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Is Pluto the biggest dwarf planet after all?

The views expressed are those of the author and are not necessarily those of Scientific American.


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Dwarf planet Eris from HSTPluto’s controversial demotion from planetary status came in 2006 after the rapid discovery of comparably sized bodies—now named Haumea, Makemake and Eris—made Pluto look rather ordinary. In particular, Eris was found to be larger in diameter than Pluto, raising the question of what separated a planet from numerous smaller bodies. The International Astronomical Union decided on a new definition for planets that resulted in a paring down the solar system’s tally of planets to eight, relegating Pluto and its ilk to dwarf planet status.

Pluto lovers of the world may take some small comfort in a new look at Eris that puts Pluto back in the running for the largest dwarf planet, diameter-wise. (Eris seems to retain a lock on the title of most massive dwarf planet for the time being.) Measurements taken as Eris temporarily blotted out the light of a distant star indicate that the dwarf planet’s diameter is on par with, and maybe even smaller than, that of Pluto.

Eris is extremely distant, orbiting much farther from the sun than even Pluto does, and it is difficult to get a good look at the relatively small world. Although initial thermal readings pegged Eris at about 3,000 kilometers (km) in diameter, later infrared observations taken with the Spitzer Space Telescope indicated a diameter of roughly 2,600 kilometers (km), whereas Hubble Space Telescope measurements pointed to a diameter of 2,400 km. Pluto, in comparison, is about 2,300 kilometers across.

On the night of November 5, a fortuitous alignment provided the new data point. As Eris cruised through its orbit, some 14 billion km from Earth, it passed in front of a distant star from Earth’s vantage point, casting a small shadow across our planet in an event known as an occultation. By timing the duration of the occultation at multiple sites, researchers can estimate the size of the shadow and hence the size of the object.

According to Sky & Telescope, three teams witnessed the occultation from sites in Chile. Based on those measurements, astronomer Bruno Sicardy of the Paris Observatory told the magazine that Eris’s diameter is "almost certainly" smaller than 2,340 km.

Mike Brown of the California Institute of Technology, one of the co-discoverers of Eris who seems to relish his role in the Pluto controversy (his Twitter handle is plutokiller), noted on his Web site that the results, though preliminary, are tantalizing. For if Pluto and Eris are approximately the same diameter, yet Eris is substantially more massive, their composition must be fundamentally different. "How could Eris and Pluto look so similar in size and exterior composition yet be totally unalike on the inside?" Brown wrote. "As of today I have absolutely no idea."

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





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  1. 1. jtdwyer 9:05 pm 11/8/2010

    This is not the first time in history that the first interpretation of initial evidence proves later not to be the best interpretation of all evidence…

    Poor Pluto – he’s such a good dog, too! Isn’t it time we correct this grave injustice?

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  2. 2. iDr Gonzo 7:41 am 11/9/2010

    Sentiment and Science are not good bed fellows.

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  3. 3. dragonasbreath 3:54 pm 11/9/2010

    Well, the only reason Pluto was ever called a planet was we were looking for planets. At the time Pluto was actually something like planet #80.
    All of these other former planets were renamed asteroids because they crossed other planets orbits, they were much smaller than the other planets, they had odd ellipses, they ran in herds (any of this sound familiar?)
    Pluto was allowed to retain the title because he did not run in a herd. Only he turns out that he does.
    And on a historical note: the name Minerva (Pluto’s wife) actually won out in the polls, but all the other planets are named for Male Gods, so Pluto was affixed rather than Minerva.
    Asteroid Chiron was discovered before moon Charon. These names are so close that Charon should yield the floor to Chiron. And it is far more appropriate for Pluto’s eternal twin-companion to be his wife Minerva than his employee the Charon ferryman, isn’t it?

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  4. 4. laurele 4:18 pm 11/11/2010

    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?

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