New Bragging Rights for Pluto? It May Be the Biggest Dwarf Planet

By Andrew Moseman | November 8, 2010 4:47 pm

erisbigPluto’s dinky diameter wasn’t the official reason it was demoted from the planetary club back in 2006, but symbolically, size was the last straw. When Caltech astronomer Mike Brown spotted the object we now call Eris back in 2005 and astronomers figured it to be larger than Pluto, the former ninth planet’s fate was sealed. Now Pluto’s reclassification as a “dwarf planet” and the subsequent public outcry is behind us, but new research suggests that the former planet’s symbolic death knell—Eris’ size advantage—was wrong.

The argument has been rekindled by astronomers who just completed detailed viewings of Eris from observatories high in the Chilean Andes. According to Bruno Sicardy of the Paris Observatory, Eris must be no wider than 1,454 miles, while the accepted value for Pluto’s diameter is 1,456.5 miles. And because of the uncertainty of measuring diameter of such a distant object, Kelly Beatty at Sky & Telescope says, Eris’ official size could decease another 30 miles or more after astronomers analyze more of this data.

Images taken in December 2005 by Brown and others with the Hubble Space Telescope indicated a diameter of 1,500 miles (2,400 km), just 5% larger than Pluto’s. But the true size remained uncertain because even Hubble’s supersharp gaze is only barely able to resolve Eris’s disk. (Remember: it’s some 9 billion miles from the Sun, twice as far away as Pluto.) [Sky & Telescope]

This month, however, the window of opportunity opened for astronomers. Eris passed directly in front of a star, which allowed them to more accurately gauge its size.

Gathering the data for the measurements was a grand astronomical feat: Three teams of scientists watched the distant star disappear when Eris crossed in front of it. By analyzing how long the star was covered over, as seen from three vantage points in Chile, the astronomers could calculate how wide Eris’ round disk was. Previous estimates were based on indirect data, such as Eris’ brightness. [MSNBC]

Mike Brown, writing on his website, notes that making this measurement was a stroke of good fortune. Eris is so small and distant that the shadow it cast when passing in front of the star covered an area 25 times smaller than the Earth itself. Fortunately, the band of our planet that the shadow passed over included the part of Chile where observatories reside.

Brown warns that there are uncertainties even with the new measurements, though larger telescopes might try to duplicate the Chile feat and bring those uncertainties down. But if the numbers are correct, it means that there’s a lot more about Eris we don’t understand, foremost being the fact that the research still indicates that it’s 25 percent more massive than Pluto. From Brown’s site:

Though Eris is substantially more massive, they are essentially the same size. Eris must be made almost entirely of rock with a little coating of frost – which we see – on the outside. How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside? As of today I have absolutely no idea.

Finally, there’s the payback for Pluto—if it’s bigger than Eris, then it’s the largest known object in its region, the Kuiper Belt. Alas, true planetary status will probably never return: Pluto will never “clear its neighborhood,” one of the three International Astronomical Union criteria, so a dwarf planet it shall remain.

Would things have gone differently for Pluto if Eris was initially identified as slightly smaller than it, rather than 5 percent larger? I put the question to Brown (who tweets as “Plutokiller“), and he replied via e-mail:

Funny, huh? My suspicion is that within the final error bars we won’t actually be able to say for sure which one is bigger. But the one thing we do know well and the one that probably matters the most to the solar system is that Eris is significantly more massive, so I suspect we would still have had to deal with Pluto!

For plenty more on what it means to be a planet, pick up the December issue of DISCOVER, which hits newsstands this week, and check out the piece “Why Size Matters” by resident Bad Astronomer Phil Plait.

Related Content:
DISCOVER: Pluto Gets Demoted. Again.
DISCOVER: Beyond the Nine Planets
80beats: The Fourth Dwarf Planet Is Officially Christened: Meet “Makemake”
80beats: New and Improved: the “Plutoid”
Discoblog: Third-Grade Students to Scientist: Pluto Is too a Planet!

Image: Wikimedia Commons

  • Jumblepudding

    Are dwarf people still people? yes. issue resolved.

  • a. Laurie

    I’m sure Pluto isn’t too concerned about its change in status.

  • Ryan

    Pluto will be here long after the IAU is gone, so I think it is Pluto that will have the last laugh on this one.

    Besides, if Pluto ever feels bad about its status, it always has Charon’s gravitationally locked shoulder to cry on.

    A Pluto by any other categorization would still be as big…

  • Andrew Moseman

    I do hear that Charon is a good listener.

  • Messier Tidy Upper

    I’d love to see the current IAU definition of “planet” rejected and all the dwarf planets added to the planets list.

    A dwarf star is still a star so why isn’t a dwarf planet counted as a planet?

    Moreover, NO planet could clear an area the size of Pluto’s orbit – not even Jupiter.

    Plus Pluto *does* gravitationally dominate its local region – hence its three moons one of them relatively huge! 😉

    I’d define planet as :

    1) Rounded by self-gravity thus too big to be an asteroid or comet,

    2) But too small to ever shine by fuse elements at its core thus not a star or brown dwarf,


    3) Not orbiting another planet but the Star thus not a moon.

