Pluto or Eris: Which is Bigger?

Article written: 14 Oct , 2011
Updated: 24 Dec , 2015


The controversy between Pluto and Eris regarding their status as “largest dwarf planet” continues. During a joint meeting of the American Astronomical Society Division for Planetary Sciences and the European Planetary Science Congress last week in Nantes, France, new data was presented that may help settle the debate. The new findings regarding this size of Eris may be a surprise to some, and to others a confirmation of what was believed to be true.

How were astronomers able to make the new measurements of Eris, and what implications will these new measurements have on the Pluto / Eris debate?

Using a celestial alignment known as an occultation, Bruno Sicardy of the Paris Observatory (University of Pierre and Marie Curie, France) and his team were able calculate the diameter of Eris in 2010. The occultation was caused by Eris moving past a background star, which blocked the star’s light and cast a small shadow on Earth. When Sicardy and his team compared the shadow’s size at two different sites in Chile, the calculations provided a diameter of 2,326 kilometers for Eris. A previous study by Sicardy in 2009 placed Pluto’s diameter to be at least 2,338 kilometers.

However, the first estimates of Eris’ size that were made shortly after its discovery put the diameter at 3,000 km, plus or minus 400 km. But a later estimate from observations with the Hubble Space Telescope said Eris might be 2,400 km in diameter, plus or minus 100 km.

If Sicardy’s data calculations hold true, this places Pluto and Eris at nearly the exact same diameter. What has continued to not be up for debate, however, is that Eris is far more massive than Pluto. Given a nearly identical diameter for Eris and Pluto, Eris’s extra mass makes it the denser of the two dwarf planets. According to Sicardy and his team the increased density of Eris, “indicates that Eris is mainly composed of rocky material, with a relatively thin ice mantle.” Since Pluto’s density indicates it comprised of about equal parts ice and rock, Eris’s extra mass would appear to validate Sicardy’s assertion.

Eris and its moon, Dysnomia. Credit: NASA, ESA, and M. Brown (California Institute of Technology)

The Co-discoverer of Eris, and noted “Plutokiller” Mike Brown (Caltech) offers an interesting thought regarding the Pluto / Eris Debate:

“Scientifically, knowing which one is bigger will teach us…. absolutely nothing. The fact that they are nearly identical in size is scientifically interesting; which one is a few kilometers bigger than the other matters not one bit.” Brown also added, “But, still, I will admit to having a bit of an emotional attachment to Eris, so, deep down inside, I want to believe it will turn out to be a little bigger.

You can read a brief synopsis of Sicardy’s findings at:

If you’d like to learn more about the Pluto / Eris debate, Brown has some great thoughts regarding the debate on his blog at:


40 Responses

  1. Ray Fowler says

    Viewing an occultation from multiple sites is another one of the fascinating ways that astronomers tease useful information out of what otherwise looks like a simple point of light.

    With regards to the size, I have always considered mass to be a more important characteristic as physical size can be greatly skewed by composition. Mercury is smaller in size than Titan, for example, but 2 1/2 times more massive!

    • laurele says

      But the critical mass to achieve hydrostatic equilibrium is not in question for Pluto, meaning according to the geophysical planet definition, it is a small planet.

  2. laurele says

    Why are you referring people only to Brown’s page to learn more about this debate? You should be referring people to sources on both sides. I am “Plutosavior” on Twitter, also an astronomer, and invite anyone interested in hearing why both Pluto and Eris are planets to visit my blog at . Other good sources are the New Horizons web site and Alan Boyle’s book “The Case for Pluto.”

    Pluto is actually estimated at 70 percent rock. And the so-called controversy over which is bigger, Pluto or Eris, is meaningless because both are clearly well beyond the minimum mass and size for attaining hydrostatic equilibrium, meaning they are large enough to be rounded by their own gravity and therefore fit the geophysical definition for a planet. What difference does it make which one is bigger? Why can’t we just study these fascinating worlds without creating an artificial contest?

