Pluto: The Biggest Dwarf Planet?

by Katie Bowell, Curator of Cultural Interpretation

New Images of Pluto from the Hubble Space Telescope

Pluto’s had a tough couple of years, hasn’t it? First Eris showed up. An object in our solar system thought to be larger than Pluto, the 2005 discovery of the dwarf planet brought Pluto’s status as a planet into question (in that regard, Eris’ name is quite appropriate: “Eris” was the Greek goddess of discord and strife). Then in 2006, the International Astronomical Union (IAU) declared that Pluto would be reclassified as a “dwarf planet.”

But now, a little bit of good news.

Last month astronomers were able to measure Eris by watching it pass in front of a star (learn more about how they measured Eris here).

By measuring how long the star disappeared behind Eris, a measurement that corresponds to Eris’ size, astronomers now believe that Eris may be no larger than 1,454 miles across. Pluto is believed to be 1,456 miles across. So is Pluto 2 miles wider? Maybe.

The size of both dwarf planets is still being explored, and with measurements so close, the winner of this size contest will likely be debated for quite some time.

However, even if Pluto is bigger than Eris, that doesn’t mean a promotion back to full planet status. Pluto still hasn’t “cleared out” its orbit, joining or consuming the smaller objects in its orbit, one of the three requirements for “planethood” established by the IAU. Pluto’s also in a neighborhood full of similarly-sized bodies, and so its dwarf status is still solid. But after five years of beat downs and demotions, if Pluto does turn out to be the largest dwarf planet, that’s still something to celebrate.

For more on the Pluto planet debate, watch PBS’s The Pluto Files.

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1 Response to “Pluto: The Biggest Dwarf Planet?”


  1. 1 Plutogirl November 23, 2010 at 11:01 am

    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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