Get Ready For The >100 Planet Solar System

Article Updated: 18 Mar , 2017

Pluto’s status as a non-planet may be coming to an end. Professor Mike Brown of Caltech ended Pluto’s planetary status in 2006. But now, Kirby Runyon, a doctoral student at Johns Hopkins University, thinks it’s time to cancel that demotion and restore it as our Solar System’s ninth planet.

Pluto’s rebirth as a planet is not just all about Pluto, though. A newer, more accurate definition of what is and what is not a planet is needed. And if Runyon and the other people on the team he leads are successful, our Solar System would have more than 100 planets, including many bodies we currently call moons. (Sorry elementary school students.)

This composite of enhanced color images of Pluto (lower right) and Charon (upper left), was taken by NASA’s New Horizons spacecraft as it passed through the Pluto system on July 14, 2015. Credits: NASA/JHUAPL/SwRI

In 2006, the International Astronomical Union (IAU) changed the definition of what a planet is. Pluto’s demotion stemmed from discoveries in the 1990’s showing that it is actually a Kuiper Belt Object (KBO). It was just the first KBO that we discovered. When Pluto was discovered by Clyde Tombaugh in 1930, and included as the ninth planet in our Solar System, we didn’t know much about the Kuiper Belt.

But in 2005, the dwarf planet Eris was discovered. It was like Pluto, but 27% more massive. This begged the question, Why Pluto and not Eris? The IAU struck a committee to look into how planets should be defined.

In 2006, the IAU had a decision to make. Either expand the definition of what is and what is not a planet to include Eris and other bodies like Ceres, or shrink the definition to omit Pluto. Pluto was demoted, and that’s the way it’s been for a decade. Just enough time to re-write text books.

But a lot has happened since then. The change to the definition of planet was hotly debated, and for some, the change should never have happened. Since the New Horizons mission arrived at Pluto, that debate has been re-opened.

A group of scientists led by Runyon has written a paper to be presented at the upcoming Lunar and Planetary Science Conference on March 20th to 24th.

“A planet is a sub-stellar mass body that has never undergone nuclear fusion…” – part of the new planetary definition proposed by Runyon and his team.

The group behind the drive to re-instate Pluto have a broader goal in mind. If the issue of whether Pluto is or is not a planet sounds a little pedantic, it’s not. As Runyon’s group says on their poster to be displayed at the upcoming conference, “Nomenclature is important as it affects how we compare, think, and communicate about objects in nature.”

Runyon’s team proposes a new definition of what is a planet, focused on the geophysics of the object: “A planet is a sub-stellar mass body that has never undergone nuclear fusion and that has enough gravitation to be round due to hydrostatic equilibrium regardless of its orbital parameters.”

The poster highlights some key points around their new planetary definition:

  • Emphasizes intrinsic as opposed to extrinsic properties.
  • Can be paraphrased for younger students: “Round objects in space that are smaller than stars.”
  • The geophysical definition is already in use, taught, and included in planetological glossaries.
  • There’s no need to memorize all 110 planets. Teach the Solar Systems zones and why different planet types formed at different distances from the Sun.

Their proposal makes a lot of sense, but there will be people opposed to it. 110 planets is quite a change, and the new definition is a real mouthful.

“They want Pluto to be a planet because they want to be flying to a planet.” – Prof. Mike Brown, from a BBC interview, July 2015.

Mike Brown, the scientist behind Pluto’s demotion, saw this all coming when New Horizons reached the Pluto system in the Summer of 2015. In an interview with the BBC, he said “The people you hear most talking about reinstatement are those involved in the (New Horizons) mission. It is emotionally difficult for them.”

Saying that the team behind New Horizons find Pluto’s status emotionally difficult seems pretty in-scientific. In fact, their proposed new definition seems very scientific.

This image from New Horizons shows the true nature of Pluto. What for a long time was just a blurry, round, blob in space, was revealed as a geologically active planet with a seasonal atmosphere. Image: NASA/JPL/New Horizons

There may be an answer to all of this. The term “classical planets” might be of some use. That term could include our 9 familiar planets, the knowledge of which guided much of our understanding and exploration of the Solar System. But it’s a fact of science that as our understanding of something grows more detailed, our language around it has to evolve to accommodate. Look at the term planetary nebula—still in use long after we know they have nothing to do with planets—and how much confusion it causes.

“It is official without IAU approval, partly via usage.” – Runyon and team, on their new definition.

In the end, it may not matter whether the IAU is convinced by Runyon’s proposed new definition. As their poster states, “As a geophysical definition, this does not fall under the domain of the IAU, and is an alternate and parallel definition that can be used by different scientists. It is “official” without IAU approval, partly via usage.”

It may seem pointless to flip-flop back and forth about Pluto’s status as a planet. But there are sound reasons for updating definitions based on our growing knowledge. We’ll have to wait and see if the IAU agrees with that, and whether or not they adopt this new definition, and the >100 planet Solar System.

You can view Runyon and team’s poster here.
You can view Emily Lakdawalla’s image of round objects in our Solar System here.
You can read the IAU’s definition of a planet here.

