Astronomer Responds to Pluto-Not-a-Planet Claim

Last week, various media reported that exhibits at the Rose Center for Earth and Space in New York City give short shrift to Pluto, with the center omitting the far-flung ice ball from planetary displays of our solar system and implying that Pluto is not a planet. On Friday, Neil de Grasse Tyson, director of the Hayden Planetarium at the center, explained the center's treatment of Pluto in an open letter to the Cambridge-Conference Network, an international network of asteroid researchers and planetary scientists who have advocated a change in Pluto's status for more than two years.

With permission from CCNet, Dr. Tyson's letter is reproduced below:

Regarding our exhibits in New York City's new Rose Center for Earth and Space, I am surprised and impressed by the amount of recent media attention triggered by our decision to treat Pluto differently from the other planets in the solar system.

I am surprised because our exhibit has been in place since opening day, 19 February 2000, and our treatment didn't seem to be newsworthy at the time. I am impressed that people feel so strongly about Pluto that much time and attention had been devoted to it in print and on the air.

The New York Times' front page article, which ignited the recent firestorm, donned a title that was somewhat afield of what we actually did, and which I would like to clarify. The title read "Pluto Not a Planet? Only in New York," which implied that we kicked Pluto out of the solar system and that we are alone in this action and that, perhaps more humorously, Pluto wasn't big enough to make it in NYC.

I have written previously on the subject, in an essay titled "Pluto's Honor" (Natural History, February 1999) where I review how the classification of "planet" in our solar system has changed many times, most notably with the 1801 discovery of the first of many new planets in orbit between Mars and Jupiter. These new planets, of course, later became known as asteroids. In the essay, arguing in part by analogy with the Asteroid Belt, I argued strongly that Pluto, being half ice by volume, should assume its rightful status as the King of the Kuiper Belt of comets. Apart from my views expressed there, I have a different sort of responsibility to the public as director of the Hayden Planetarium and as project scientist of the Rose Center for Earth & Space.

That responsibility is as an educator for a facility that has received an average of 1,000 people per hour over the past eleven months. For the exhibit on planets in our "Hall of the Universe," rather than use the word planet as a classifier, we essentially abandon the ill-defined concept and simply group together families of like-objects. In other words, instead of counting planets or declaring what is a planet and what is not, we organize the objects of the solar system into five broad families: the terrestrial planets, the Asteroid Belt, the Jovian planets, the Kuiper Belt and the Oort Cloud. With this approach, numbers do not matter and memorized facts about planets do not matter. What matters is an understanding of the structure and layout of the solar system. On other panels, in an exercise in comparative planetology, we highlight rings, storms, the greenhouse effect, surface features and orbits with discussions that draw from all members of the solar system where interesting and relevant.

Our intro-exhibit panel meets the visitor's expectations head-on:

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"What is a planet?"

"In our solar system, planets are the major bodies orbiting the Sun. Because we cannot yet observe other planetary systems in similar detail, a universal definition of a planet has not emerged. In general, planets are massive enough for their gravity to make them spherical, but small enough to avoid nuclear fusion in their cores."

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A second panel, describes and depicts the layout of the solar system:

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"Our Planetary System"

Five classes of objects orbit our Sun. The inner terrestrial planets are separated from the outer gas giant planets by the Asteroid Belt. Beyond the outer planets is the Kuiper Belt of comets, a disk of small icy worlds including Pluto. Much more distant, reaching a thousand times farther than Pluto, lies the Oort Cloud of comets."

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Our goal was to get teachers, students and the average visitor to leave our facility thinking about the solar system as a landscape of families rather than as an exercise in mnemonic recitation of planet sequences.

That being said, I have benefited from some reasoned feedback on what we have done. As many are already aware, we use our giant 87-foot (26.5-meter) sphere (housing the Hayden Space Theater in the upper half and a recreation of the first three minutes of the Big Bang in the lower half) as an exhibit unto itself. We invoke it to compare the relative sizes of things in the universe for a walk-around "powers of 10" journey that descends from the observable universe all the way to atomic nuclei. About midway in the journey you come upon the size scale where the sphere represents the Sun. On that scale, hanging from the ceiling, are the Jovian planets (the most highly photographed spot in the facility) while a set of four small orbs are also on view, attached to the railing. These are the terrestrial planets. No other members of the solar system are represented here. This entire exhibit is about size, and not much else. But the absence of Pluto (even though the exhibit clearly states that it's the Jovian and terrestrial planets that are represented) has led about 10 percent of our visitors to wonder where it is.

In the interest of sound pedagogy we have decided to explore two paths: 1) Possibly add a sign at the right spot on the size scales exhibit that simply asks "Where's Pluto" and gives some attention to why it was not included among the models. 2) We are further considering a more in-depth treatment of the life and times of Pluto to add to our kiosks, which contain our computer-searchable data base of current astrophysics news that we display in a timely fashion on a video "bulletin" wall. This material might even contain a sampling of the various points-of-view expressed on how planets should be counted for those who feel compelled to do so.

I close with the opinion that a mid-ex style mission to Pluto might resonate much more deeply with the public and with Congress if instead of saying "we must complete the reconnaissance of solar system's planets by sending probes to Pluto," we say "we must begin the reconnaissance of a newly discovered, and hitherto uncharted swath of real estate in our solar system called the Kuiper Belt, of which, Pluto reigns as king."

Respectfully Submitted,

Neil deGrasse Tyson

Department of Astrophysics & Director, Hayden Planetarium Division of Physical Sciences

American Museum of Natural History