Do You Think Pluto Should Be a Planet?

Pluto was the ninth planet of the solar system until 2006, when the International Astronomical Union knocked it down to “dwarf planet” status because a similarly-sized body, Eris, had been discovered orbiting the sun.

Do you think Pluto should still be considered a planet? Should Eris be one, too? What if we find another big object in the distant solar system?

And what is a planet, anyway? Should planets be defined by their size? Their roundness? What they orbit?

Is it possible that we do not yet know enough about the solar system to define a planet at all?

In “Is Pluto a Planet?,” Kenneth Chang digs into the lively debate between astrophysicists and planetary scientists:

Who doesn’t love Pluto? It shares a name with the Roman god of the underworld and a Disney dog.

But is it a planet?

This debate about the icy world that used to be the ninth planet of the solar system is as much linguistics as science, and it is not a question with simple answers.

But it is a fascinating story about an ancient word with shifting meanings, humanity’s evolving views of its place in the cosmos and the process of scientific discovery, which like all human endeavors, can be messy and contentious.

In the interactive article, the author continues, explaining each side of the issue:

Using Pluto as an arbitrary cutoff for planethood made sense only for the sake of sentimentality, and that did not appeal to astronomers on either side of this issue.

But there really are only two scientifically reasonable possibilities: the current definition that embodies the idea that planets are the big things that orbit the sun, and the other idea that planets should be anything that is big enough to be round and thus to undergo interesting geologic processes.

Perhaps the best answer is that it depends.

There are two opposing views, and in contrast to evolution and climate change, the different scientific views are both reasonable and scientifically valid.

For astrophysicists, planets are like the detritus left over from the formation of much, much larger structures, like the clumping of dark matter and the collapsing of gas clouds into galaxies.

In our solar system, for example, the sun accounts for more than 99.8 percent of the mass.

For the astrophysicists, planets are essentially the solar system’s equivalent of continents: the few big pieces, decided upon somewhat arbitrarily by historical convention.

Why are Europe and Asia separate continents when they are solidly connected? Why is Australia a continent and not just a big island? How much larger would Greenland need to be to be called a continent? Yes, it turns out that Australia sits on its own tectonic plate, and Greenland is part of the North American plate, but that only became clear in the 1960s, centuries after people started using the word “continent.” Geologists, at least, do not feel the need to vote on a definition of “continent.”

Hal Levison, an astrophysicist at the Southwest Research Institute in Boulder, Colo. points out a problem with using roundness as the defining quality of a planet: How round is round enough?

No matter how one devises a definition of roundness, “there will be two objects, almost identical to one another, one that will be slightly just epsilon — bigger than the other,” Dr. Levison said. “And one of them will be a planet, and the other one won’t.”

Planetary scientists, not surprisingly, have a decidedly planet-centric view of the universe. All of the really interesting stuff, they say, happens on planets.

“Planets are engines of complexity,” said Phil Metzger, a planetary scientist at the University of Central Florida and the lead author of a newly published paper in the journal Icarus that argues for a more inclusive definition of “planet.”

“When you get a body of a certain size, then these geological and chemical and even biological processes begin,” he said. “And you have this emerging, this flowering of complexity. And that’s really what the useful definition of a planet has always been.”

That complexity, Dr. Metzger argued, does not depend on the orbit, such as whether the body originally orbited the sun and was then captured as a moon, which is what occurred with Triton, Neptune’s largest moon.

In that sense, they are using the word “planet” in the way “river” is used. There is not an infinite number of rivers on Earth, but rivers are also not a countable quantity. What’s the dividing line between a stream and a river? If a river sometimes dries up, is it still a river? And scientists who study rivers do not worry about particulars like this.

Perhaps the International Astronomical Union should have just left “planet” undefined.

Students, read the entire article, then tell us:

• Do you think Pluto should be considered a planet? Why or why not?

• The author asks: “Should a planet be defined by what it is — some specification of size or composition or shape? Or should it be defined by what it does — that it goes around the sun?” Based on the arguments presented in the article, what do you think?

• The author suggests that we may not yet know enough to define what a planet is or isn’t at all. Do you think having a working definition of a planet is valuable? Why or why not?

• In your lifetime, can you think of a time when the scientific consensus about something has changed? How did people react? How do you think changes in scientific opinion should be communicated to the public?

• Why do you think this topic inspires such passionate debate among both scientists and nonscientists? How much does it matter to you?

Want more writing prompts? You can find all of our questions in our Student Opinion column. Teachers, check out this guide to learn how you can incorporate them into your classroom.

Students 13 and older in the United States and Britain, and 16 and older elsewhere, are invited to comment. All comments are moderated by the Learning Network staff, but please keep in mind that once your comment is accepted, it will be made public.