    It then make sense, I think, to divide this broad category of planets into sub-classes – the gassy, the rocky and the icy, Hot Jupiters, SuperEarths, gas dwrafs, ice dwarfs, water worlds and so forth. Instead of teaching all the planets name we then teach kids the sub-classes and a few examples of each eg. Jupiter, Earth, Pluto, 51 Pegasi b (aka “Bellopheron”), Gliese 581 g, etc .. as examples.

    The whole “orbital clearnance”criterion strikes me as raising and creating more problems and confusion than it solves. It also seems superflous & really pretty needless thus violating Occams razor.

    I think the IAU got it badly wrong and the sooner they correct their error on the Pluto / Planet question the better.

  • Laurel Kornfeld

    Pluto’s fate was never “sealed.”

    Pluto and Eris are both planets and Kuiper Belt Objects. One does not preclude the other. They are planets because they are large enough to be rounded by their own gravity. They are Kuiper Belt Objects because they are located in the Kuiper Belt. Ceres too is a small planet because it is large enough for its gravity to pull it into a spherical shape. The IAU misappropriated the term “dwarf planet,” which was first coined by Dr. Alan Stern, Principal Investigator of NASA’s New Horizons mission to Pluto, to indicate a third class of planets which are large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never intended for “dwarf planets” to be classed as not planets at all. The IAU did not “have” to do anything other than allow Eris’s discoverer to name it while holding off on any additional classification until more information is discovered about remote planets in this solar system and all planets in other solar systems.

    Significantly, there are quite a few exoplanet systems in which multiple planets orbit the host star in various different planes. Some have orbits far more eccentric than Pluto’s, yet they are giant planets the size of Jupiter or larger. According to the IAU definition, none of these objects are planets!
    Saying there are more differences between Pluto and the eight closer planets to the Sun depends on what aspects one considers. Earth actually has far more in common with Pluto than with Jupiter. Both have surfaces on which we can place rovers and landers. Both have a large moon formed by giant impact; both are geologically differentiated into core, mantle, and crust, and both have nitrogen in their atmospheres. Other than orbiting the Sun, what do Earth and Jupiter have in common?

    It is premature to pronounce declarations that these faraway objects are definitively not like the other planets or that one is larger than the other. We just do not have enough data at this point to do more than make educated estimates. What we really need to do is send robotic missions like New Horizons to Eris as well as Haumea and Makemake. Yes, that will take time and money, but it is a far better investment than the black holes the endless wars in Iraq and Afghanistan have become.

    Also, memorization is not important. It is much more important to teach the characteristics of each category of planet than to ask kids to memorize a bunch of names. We don’t ask them to memorize the names of rivers or mountains on Earth, so why do so with planets, and why allow a need for convenient memorization to determine how we classify them?

    Note: I tweet as @plutosavior

  • Mike Wrathell

    Pluto was too quickly dissed by the IAU after the discovery of the pipsqueak planet Eris. It has been concluded, by my recent readings, that Pluto is definitely larger than the pipsqueak planet discovered by a man who first named it after a teevy show and whose Twitter name is “plutokiller.”

    Pluto and its three moons deserve more respect than that from someone who claims to be a professional astronomer. This is the best story of 2010. What beautiful poetic justice!

    Please consider coming to Honolulu in August 2015 to fight for the replanetization of Pluto during the IAU’s trienniel General Assembly. I hope to be there.

    Also, check out my space art and other art on my page. I have made art from New Horizons images, too, during its Jupiter encounter.

    Nice post, Laurel! Would love to see high res photos of Eris and its moon, even though it is a pipsqueak planet! LOL!

  • Messier Tidy Upper

    @ ^ Mike Wrathell : Pipsqueak planet? Actually Pluto is an average size planet and is bigger than most if we include the ice dwarfs too as I consdier makes the most logical and semantic sense :

    Planets in order largest to smallest diameter counting just some of the ice dwarfs and asteroidal / rock dwarfs candidates :

    Jupiter, Saturn, Ouranos, Neptune, Erath, Venus, Mars, Mercury,

    *Pluto* (# 9 now upgraded from #10)

    Eris, Makemake, Haumea, Sedna, Quaoar, Varuna, Orcus, “Buffy” or 2004 XR190, Ceres, Huya, Ixion, 2002 TC302, “Snow White” or 2007 OR10, 2002 AW197, Vesta and maybe Pallas.

    So that’s 15 or 16 worlds *smaller* than Pluto versus only eight that are larger. Not so small relatively at all but right in the middle of the planetary size range! 😉

    PS. No, I wouldn’t make the schoolchildren learn all the names of the likely 30 + planets – just the key one’s incl. Pluto and breaking them into major types : the gas giants, the rocky terrestrial types and the ice dwarfs. Or the gassy, the rocky and the icy! 😉

    @ Laurel : Seconded – great comment there. I think @plutosavior is a great username although I’m not on twitter just email, facebook and here. (I was posting formerly as ‘StevoR’ btw.) Wishing you all the best & please let me know if I can help – you have my full support on this issue. :-)


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