    • IVAN3MAN_AT_LARGE says

      Here’s how to end the Pluto ‘debate’ once and for all: Click Here.

      • laurele says

        Begun, the planet wars have!

      • From the reaction of many people about the IAU’s decision, it kinda makes me wish that is what happened. The decision doesn’t make Pluto any less interesting from a scientific standpoint. The reality is it *NEVER* really fit in with the Solar System…

      • IVAN3MAN_AT_LARGE says

        Indeed! I remember reading in an astronomy book from the 1970s which speculated that Pluto, due to its eccentric orbit, may be just an escaped moon of Neptune, so even back then it was not considered a proper planet.

      • laurele says

        That was speculation, and it is probably wrong. But back then, no one knew that our solar system has many dwarf planets, not just Pluto. Eccentric orbits cannot preclude objects from being considered planets because many exoplanets have orbits far more eccentric than Pluto’s, and these are mostly giant planets.

      • laurele says

        Yes, Pluto DOES fit in with the rest of the solar system if you recognize that the solar system has three, not two classes of planets–terrestrials, jovians, and dwarf planets. Dwarf planets are a subclass of planets because they are much more akin to the bigger planets than to asteroids. They are shaped by their own gravity, are geologically differentiated, have atmospheres and weather, and are complex bodies–unlike asteroids and tiny KBO iceballs. In fact, Earth has a lot more in common with dwarf planets like Pluto than with jovians like Jupiter. Both Earth and Pluto have large moons formed via giant impact; both have solid surfaces and are differentiated into core, mantle, and crust; and both have nitrogen in their atmospheres. Pluto may even have a subsurface ocean. In contrast, Jupiter’s composition is much more like that of the Sun than like that of Earth–mostly hydrogen and helium. It has its own “mini solar system” going of moons that formed not by impacts but with the planet. Unlike Earth, it has not solid surface. To put Earth and Jupiter in the same category while excluding Pluto (and all dwarf planets) amounts to cherry picking characteristics that Earth and Jupiter have in common while ignoring the differences between them as well as the characteristics Earth and Pluto have in common. Dwarf planets are actually the most numerous class of solar system planets, so saying they don’t fit in with the rest of the solar system makes absolutely no sense.

      • IVAN3MAN_AT_LARGE says

        “Aye, and if my grandmother had wheels, she’d be a wagon!”
        —Montgomery Scott, Star Trek III: The Search For Spock.

      • Interesting viewpoint. So let’s look at the Solar System, shall we? We’ve got four inner planets made of metal and rock ( and a little bit of water/ice on the outer two.), an asteroid belt of objects made of metal and rock ( and a bit of ice), a couple gas giants, and then a couple of “ice” giants. Past the ice giants you have dusty, icy objects in the region beyond Neptune, all the way out to the Oort cloud.

        When you look at the physics of solar system formation, this layout makes sense. (I won’t go into a full lecture on said physics)

        You say Earth has more in common with Pluto than Jupiter? That side steps the issue a bit, Doesn’t it? Jupiter and Saturn aren’t stars, therefore are classified as “gas” giant planets. Apples and Oranges…

        There’s several classifications of planets, and “dwarf” planet is simply a new addition that helps us to classify Pluto, Eris and all the other dwarf planets, since they themselves all have similar characteristics. So we have terrestrial, gas giant, ice giant, and dwarf planet categories. Seems logical.

        Ever watch sesame street? There was a skit called “one of these things doesn’t belong”. Pluto is far more similar to Eris and the other Dwarf planets than it is to say, Earth or Venus.

        Sure, Pluto likely has a crust, mantle and core. Pluto is pretty much at hydrostatic equilibrium too. It takes far less ice, mass wise, to reach hydrostatic equilibrium than rock or metal.

        As for an “ocean” on Pluto, I’m highly skeptical of that theory. A more likely candidate for subsurface water is Ceres. New Horizons will give us more info in a few years, though so anything at this point is speculation.

