New Evidence that Pluto should still be classed as a Planet

University of Central Florida scientist Philip Metzger, who is with the university’s Florida Space Institute, makes the case that Pluto should be reinstated as a planet.

Metzger, who is lead author on the study, reviewed scientific literature from the past 200 years and found only one publication – from 1802 – that used the clearing-orbit requirement to classify planets, and it was based on since-disproven reasoning.

He said moons such as Saturn’s Titan and Jupiter’s Europa have been routinely called planets by planetary scientists since the time of Galileo.

“It’s a sloppy definition,” Metzger says of the IAU’s definition. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

The planetary scientist says that the literature review showed that the real division between planets and other celestial bodies, such as asteroids, occurred in the early 1950s when Gerard Kuiper published a paper that made the distinction based on how they were formed.

However, even this reason is no longer considered a factor that determines if a celestial body is a planet, Metzger says.

Study co-author Kirby Runyon, with Johns Hopkins University Applied Physics Laboratory in Laurel, Maryland, says the IAU’s definition was erroneous since the literature review showed that clearing orbit is not a standard that is used for distinguishing asteroids from planets, as the IAU claimed when crafting the 2006 definition of planets.

Icarus – The Reclassification of Asteroids from Planets to Non-Planets

Highlights

• The literature on asteroid classification is reviewed from 1801 to the present.
• Scientists considered asteroids to be planets until the 1950s.
• Thousands of asteroids sharing orbits was not debarring from planet status.
• They were reclassified as non-planets on the basis of geophysical characteristics.
• Taxonomical terms like planet are determined by the scientific process, not voting.

Abstract

It is often claimed that asteroids’ sharing of orbits is the reason they were re-classified from planets to non-planets. A critical review of the literature from the 19th Century to the present shows this is factually incorrect. The literature shows the term asteroid was broadly recognized as a subset of planet for 150 years. On-going discovery of asteroids resulted in a de facto stretching of the concept of planet to include the ever-smaller bodies. Scientists found utility in this taxonomic identification as it provided categories needed to argue for the leading hypothesis of planet formation, Laplace’s nebular hypothesis. In the 1950s, developments in planet formation theory found it no longer useful to maintain taxonomic identification between asteroids and planets, Ceres being the primary exception. At approximately the same time, there was a flood of publications on the geophysical nature of asteroids showing them to be geophysically different than the large planets. This is when the terminology in asteroid publications calling them planets abruptly plunged from a high level of usage where it had hovered during the period 1801 – 1957 to a low level that held constant thereafter. This marks the point where the community effectively formed consensus that asteroids should be taxonomically distinct from planets. The evidence demonstrates this consensus formed on the basis of geophysical differences between asteroids and planets, not the sharing of orbits. We suggest attempts to build consensus around planetary taxonomy not rely on the non-scientific process of voting, but rather through precedent set in scientific literature and discourse, by which perspectives evolve with additional observations and information, just as they did in the case of asteroids.

Defining “Planet”

Metzger says that the definition of a planet should be based on its intrinsic properties, rather than ones that can change, such as the dynamics of a planet’s orbit.

“Dynamics are not constant, they are constantly changing,” Metzger says. “So, they are not the fundamental description of a body, they are just the occupation of a body at a current era.”

Instead, Metzger recommends classifying a planet based on if it is large enough that its gravity allows it to become spherical in shape.

“And that’s not just an arbitrary definition,” Metzger says. “It turns out this is an important milestone in the evolution of a planetary body, because apparently when it happens, it initiates active geology in the body.”

Pluto, for instance, has an underground ocean, a multilayer atmosphere, organic compounds, evidence of ancient lakes and multiple moons, he says.

“It’s more dynamic and alive than Mars,” Metzger says. “The only planet that has more complex geology is the Earth.”

50 thoughts on “New Evidence that Pluto should still be classed as a Planet”

  1. I would personally define “planet” as any body sufficiently large that gravity dictates its shape, which is not a star, and is not in orbit around another planet with their barycenter within the other body. (f the barycenter is inside the other body, you get demoted to “moon”. I see no reason why a few asteroids sharing Earth’s orbit would be a reason to deny Earth planetary status. But it’s all largely arbitrary taximony; Again, “planet” is not a natural category.

    Reply
  2. I would personally define planet”” as any body sufficiently large that gravity dictates its shape”” which is not a star and is not in orbit around another planet with their barycenter within the other body. (f the barycenter is inside the other body”” you get demoted to “”””moon””””.I see no reason why a few asteroids sharing Earth’s orbit would be a reason to deny Earth planetary status.But it’s all largely arbitrary taximony; Again”””” “”””planet”””” is not a natural category.”””

