KAGRA consortium released the next edition of their gravitational wave catalog. I find it really fascinating that we’ve got enough data for not just one catalog, but three! And, the waves continue to roll in. Which tells us it’s a busy universe out there, collision-wise.

This latest edition categorizes the objects involved in collisions that produce “ripples in space”. Could the 90 known events all be black holes colliding? Or, are they neutron stars? What do the collisions tell us about the masses of the objects that did the colliding? The answers to those and other questions are all in the data.

For example, the collaboration released data on 35 new events detected since the last catalog update. A huge number of them—32—were probably the collisions of black holes. To refresh our collective memory: a black hole is a region of spacetime where gravity is incredibly strong. It’s so strong, in fact, that nothing — including light—can escape. The high gravity stems from the mass of material inside the black hole’s event horizon (its outer boundary). In short: these are incredibly massive places with strong magnetic fields.

The black holes in the latest batch of grav-wave discoveries weren’t limited to just one size or mass range. Some were about 90 times the mass of the Sun. Others were more than a hundred solar masses. That’s a lot of mass, and when you slam two of them together, it generates gravitational waves.

Neutron Stars Get into the Act, Too

A couple of collisions in the latest detections were between pairs of neutron stars. These are odd astrophysical objects made of tightly packed neutrons, with incredibly strong magnetic fields. As we used to say in the planetarium lectures, a tablespoon of neutron star “stuff” would weigh nearly a trillion kilograms.

So, try to imagine a sphere of neutrons about the size of New York or London. THAT would be massive. Now, imagine two of them colliding. Or, even better yet, a neutron star and a black hole. A lot of energy—and gravitational waves—get released. The consortium suspected that was at the root of at least one collision.

Gravitational Wave Implications

Aside from the fact that these collisions shove gravitational waves out into the universe, what else do they tell us? A lot, actually.

First, they tell us there’s a population of objects out there that can and do collide. Massive black holes and neutron stars exist throughout galaxies. They’re both the dead ends of massive stars that ended their lives in supernova explosions.

Also, there’s information to be gained about the masses of “pre-merger” objects. Those would be the massive stars that ultimately became the black holes or neutron stars that eventually collided. In addition, measurements of the gravitational waves also help scientists figure out how far away the event took place.

Data from many such distant events can also help scientists as they model the history of the universe itself. In addition, the detections of very distant mergers may give some insight into the expansion of the universe. Some scientists suggest merger data might shine some light on the nature of dark energy. That’s an unknown “thing” that influences the rate of expansion.

Those are just a few implications of the study of gravitational waves. Our takeaway here is that there are now 90 known mergers. Six years ago, there was only one. So, the universe is giving yet more clues about itself. This time, it’s in the form of dead and dying stars. Eventually, they will crash together and send out massive amounts of data hidden inside gravitational ripples.

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.

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