The Solar System: Course Preview

Pondering Pluto

This discussion was selected from Week 5 of the AMNH online course The Solar System, part of Seminars on Science, a program of online graduate-level professional development courses for K-12 educators. This is an excerpt from an actual course discussion, but learner names have been changed. Explore more sample resources...

In what ways does Pluto resemble a terrestrial planet? In what ways does it resemble a Jovian planet? In what ways does it resemble neither? Do you think that Pluto should be considered a planet? Support your answer with data. From what you have read about how science works, discuss how scientists can come to different conclusions with the same data.

Larry 7 Jul 08 2:41 PM

Demote it

I agree with the demotion of Pluto and the IAU's reasoning. Attempting to keep Pluto a planet goes against the newfound discoveries of astrophysicists. I believe change is difficult for humans, especially those who for so long held these notions which become ingrained in their mind. I believe this resistance to be expected, as previous scientific discoveries (flat vs. round Earth, Geocentric vs. Heliocentric model) were not initially accepted.

According to the IAU (Astronomy Today, p.379 & Essay 5.1) a planet needs to:
1. Orbit the sun
2. Be massive enough to have gravity create a spherical shape
3. "Clear the neighborhood" of other bodies.

I do believe the third prerequisite is valid and vital towards the determination of planets. My rational lies in the evolution of a solar system from a nebula. Over billions of years ago there was a time when the Earth would not fall under the current designation of planet. There was however enough material for gravity to pull these smaller pieces together to create a single planet in this zone. The may be the case for the asteroid belt and the Kuiper belt as they may one day accrete into on single object. However, this is highly unlikely as the total mass of these areas combined is less than that of Earth. (Astronomy Today, p.378)

I also believe that there should be a designation for objects that have enough mass and gravity to create a spherical shape. To me this level of size indicates a significant difference from large asteroids.

The only grey area I still wonder about is objects with a highly eccentric orbit, such as Sedna. Objects such as these are not located in zones that need "clearing." Should these objects be considered planets, dwarf planets, or extremely large comets?

Mike 7 Jul 3:43 PM

Larry, I really like your introduction of Sedna to this conversation. You're right, what if the neighborhood doesn't need clearing? It seems to fulfill the current requirements to be a planet. However, it doesn't seem to pass the "sniff" test, but that isn't the stuff science is made of.

Jeff 7 Jul 5:30 PM

I think we should avoid using terms like "demote" when describing Pluto's classification. Words and phrases like "demote", "downgraded", or "being plutoed" tends to support those who argue that Pluto should remain a planet based on history and tradition, and not on science. Pluto has not changed - science has gained additional information about the Solar System that may in turn lead to better ways of classifying the objects within it.

Instructor Zohar 8 Jul 08 5:11 AM MST

So much is still unknown with our Kuiper belt! Another issue we must deal with is the mere fact that no direct observations of an Oort cloud object in-situ have ever been made!

As for "demoting" the status of Pluto... Jeff brings up some great points as well on this topic.

Why does public perception and expectations of Planets affect our view of what Planets really are all about?

Jeff 8 Jul 4:35 PM

Public perception and our expectations of planets has become newsworthy of late, in light of Pluto's "demotion", but it is not necessarily new. Kepler, for example, expected the planets to reveal God's geometric plan for the Universe, and tried to describe their orbits as spheres that encased five Platonic solids ("the music of the spheres"). The model he created actually fit quite well with the observational data of the time, but eventually he had to discard it. My guess is that Kepler, at that time, would not have minded if you called him an astronomer or an astrologer. Imagine how hard it would have been to get school kids to think differently about the Platonic Solid Model of the Solar System if it had become a cute mnemonic saying ... and then a new planet were to be discovered.