    Reply
  3. I would personally define “planet” as any body sufficiently large that gravity dictates its shape, which is not a star, and is not in orbit around another planet with their barycenter within the other body. (f the barycenter is inside the other body, you get demoted to “moon”. I see no reason why a few asteroids sharing Earth’s orbit would be a reason to deny Earth planetary status. But it’s all largely arbitrary taximony; Again, “planet” is not a natural category.

    Reply
  4. I would personally define planet”” as any body sufficiently large that gravity dictates its shape”” which is not a star and is not in orbit around another planet with their barycenter within the other body. (f the barycenter is inside the other body”” you get demoted to “”””moon””””.I see no reason why a few asteroids sharing Earth’s orbit would be a reason to deny Earth planetary status.But it’s all largely arbitrary taximony; Again”””” “”””planet”””” is not a natural category.”””

    Reply
  5. I would personally define “planet” as any body sufficiently large that gravity dictates its shape, which is not a star, and is not in orbit around another planet with their barycenter within the other body. (f the barycenter is inside the other body, you get demoted to “moon”.

    I see no reason why a few asteroids sharing Earth’s orbit would be a reason to deny Earth planetary status.

    But it’s all largely arbitrary taximony; Again, “planet” is not a natural category.

    Reply
  6. The definition matters in the way in which it shapes our view of the solar system and the impact it has on our future endeavors. Demoting Pluto to a minor planet resulted in a shrinking of the solar system in the mind of the public. It is seen as less worthy of our attention. Alternatively, if the IAU had established a more expansive definition, the narrative would be that scientists have discovered new planets. Schoolchildren would learn their names and study them. Requests for funding for missions to explore them would be more politically popular. The IAU decision was viewed as largely capricious by the general public and has diminished their opinion of science and space exploration. Frankly I struggle to think of any positive result of the decision. If you look to our future as a space fairing species, these bodies will play an important role. Our definitions should reflect that.

    Reply
  7. The definition matters in the way in which it shapes our view of the solar system and the impact it has on our future endeavors. Demoting Pluto to a minor planet resulted in a shrinking of the solar system in the mind of the public. It is seen as less worthy of our attention. Alternatively if the IAU had established a more expansive definition the narrative would be that scientists have discovered new planets. Schoolchildren would learn their names and study them. Requests for funding for missions to explore them would be more politically popular.The IAU decision was viewed as largely capricious by the general public and has diminished their opinion of science and space exploration.Frankly I struggle to think of any positive result of the decision. If you look to our future as a space fairing species these bodies will play an important role. Our definitions should reflect that.

    Reply
  8. The definition matters in the way in which it shapes our view of the solar system and the impact it has on our future endeavors. Demoting Pluto to a minor planet resulted in a shrinking of the solar system in the mind of the public. It is seen as less worthy of our attention. Alternatively, if the IAU had established a more expansive definition, the narrative would be that scientists have discovered new planets. Schoolchildren would learn their names and study them. Requests for funding for missions to explore them would be more politically popular. The IAU decision was viewed as largely capricious by the general public and has diminished their opinion of science and space exploration. Frankly I struggle to think of any positive result of the decision. If you look to our future as a space fairing species, these bodies will play an important role. Our definitions should reflect that.

    Reply
  9. The definition matters in the way in which it shapes our view of the solar system and the impact it has on our future endeavors. Demoting Pluto to a minor planet resulted in a shrinking of the solar system in the mind of the public. It is seen as less worthy of our attention. Alternatively if the IAU had established a more expansive definition the narrative would be that scientists have discovered new planets. Schoolchildren would learn their names and study them. Requests for funding for missions to explore them would be more politically popular.The IAU decision was viewed as largely capricious by the general public and has diminished their opinion of science and space exploration.Frankly I struggle to think of any positive result of the decision. If you look to our future as a space fairing species these bodies will play an important role. Our definitions should reflect that.

    Reply
  10. Looks like someone is unhappy with the removal of pluto from the planet list and wants to find any reason to go against it. I’ve heard of Pluto’s controversial planet status over a decade before it was removed, and I totally agree with it, though to be honest, it’s an inconsequential matter of classification.

    Reply
  11. Looks like someone is unhappy with the removal of pluto from the planet list and wants to find any reason to go against it.I’ve heard of Pluto’s controversial planet status over a decade before it was removed and I totally agree with it though to be honest it’s an inconsequential matter of classification.

    Reply
  12. It’s just politics, “planet” is not a natural category, like “atom”. They removed Pluto from the list of planets because they could, and they wanted to shock people, and be contrarian. Not because Science demanded it. Might as well argue about whether a particular hunk of rock is a “stone”, a “pebble”, or a “boulder”. It’s just words, Pluto doesn’t change.