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26 Responses

  1. laurele says:

    Your very first sentence is in error. Pluto is not a “non-planet,” and Mike Brown had absolutely nothing to do with the controversial 2006 IAU decision. First, an object’s status does not change because 424 non-experts take a vote. That is what the IAU decision was. Only four percent of the IAU voted, and most were not planetary scientists but other types of astronomers. Their decision was immediately opposed by an equal number of professional planetary scientists led by Alan Stern. The media did a disservice to the public by treating the IAU decision as some sort of gospel truth as opposed to what it really was–just one view in an ongoing debate. They should never have given the IAU this much power, as science is NOT decided by decree of “authority.”

    Furthermore, the media has enabled Mike Brown in his nonsensical and repetitive claim that he “killed” Pluto. Not only is this claim untrue; it is also unprofessional and unscientific. Brown was one of a team of three who discovered Eris, a small planet beyond Pluto. He is not an IAU member and could not vote at the 2006 General Assembly. He had no influence whatsoever on that vote. He does not study Pluto. The “killing Pluto” idea is nothing more than a way of branding himself to sell books, make money, and become famous. Of the other two discoverers of Eris, one Dr. David Rabinowitz, signed the petition rejecting the IAU decision.

    The IAU never had to make a decision. The only reason some people felt they did was because they had a problem with the notion of our solar system having “too many planets.” This has no scientific basis whatsoever. The solar system has whatever number of planets it has. And there is no need to apologize to elementary school students because there is no need for them to memorize a list of names. That is an archaic mode of teaching from the days when we knew little more about the planets than their names. Today, the important thing is teaching the different types of planets and their characteristics. After all, we don’t ask kids to memorize the names of all the elements in the Periodic Table or the names of all the rivers and mountains on Earth.

    The best thing for textbooks and teachers is to teach the controversy. And there is no need for the IAU to give the geophysical definition its stamp of approval. No one voted on whether gravity is real or whether the theory of relativity is true or whether the universe has one or billions of galaxies. Let the data do the talking, not wannabe dictators.

    • Code 80 says:

      Lots of very good points made here.


    • Pvt.Pantzov says:

      the Plutonians (from “Rick and Morty”) will be pleased.

    • BCstargazer says:

      Thank You Laurele
      You’re a great advocate for planetary scientists
      but to me you’re a star 🙂

    • SteveZodiac says:

      hear hear!

    • tsuchan says:

      Hi laurele… I want you to know that I had a premonition you’d have responded to this article and that I made my way straight down to read your commentary. (^_^)/

    • You’ve GOT to be kidding me….. FINALLY, you’re getting what you’ve been whining about for over a decade…. and you’re STILL throwing a hissy fit!! People like you are only happy when the world is seen in opposition to what you “know” is right. This is cool in a big way, in my opinion. In fact, I came to this article’s comments, knowing you’d be actively discussing. I envisioned my finally being able to agree with you and rejoice together in a new, more clear and in line with what most planetary scientists already use as internal jargon nomenclature anyhow… and I find you still screaming at everyone. *boggled* What-ev…

  2. dal_lemos says:


    I’m not an astronomer, I’m a musician (with academic degree). But I’m a fan of astronomy. However, about this “planet stuff”, I would make one question:

    In Geography, what is the concept of “continent”? Did the “world association of Geography” had the pretension to redefine the concept of “continent”?


    But in Astronomy, the “intelectuals” did.

    Sometimes, there are concepts that are so socially widespread that any attempt to change it will result in controversy. “Planet” is one of them.

    Also, how an association may simply create a rule for English language? Impossible… rules come from the current use of the language. So, why a concept as “planet”, in which Pluto is widespread as a planet, can be simply vanished just because a bunch of specialists say that Pluto isn’t a planet anymore?

    I’m not here to give answers. But just to show how the discussion is deeper than most people imagine. In Humanities, we can’t (and we don’t) accept concepts just because it is approved by an “association of intelectuals” nor published in an “scientific” article. Concepts are built through much more complex processes. I like Astronomy just because it has so much of Humanities: where did we came from? Is there any life besides our beloved “planet” Earth? How do we can interact with “intelligent” life? These questions can not be answered so easily by the “rational” or “scientific” part of Astronomy: and this is what pushes me closer to it. Science can not provide all the answers we need to be human beings.

    • SteveZodiac says:

      @DAL_LEMOS Your points about language and concepts are probably the most cogent as the term “planet” derives directly from the Greek word for wanderer. Anything not fixed in the sky is named “planet”. Whilst any other name would smell as sweet, that is the one that has endured through Latin, French and English. if you want a word that expresses a fixed set of scientifically defined criteria of your own invention then invent your own word, oh, and the international association of nomenclature insists it must be no more than 2 syllables.