At the time of Pluto's "demotion" there were surveys galore, and the public seem to overwhelmingly vote for Pluto to be restored to full "planetdom", basely primarily on tradition, history, and, I guess, just rooting for the underdog. (I'm sure that Neil Tyson received a bunch of angry emails). I was once taught that George Washington chopped down a cherry tree and never told a lie, but I don't (necessarily) believe it to be true today. Press Secretaries and PR Directors should be hired by Presidents to paint pictures of themselves that meet the public's expectations, but planets should not. Generally, there was a good deal of humor when it came to pubic perception and expectations involving the "Pluto issue". My guess is that there was not a drop in funding for astronomers doing research involving KBOs, TNOs, and the Kuiper Belt. However, with issues like global warming and evolution, science may be affected greatly by the public's expectations and perceptions.

Sheryl 9 Jul 08 10:34 PM MST

I believe that we have to be an open-minded community. We have to understand that as in anything, nothing is set in stone. We have to be able to break away from our paradigms in order to grow and further our learning.

I like how Thomas Kuhn, a historian of science, explained the meaning of the word paradigm as "the set of practices that define a scientific discipline during a particular period of time." ( I believe that it does deal with time. For over time, we learn so much more and have to be willing to change our ideas and beliefs. Even in the case of Pluto. "One important aspect of Kuhn's paradigms is that the paradigms are incommensurable, which means that two paradigms can not be compared to each other. A new paradigm which replaces an old paradigm is not necessarily better, because the criteria of judgment depend on the paradigm". ( Some in the general public may be "stuck" in their old paradigms due to it just being comfortable. Pluto being a planet fits into their lives as they remember learning it in school, and some do not want to leave that comfort zone because this would in turn change the way that they have perceived something their entire lives. Kuhn also describes this way of thinking as a mindset.

Alan 10 Jul 7:55 AM

The whole Pluto controversy is very interesting from a nature of science (NOS) point-of-view. The point being that scientists could have all the data ever desired, and still they would come up with different interpretations and conclusions based on their own personal background and experiences. Scientists are not without bias, after all they are only human. A scientist could have his or her entire career invested in a certain line of reasoning and when new information comes along they aren't always able to look at the new evidence with an objective mind. Davis (2000) states that the passion of a scientist in pursuit of an exciting idea places a strain on his objectivity. He may fail to consider alternative hypotheses, he may unconsciously avoid designing experiments that might threaten the favored hypothesis, he may clutch at straws in the face of decisive evidence, or he may even distort the very process of observation.

In my opinion, this is the true nature of science that many inside and outside the area of science don't always understand or want to admit exists. This is why some of the most exciting, and revolutionary discoveries in science have come to people early in their careers or when they are new to a particular field of study. McComas (2004) states that the prior insights that some scientists bring to the process of investigation explain why some individuals make monumental breakthroughs while others do not. It wouldn't surprise me if part of the reason Pluto was named a planet in 1930 was because scientists "wanted" a new planet to be in that particular location of the solar system; therefore, that may have clouded their judgement or ability to look at all the available data, lack of data, and/or quality of evidence.

Davis, B.D. 2000. The scientist's world. Microbiol. Molecul. Biol. Rev. 64:1.
McComas, W.F. 2004. Keys to teaching the nature of science. Science Teacher. 71:24.

Sebastien 11 Jul 11:25 PM

Planet X

Alan is touching a critical point about Pluto's discovery and initial classification as a planet. Indeed when Pluto was discovered, some astronomers were really expecting that a planet would be discovered over there! This has certainly contributed to Pluto being hailed as the 10th planet.

The story is that shortly after Neptune was discovered (1846), initial calculations suggested that its orbit was irregular. Some postulated that a planet the size of Earth was causing a perturbation in Neptune's orbit. At the time, the hypothetical 10th planet was called "Planet X" (from the roman numeral X=10).

This was actually quite controversial. But the very colorful and wealthy amateur astronomer Percival Lowell (better known for his ideas about life on Mars -- the infamous "canals") set out on a personal quest to find that mysterious Planet X, devoting considerable resources to the program.