        So when everyone classifies Pluto as a dwarf planet, that is because its composition, orbital distance, orbital parameters, and other characteristics are far more similar to the rest of the dwarf planets, than say Earth, Saturn or Uranus.

        Pluto isn’t a terrestrial planet, it’s not a gas giant, nor is it an ice giant. Where does it best fit in our current planetary classification system? with Ceres and Eris and the other dwarf planets. Twenty years ago we didn’t HAVE a dwarf planet classification, so Pluto didn’t fit in.

        Personally I wish people would stop seeing Pluto’s reclassification as an attack on the poor thing. Pluto now has a family of dozens, possibly hundreds of similar objects. There’s a place for Pluto to “fit” in our solar system!

        While we’re on the topic, a helpful memorization phrase:

        My Very Eccentric Mother Just Served Us Nachos.

      • laurele says

        I outright reject that memorization phrase, and so should you.

        Everything you say makes sense except for one thing: the IAU decision specifically states that dwarf planets are NOT planets at all (a position you reinforce with that mnemonic). Pluto may not belong with terrestrials or jovians, but that doesn’t mean it or other dwarf planets are not planets. In astronomy, dwarf stars are still stars, and dwarf galaxies are still galaxies.

        The question isn’t an attack on Pluto; it’s a wrongful lumping of Pluto and dwarf planets with asteroids and comets in the same category as “minor planets.” Structurally and compositionally, dwarf planets are much more akin to the bigger planets than to asteroids or comets.

        What I really want to see is an amendment of that resolution to incorporate dwarf planets as a subclass of planets.

        Memorization is not important. We don’t ask kids to memorize all the rivers or mountains on Earth or all the moons of Jupiter. At one time, we knew little more about the planets than their names. Today, we have so much data on the planets and their moons. Instead of asking kids to memorize a list of names, we should teach them the types of planets and the characteristics of each one. The notion that we cannot have too many planets because kids wouldn’t be able to memorize them has no scientific merit.

    • Torbjörn Larsson says

      What “both sides”? “Planets” isn’t a debate, it is a (settled for now) definitional issue.

      The important issue of science debates should be reserved for when they actually occur. Say, how planets form in the first place.

      • laurele says

        No, the question of what is a planet is NOT a settled definitional issue, which is why both sides of the ongoing debate should be presented. There are as many astronomers who reject the definition adopted by four percent of the IAU, a solely dynamical definition, and favor a geophysical definition instead. The petition by 300 professional astronomers opposing the IAU decision is just as valid a statement as is the initial vote. It is a disservice to pretend that only one side is legitimate when this is not the case. The debate between advocates of the dynamical and geophysical definitions is still occurring, as can be seen from books that continue to be published on both sides of the issue, debates at conferences such as those of the American Geophysical Union and the European Geophysical Union, the continued reference by the New Horizons mission to Pluto as a planet, and new data learned about Pluto, Eris, and exoplanets. We certainly should discuss planet formation, but it’s not an either/or. There is no reason to arbitrarily cut off debate on planet definition because proponents of one view do not want that view to be challenged.

      • IVAN3MAN_AT_LARGE says

        You remind me of those Metric Martyrs in the U.K. who stubbornly refuse to conform to the more logical metric system. Enough already!

      • laurele says

        No, not enough! You don’t get to arbitrarily decide the debate is over because you want it to be. The geophysical planet definition is not a fringe position; it is one held by many professional astronomers.

      • IVAN3MAN_AT_LARGE says

        I think that the majority of professional astronomers are more concerned about getting the J.W.S.T. into space than to get their knickers in a twist, like you have, over Pluto’s “dwarf planet” status!

      • laurele says

        Please stop trying to demean those of us who disagree with you with terms like “get their knickers into a twist.” It does not help your argument. And even JWST is a subject of controversy. I don’t trust the US Congress to make good scientific decisions, but their proposal to defund JWST has caused its own rift among astronomers at NASA, and I’ve seen some very heated arguments between those who support JWST and those who prefer the money be spent on more robotic planetary explorations.