    • Actually, as the newly discovered 8th continent, Zealandia highlights, there IS a specific list of ‘rules’ that Geologists (not Geographers, thank you) use to define what a continent is (as opposed to say, a sub continent like India) and it was put to the test in the last few months with this discovery. Know your topics before you make erroneous claims to back your pet arguments. PS: HOw, as you sem to suggest) do Astronomers get the apparently ad hominem, in your usage, tag of “intellectuals” and Geographers (sic)/Geologists get spared from being called a name to describe intelligent people? Thart’s silly.,

    • Actually, as the newly discovered 8th continent, Zealandia highlights, there IS a specific list of ‘rules’ that Geologists (not Geographers, thank you) use to define what a continent is (as opposed to say, a sub continent like India) and it was put to the test in the last few months with this discovery. Know your topics before you make erroneous claims to back your pet arguments. PS: How, as you seem to suggest) do Astronomers get the apparently ad hominem, in your usage, tag of “intellectuals” and Geographers (sic)/Geologists get spared from being called a name to describe intelligent people? That’s silly.

    • PS: With the original decision to excise Pluto’s title, there never WAS a definition at the onset TO change… that’s what caused this dilemma in the first place.

    • tsuchan says:

      @dal_lemos I think your point is very well made: there should be no authorities in science; and aside from that it is intrinsic to science that we define things from the changing nature of what we observe, not to make an non-objective definition specifically designed to fit withinthe number of fingers of a school-child. They could have introduced a suitably abstract sub-division such as “First discovered planets” to meet that kind of purpose.

  3. In addition to all other planetary descriptions, I think the direct orbit around a star should be also part of the description. However, a body that orbits a planet should be called a moon, not a planet.

    I firmly believe Pluto is a planet. So are Ceres, Eris and Makemake, because all four of these bodies orbit the sun directly, and are large enough to form a sphere under their own gravity.

    Therefore, the solar system should be stated to have 12 planets.

    • SteveZodiac says:

      Agreed about moons but for every definition there will always be a difficult candidate, e.g. does Pluto orbit Charon or vice versa. Classification is just just our ape brains putting things in boxes for the convenience of dealing with them in groups but the universe doesn’t give two hoots. There will always be an entry under miscellaneous.

  4. BlackWolfStanding says:

    I am not going to war over any idea. But In school, I did memorize the complete Periodic Table. It was a defining moment in which direction I would choose to take my life. I also memorize the location and important facts about nearly 3,000 stars, constellations, nebulae, and galaxies. This became a Hobby of mine throughout my life.
    Learning different theories expanded my horizons. Knowing the theories place in science also expanded my horizons. Learning that change is inevitable and needed also expanded my horizons. The only thing I can say is NEVER undervalue the memorization of facts. A person cannot form ideas or opinions without having some facts memorized. And the fewer facts memorized, the less substance the opinion will have.
    When the IAU made their decision, yes it was valid for what facts they knew. It was also very political because of what facts they knew. This is why they are getting ready to change our solar system into a hodgepodge of various planet definitions. Be ready for Planets to be redefined several times by the IAU until they get it right.

    We are all creatures on this world. We are all bipeds on this planet. We are all homo sapiens on Earth. We are all stewards of the Earth. All the prior sentences are true. Why can’t we look at Planets the same way and not get caught up in knots and battles?

  5. neilp says:

    Much ado over nothing; a tempest in a teapot. It’s akin to the debate over when a lake is a pond, when is a hill a mountain? I preferred the latest definition of these last few years and thought it fit within the spirit of scientific logic and classification but either way it comes down to one of semantics and as long as we all know what we’re talking about… who really cares?

  6. Keatah says:

    Why can’t we just stick with these requirements?

    1. pull itself into a sphere.
    2. orbit its parent star

    And moons would orbit planets like they always have.

  7. rc davison says:

    This proposed new definition of a planet only serves to confuse the issue by it’s broad, sweeping scope. This is very evident when you are now classifying natural satellites of planets – moons – as planets themselves. Whether Titan was a “planet” captured by Saturn or it formed naturally around Saturn is a moot point at this in the evolution of our solar system. Titan is gravitationally bound to Saturn and not orbiting the Sun on an independent orbit. Pluto and Charon are different in that they orbit a barycenter (the center point which two masses will orbit each other) which is located outside the radius of either body. To me this is a binary planetary system, just like a binary star system, but in this case the coupled celestial objects orbit the local star. Maybe we need to revise the “clearing its orbit” clause in the IAU definition, which seems to be the biggest hang up for Pluto and its brethren.

  8. Using logic and the laws of nature, it is clear what the planets in our solar system, and what are the companions of the planet. Planets are celestial bodies whose center of mass (of the planet and its satellites), rotates around its parent star (our sun). Everything rotates around the center of mass of the planetary system, the companion planet or its months. It does not matter how much weight a month or a companion planet on its moon. What are the celestial bodies that have their own planetary center of mass?
    Only in this way should be viewed celestial bodies, and call them planets or months, if they rotate around the planet (planet pericenter)

  9. mcvey68 says:

    I would go back to the original definition of planet. Remove the part about clearing its neighborhood because it’s possible we might find an object larger than Mercury in the Kuiper Belt, and I also agree with defining a planet only be intrinsic characteristics; but maintain the criteria that it has to be orbiting the sun and not another planet (that way moons don’t become planets). An exception could be made if two objects gravitationally bound to one another are so similar in size that it would be arbitrary to call one a planet and the other a moon (say, the smaller object has at least 80% the mass and/or diameter of the other). In that case it might make more sense to call it a double planet.

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