Lowell died in 1916, but left a large endowment to continue the search for Planet X. Clyde Tombaugh was hired by the "Lowell Observatory" in 1929, and hit pay dirt in 1930. He actually discovered Pluto very close to where some astronomers had predicted that "Planet X" should be found. The rest is history.

As in turns out, there really were no deviations in Neptune's orbit. The supposed "perturbations" were only measurement errors! However, it took decades to finally figure out that there was absolutely no need a "Planet X"...

In any case, Pluto was initially thought to be about as massive as Earth. Only when the satellite Charon was discovered could the mass of Pluto be calculated with some precision, and it was then found that it was much less massive than anyone ever thought. Not even massive enough to have been the hypothetical Planet X.

Talk about shattered expectations!

Cheryl 14 Jul 8:08 PM

Since Pluto is a large member of the Kuiper belt then I do not think that it should be categorized as a planet. Yes, It does have "roundness" therefore exhibiting a balance between the pushing of pressure and the pulling of gravity, but it does not quite fit the entire description of "today's planet". Its roundness does prevent it from being considered a small solar system body, but it still lacks at least one aspect. "A planet must also be the most massive object in its orbital zone". (The Pluto Controversy: What's a Planet, Anyway?- Neil Tyson) Pluto is not alone. It is among many other bodies. This causes Pluto to be categorized with the term dwarf planet. I agree with this decision. I can see how it is quite different than the other planets, even though "traditional planets don't fit into one neat category" either (The Pluto Controversy: What's a Planet, Anyway?). The following description of a dwarf planet sums it up nicely. "A dwarf planet may also orbit in a zone that has many other objects in it. For example, an orbit within the asteroid belt is in a zone with lots of other objects." (Questions and Answers on Planets-

Alissa 10 Jul 11:43 AM

This discussion is generating many interesting responses. There is something about Pluto that makes people very passionate- is it tradition, a Disney character named after it, the wonderful name or maybe all of the above! I know that the changing of Pluto's place in the Solar System hierarchy has made many people very upset. It is up to us - who know something about how science works - to explain it to others, either the public or our students. As a part of our science test, students are to be familiar with the nature of science. This whole controversy is a way to show students how science works.

As far as I am concerned, Pluto is neither like a terrestrial or a Jovian planet since it is a very large comet in the Kuiper belt. It is icy, fairly small, and it crosses Neptune's orbit. Its orbit is far more elliptical than the planets. So there is no comparison- it is not a planet at all.

Keith 10 Jul 2:32 PM

Nature of Science

Through my courses in getting my M.Ed. I have learned the value in showing students the nature of science and giving them an idea of how theories and ideas can change over time. Shelley brings up the great point that this is a wonderful topic to explore not only because people have strong feelings about it (I know nearly all of my students from student teaching were very much aware of Pluto's change in classification), but because it demonstrates that science is fluid and up for scrutiny and revision. It is good for students to see examples of people looking back at past ideas, challenging them, and forming new ideas out of that process. I've noticed that many students can't stand being told that science shouldn't be questioned and that it is always right(I know some teachers do this!); they like knowing that part of the nature of science is the questioning, the skepticism, and the revision.

Sebastien 11 Jul 11:47 PM

Science is fluid

Well said. And thanks so much for teaching your students about what science is really about!

I must add that, having evolved in the world of professional scientific research for many years now, I can't understand why scientists are so often depicted as axiomatic and inflexible. It is all the contrary! It is part of our training to constantly challenge old and new ideas, and put them all to the test. I see my fellow astronomers at their happiest when something new and unexpected is revealed. This whole reclassification of Pluto (which by the way had been brewing for over 15 years) has had everyone so excited.

Chery 14 Jul 8:17 PM

I feel the same way. If anyone is flexible, it is a scientist! I think in order to be a scientist, flexibility has to be a given trait. It is ingrained into them. They are constantly looking for new ideas and either building on or changing the old. I can't wait until the new information about Pluto allows astronomers to spark new ideas again. It is so interesting to see how only minute pieces of information can change a large amount of ideas.