    Reply
  13. It’s just politics planet”” is not a natural category”””” like “”””atom””””. They removed Pluto from the list of planets because they could”” and they wanted to shock people”” and be contrarian. Not because Science demanded it.Might as well argue about whether a particular hunk of rock is a “”””stone”””””””” a “”””pebble”””””””” or a “”””boulder””””. It’s just words”””” Pluto doesn’t change.”””

    Reply
  14. I would define a planet as any celestial body massive enough that no terrestrial creature can launch itself into orbit with its fastest running jump, all other factors being equal.

    Reply
  15. I would define a planet as any celestial body massive enough that no terrestrial creature can launch itself into orbit with its fastest running jump all other factors being equal.

    Reply
  16. The definition matters in the way in which it shapes our view of the solar system and the impact it has on our future endeavors. Demoting Pluto to a minor planet resulted in a shrinking of the solar system in the mind of the public. It is seen as less worthy of our attention. Alternatively, if the IAU had established a more expansive definition, the narrative would be that scientists have discovered new planets. Schoolchildren would learn their names and study them. Requests for funding for missions to explore them would be more politically popular.

    The IAU decision was viewed as largely capricious by the general public and has diminished their opinion of science and space exploration.

    Frankly I struggle to think of any positive result of the decision.

    If you look to our future as a space fairing species, these bodies will play an important role. Our definitions should reflect that.

    Reply
  17. When Pluto was discovered in 1930, it was thought to be larger than Mercury. That turned out not to be the case, in fact it is smaller than our moon. Eris has more mass than Pluto, and there are many trans-neptunian objects of similar size as Pluto. If Pluto stays, so do likely a dozen new ones. In any case, the term planet is purely arbitrary and doesn’t matter. Let it die.

    Reply
  18. When Pluto was discovered in 1930 it was thought to be larger than Mercury. That turned out not to be the case in fact it is smaller than our moon. Eris has more mass than Pluto and there are many trans-neptunian objects of similar size as Pluto. If Pluto stays so do likely a dozen new ones.In any case the term planet is purely arbitrary and doesn’t matter. Let it die.

    Reply
  19. Some people consider the Earth-Moon as a binary planet because the masses are not as dramatically different as all the other planet-moon scenario in our solar system. Pluto-Charon is also binary, and the mass should be considered collectively. Location, particularly what something orbits, I consider to be fundamental to the issue. I think you need a minimum mass, even though that seems arbitrary. I think there should be strong indicators that an object’s formation retained heat. We see this by the shape and composition primarily, but there are other indications such as present heat, layering, mass, and surface geology. Most asteroids would be excluded because they were part of something that shattered (not necessarily the same something), given their appearance, rather than coming together. Ceres almost certainly should be considered a planet. But I don’t think any of the other asteroids is sufficiently large to be called a planet. It is almost 1000 km in diameter. The next largest is Vesta which is only about half that diameter. Pluto, Eris, Ceres, and Makemake are near or exceed this rough 1000km size limit, and should, in my opinion, be deemed planets, if it turns out they exhibit the necessary attributes. If they never melted during formation, I am hesitant to include them with the planets proper. And that includes Pluto. We need to be sure that it melted during formation and formed layers before we add it back to the list. I think there is a very high probability that it did. I think part of the fear is that there may be dozens, hundreds or even thousands of Pluto sized balls of Ice out there in our solar system way out there. But, until we find them, and they meet all the criteria I have given, I would not be too bothered.

    Reply
  20. Some people consider the Earth-Moon as a binary planet because the masses are not as dramatically different as all the other planet-moon scenario in our solar system. Pluto-Charon is also binary and the mass should be considered collectively.Location particularly what something orbits I consider to be fundamental to the issue.I think you need a minimum mass even though that seems arbitrary. I think there should be strong indicators that an object’s formation retained heat. We see this by the shape and composition primarily but there are other indications such as present heat layering mass and surface geology. Most asteroids would be excluded because they were part of something that shattered (not necessarily the same something) given their appearance rather than coming together.Ceres almost certainly should be considered a planet. But I don’t think any of the other asteroids is sufficiently large to be called a planet. It is almost 1000 km in diameter. The next largest is Vesta which is only about half that diameter. Pluto Eris Ceres and Makemake are near or exceed this rough 1000km size limit and should in my opinion be deemed planets if it turns out they exhibit the necessary attributes. If they never melted during formation I am hesitant to include them with the planets proper. And that includes Pluto. We need to be sure that it melted during formation and formed layers before we add it back to the list. I think there is a very high probability that it did.I think part of the fear is that there may be dozens hundreds or even thousands of Pluto sized balls of Ice out there in our solar system way out there. But until we find them and they meet all the criteria I have given I would not be too bothered.