  3. Anonymous says

    Ray: Good article.. but here is what confuses me.. I did read Dr. Mike Brown’s blog.. Mike is a very respected astronomer and has co-discovered several celestial objects that do deserve status as a “Planet” ie: as #10 onwards ie: whatever the sequence # is.. shame on the IAU.
    In his blog he does mention the following ia Para titled…
    ” So what is the actual size of Pluto?

    As a scientist you don’t get an answer key. You have to look at all of the measurements, read the scientific papers, and try to understand the discrepancies. Sometimes you simply might have more faith in one type of measurement than another. Sometimes you trust certain scientists more than others. ”

    And then I ask you to see 30 secs of his Keck Observatory video Part 2

    ie: from 23.00 mins to 23.30 mins (especially the last 15 secs of the 30 secs)..

    I compare Mike’s statement on that 30 sec video vs. his blog about “trusting certain scientists more than others”.. I am confused…

    Is there a deeper BIAS with this reputed astronomer/scientist (with so much experience in astronomy) than “normal”.. Killing Pluto seems a little too personal.. Why should we believe or better still “Trust” what he says more than with a pinch of salt about this Pluto/Eris, planet status debate. Maybe he is still trying to sell his book..

    Dr. Brown should continue his excellent research in co-discovering KBO’s.. I hope when the planet Eris closer to Earth, there would be a robotic mission to the Planet Eris..

    • Frances A says

      These personal attacks reflect far more on you than they do your target. I am not interested in your wish to smear the personal character of a scientist, I am interested in SCIENCE. If you are having personal TRUST issues, find a good therapist, if you want to debate science, then stick to the FACTS.

      • laurele says

        I think Brown owes an explanation of his statement about trusting some scientists more than others. Is he implying that any particular astronomers or those that hold views that differ from his untrustworthy?


    Yo Ray, in the fourth paragraph, there’s a rogue apostrophe at the end of the first line:
    “… after its discovery put it’s…”

  5. Torbjörn Larsson says

    This is why I applaud the growing population of female astronomers. I don’t see them arguing interminably over who has the greatest balls.

    • Sowff says

      Clyde Tombaugh has bigger balls than Brown who defames a dead man, calling him a hick farmer boy. Shame on Brown and his little balls. His delusion that he killed Pluto is an embarassment to all scientists. To call him a doctor suggests someone has a crush on him. Very poor taste.

  6. Anonymous says

    What happens if they find a “dwarf” planet larger than Mercury?

    • Interesting scenario. It is entirely possible that something that large could have been ejected out to the edges of the solar system. I’d have to do the math and/or a simulation.

    • laurele says

      Herein you reveal a major problem with the IAU definition, namely, it could well result in the same object being labeled a planet in one location and not a planet in another location. If Earth were in Pluto’s orbit, it would not clear that orbit either and therefore would not meet the IAU criteria for being a planet. A definition that classifies an object solely by where it is while completely excluding what it is simply makes no sense.

  7. Sowff says

    Ivan, that is a silly video that is not the end of the debate.

    Not sure why so many articles only quote Mike Brown who has nothing to do with Pluto. The New Horizons mission to Pluto blasted off when Pluto was still the 9th planet of our Solar System back on January 19th, 2006. Many distinguished scientists are on the mission team and are available to be interviewed. Why someone who inanely claims to have killed Pluto is repeatedly given a soapbox to defame Pluto is beyond me, especially now that Bruno Sicardy’s data suggests that Pluto is larger than Eris. I hope after Nature publishes his paper that the question of which planet is larger will be put to rest, at least until July 14, 2015, when New Horizons reaches Pluto and takes a definitive measurement of its diameter.

    • No one, especially Dr. Brown have any negative opinions of Pluto, scientifically speaking. Pluto and the rest of the TNO’s/KBO’s/Dwarf Planets are all VERY worthy of scientific study.

