January 5, 2005: What is a Dwarf Planet?

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A Brief History

On January 5, 2005, American astronomer Michael E. Brown (CalTech, Princeton,Berkeley ) with fellow astronomers David L. Rabinowitz (Yale University and University of Arizona) and Chad Trujillo (University of Hawaii, Gemini Observatory and Northern Arizona University) were given credit for their discovery of a planetoid they called Eris, at the time, the largest dwarf planet known in the Solar System.  The astronomical team actually discovered Eris in 2003, but its (relatively) small size and distance from Earth required rigid documentation for the discovery to be accepted.  Larger than Pluto, the dwarf planet that used to be considered just a “planet” from its discovery in 1930 until the discovery of Eris and other heavenly bodies in our solar system beyond the orbit of Neptune.  With the discovery of Eris and other objects orbiting the Sun at distances beyond Neptune, Pluto was “downgraded” from a planet to a dwarf planet along with Eris and other such objects.

Digging Deeper

Eris is more than a fourth more massive than Pluto, although Pluto has more volume than Eris, so astronomers were faced with possibly calling Eris a “planet” or creating a new genre of heavenly bodies.  Brown and his colleagues argued that because of the orbit these space rocks make not on the same plane as the orbit of the other 8 planets, they should not be considered “planets” but should be called “dwarf planets” instead.  Eris and Pluto are to be found in the so called  Kuiper belt, a ring of objects such as comets, meteors and asteroids (as well as dwarf planets) that orbit the sun beyond the orbit of Neptune.  The objects found in the Kuiper belt are made of a variety of materials such as ice, rock and iron.  Objects found in the Kuiper belt are referred to by scientists as “trans-Neptunian objects” or more familiarly as TNOs.  Such objects are also referred to as “Plutoids” in lieu of being called dwarf planets.

Artistic comparison of Pluto, Eris, Haumea, 2007 OR10, Makemake, Quaoar, Sedna, 2002 MS4, Orcus, Salacia, and Earth along with the Moon.  Image by Lexicon.

Brown, Rabinowitz and Trujillo worked together on the Quasar Equatorial Survey Team, a collaboration of Yale University, Indiana University, and Centro de Investigaciones de Astronomia (Venezuela).  At work mapping objects in space through the use of ultra-high technology cameras and telescopes (from 2003 to 2007 they used the 48 inch Samuel Oschin telescope at the Palomar Observatory and since 2009 they have used the  1 m ESO Schmidt Telescope (Chile).  Camera work is accomplished using digital cameras in an array of 112 charge-coupled devices.

When the debate about “downgrading” Pluto to a lesser status as a dwarf planet raged among the astronomers of the world, many fiercely defended Pluto as being a “true” planet.  Ah, but people are so attached to the familiar!  The new discoveries of Eris and other TNOs more or less necessitated a re-thinking of how we view our Solar System, and the dwarf planet genre became the accepted way of describing Pluto and Eris.  (The author is happy to have fewer planets to memorize as he has too many things to remember as it is!)

The Sun and planets (distances not to scale).  Image by WP.

Eris has a diameter of about 2326 kilometers and takes a whopping 558 Earth years to orbit the Sun.  Compared to the mean density of the Earth (5.5 grams per cubic centimeter), Eris is less dense with a mean density of only 2.52 grams per cubic centimeter.  The total mass of Eris is less than a fourth of that of the Moon.  Eris was at first believed to have a slightly larger diameter than Pluto, but later calculations proved Eris has a diameter slightly less than that of Pluto.  Scientists as NASA were inclined at first to call Eris the “tenth planet,” but scientific debate came up with dwarf planet label instead.  In spite of its relatively small size, like the Earth, Eris also has a single moon orbiting it.

So why did it take so long for us to discover Eris?  Because Eris is an incredible 3 times farther away from the Sun than Pluto, a distance measured as 96.3 astronomical units.  (An “astronomical unit is defined as 8.95×10 to the 9th power miles.  You can do the math if you would like!)  Eris and another dwarf planet, Dysnomia, are among the farthest objects that orbit our Sun, eclipsed only by some comets.  The mathematics involved in studying various photos of space and the relationship of objects than can be seen (and not seen) is mind boggling.  When the term “rocket scientist” is used to describe a genius, it probably includes astronomers in that grouping.

Eris (center) and Dysnomia (left of center), taken by the Hubble Space Telescope.   Image by NASA, ESA, and M. Brown.

As they used to say on the television show, Star Trek, space is the last frontier, with an untold amount of facts, figures and things yet to be discovered.  Will we ever travel through space like the Starship Enterprise?  Probably not in my lifetime, but perhaps in yours!

Questions for Students (and others): Have you ever heard of Eris?  Were you aware of the designation of “dwarf planet” for Pluto?  Do you believe humans will land safely on Mars in your lifetime?  How many years do you think it will be before humans could safely leave our solar system?

Northern hemisphere of Pluto in true color.  Image by NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker.

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Historical Evidence

For more information, please see…

Black, Vanessa. Pluto and the Dwarf Planets (Bullfrog Books: Space Voyager). Jump!, Inc., 2018.

Kortenkamp, Steve. The Dwarf Planets (The Solar System and Beyond). Capstone Press, 2011.

The featured image in this article, an image by NASA, ESA, and M. Brown from http://www.nasa.gov/mission_pages/hubble/news/eris.html (Originally uploaded to en.wikipedia by en:User:Serendipodous.  For more information, see the description page.) of the dwarf planet Eris and its moon Dysnomiais in the public domain in the United States because it was solely created by NASA.  NASA copyright policy states that “NASA material is not protected by copyright unless noted“.

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About Author

Major Dan

Major Dan is a retired veteran of the United States Marine Corps. He served during the Cold War and has traveled to many countries around the world. Prior to his military service, he graduated from Cleveland State University, having majored in sociology. Following his military service, he worked as a police officer eventually earning the rank of captain prior to his retirement.