    Reply
  21. Imbeciles. ” Yes you are. Ceres is too small and seems to have no satellites, the others you name are far smaller still. If an arbitrary line is to be drawn, and it must then Pluto is on the Planet side of it. voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae

    Reply
  22. Imbeciles. “”Yes you are. Ceres is too small and seems to have no satellites”” the others you name are far smaller still. If an arbitrary line is to be drawn and it must then Pluto is on the Planet side of it.voluptatem accusantium doloremque laudantium totam rem aperiam”” eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae”””

    Reply
  23. Can we move on with the Pluto argument, please? I don’t see this Metzger fellow defending the planethood of Haumea, Makemake, or Ceres. Why just Pluto? Why not the dozens of other dwarf planets we know in the Solar System? Why not all of the spherical satellites? The Moon should be a planet! It’s unfair to classify it based on where it happens to be at the moment! Imbeciles.

    Reply
  24. Can we move on with the Pluto argument please? I don’t see this Metzger fellow defending the planethood of Haumea Makemake or Ceres. Why just Pluto? Why not the dozens of other dwarf planets we know in the Solar System? Why not all of the spherical satellites? The Moon should be a planet! It’s unfair to classify it based on where it happens to be at the moment!Imbeciles.

    Reply
  25. Looks like someone is unhappy with the removal of pluto from the planet list and wants to find any reason to go against it.

    I’ve heard of Pluto’s controversial planet status over a decade before it was removed, and I totally agree with it, though to be honest, it’s an inconsequential matter of classification.

    Reply
  26. It’s just politics, “planet” is not a natural category, like “atom”. They removed Pluto from the list of planets because they could, and they wanted to shock people, and be contrarian. Not because Science demanded it.

    Might as well argue about whether a particular hunk of rock is a “stone”, a “pebble”, or a “boulder”. It’s just words, Pluto doesn’t change.

    Reply
  27. I would define a planet as any celestial body massive enough that no terrestrial creature can launch itself into orbit with its fastest running jump, all other factors being equal.

    Reply
  28. When Pluto was discovered in 1930, it was thought to be larger than Mercury. That turned out not to be the case, in fact it is smaller than our moon. Eris has more mass than Pluto, and there are many trans-neptunian objects of similar size as Pluto. If Pluto stays, so do likely a dozen new ones.

    In any case, the term planet is purely arbitrary and doesn’t matter. Let it die.

    Reply
  29. Some people consider the Earth-Moon as a binary planet because the masses are not as dramatically different as all the other planet-moon scenario in our solar system. Pluto-Charon is also binary, and the mass should be considered collectively.

    Location, particularly what something orbits, I consider to be fundamental to the issue.

    I think you need a minimum mass, even though that seems arbitrary. I think there should be strong indicators that an object’s formation retained heat. We see this by the shape and composition primarily, but there are other indications such as present heat, layering, mass, and surface geology. Most asteroids would be excluded because they were part of something that shattered (not necessarily the same something), given their appearance, rather than coming together.

    Ceres almost certainly should be considered a planet. But I don’t think any of the other asteroids is sufficiently large to be called a planet. It is almost 1000 km in diameter. The next largest is Vesta which is only about half that diameter. Pluto, Eris, Ceres, and Makemake are near or exceed this rough 1000km size limit, and should, in my opinion, be deemed planets, if it turns out they exhibit the necessary attributes. If they never melted during formation, I am hesitant to include them with the planets proper. And that includes Pluto. We need to be sure that it melted during formation and formed layers before we add it back to the list. I think there is a very high probability that it did.

    I think part of the fear is that there may be dozens, hundreds or even thousands of Pluto sized balls of Ice out there in our solar system way out there. But, until we find them, and they meet all the criteria I have given, I would not be too bothered.

    Reply
  30. ” Imbeciles. ”

    Yes you are. Ceres is too small and seems to have no satellites, the others you name are far smaller still. If an arbitrary line is to be drawn, and it must then Pluto is on the Planet side of it.
    voluptatem accusantium doloremque laudantium, totam rem aperiam, eaque ipsa quae ab illo inventore veritatis et quasi architecto beatae vitae

    Reply
  31. Can we move on with the Pluto argument, please? I don’t see this Metzger fellow defending the planethood of Haumea, Makemake, or Ceres. Why just Pluto? Why not the dozens of other dwarf planets we know in the Solar System? Why not all of the spherical satellites? The Moon should be a planet! It’s unfair to classify it based on where it happens to be at the moment!

    Imbeciles.

    Reply

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