      • Sowff says

        Brown is very negative toward Pluto. You might want to smoke less powerful weed, dude.

      • I think that comment towards me is a bit uncalled for… Re-read my statement. Better yet, let me elaborate: Dr. Brown might not personally like Pluto, but even he admits that Pluto is worth studying from a scientific standpoint.

        Five years later and people are still throwing a fit over Pluto’s demotion. Science isn’t dictated by “popular” opinion. If it were, we’d still be learning that Earth is the center of the universe.

      • laurele says

        Science isn’t dictated by a vote of four percent of the IAU, most of whom are not even planetary scientists, whose motive in the decision was largely political (plus they violated the IAU’s own bylaws in voting on a resolution without vetting it first by the proper committee. Stop trying to discredit those of us who reject the decision with personal attacks saying we’re “throwing fits.” We’re not; we’re legitimately objecting to something we believe is wrong and has been misrepresented to the public. In fact, I’m studying to be a professional astronomer partly to bring establish a new, more inclusive and braod planet definition.

      • Once again, I’ll let the rude comments slide…

        On the topic of you studying to become a professional astronomer: With all sincerity, I applaud your efforts.

        If you are willing to put in the time and effort to obtain a PhD from a credible university that’s awesome in it’s own right.

        After your PhD if you start publishing papers on a new planetary “standard” in peer-reviewed journals, that’s even more awesome.

        Twenty years ago, there were three classes of planets, and now there are four. Will you be able to get down to one?

      • laurele says

        Sorry, but I don’t think I said anything rude. My only attack was on the reference to those who reject Pluto’s demotion as “throwing a fit.”

        I am currently in a Masters program at Swinburne University. I may or may not transfer to a PhD program after that, but I certainly am not going to wait until then to publish my position on Pluto and planet definition. I am already attending conferences and staying in regular contact with Dr. Alan Stern and others who are NOW working on a better planetary standard, and I have no intention of giving that up.

        By four classes of planets, do you mean terrestrials, gas giants, ice giants, and dwarf planets? I counted three–terrestrials, jovians, and dwarf planets, but I do see the sense in dividing the jovians into two classes.

        I don’t think we need to get down to one class of planets. Why is that even desirable? Planet should stay a broad umbrella term under which there are many subcategories, the more, the better. How many types of stars are there? How many types of galaxies? Would you say there can only be one type of star or one type of galaxy? To quote a very well-respected planetary scientist, Dr. Mark Sykes, “that’s why God invented subcategories.”

      • squidgeny says

        What I would like to know is; why does it matter? If planetary scientists feel that Pluto’s “diminished” status will have a negative impact on funding for science programs that target Pluto, then OK, I can understand that. But beyond that, what actual impact in the real world does Pluto’s classification have?

        I couldn’t give a rat’s arse if Pluto was officially classified as a pebble, as long as it remained the target of scientific interest it always has. Pluto doesn’t have feelings to hurt so as I see it, it just doesn’t matter.

      • laurele says

        It matters because planetary scientists want to see objects classified as accurately as possible. It has nothing to do with funding. Someone who studies planets understands the primary characteristics that planets share and views it as important to distinguish these bodies from tiny, shapeless asteorids and comets, which are held together only by their chemical bonds. It matters the same way as it does for brown dwarfs to be as stars, even failed stars, rather than planets. That tells us these objects at least at one time fused hydrogen or deuterium. Blurring the distinction between dwarf planets and asteorids/comets is simply bad science. An arbitrary decision based on convenience, namely the notion that “we can’t have too many planets” has no scientific merit. This has nothing to do with hurt feelings. It has to do with the need for a classification system that recognizes the unique characteristics of these bodies and the great diversity of subtypes. Why not create a system similar to the Herzsprung Russell Diagram we have for stars to incorporate and classify the wide spectrum of planets we are discovering every day?

      • squidgeny says

        Ok, that seems fair enough.

      • laurele says

        Thanks! Glad we could come to an understanding!

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