Friday, November 21, 2008

Planet Pluto, Rest In Peace...

Tonight I attended a public lecture - the Marc Aaronson Memorial Lecture given this year by Michael Brown of Cal Tech. The title was a provocative "How I Killed Pluto And Why It Had It Coming".

Marc Aaronson was an astronomer at the University of Arizona who died in 1987 while observing at Kitt Peak National Observatory. While on the staff there, I worked with Marc many times, so it is always poignant to attend the lectures named for him.

Dr. Brown gave a great, entertaining talk. Unfortunately, Melinda was unable to attend as she was working tonight, but she would have enjoyed it. She was strongly in the camp of keeping Pluto a planet, so she pretty much wanted me to make a sign and picket the talk. I suspect she would have been swayed by his arguments...

The gist of the controversy is that Pluto is small, smaller, in fact, than our moon. Unlike the other, much larger planets, it also has an eccentric, inclined orbit, and due to the semi-automated search Michael is conducting, there are now over 1200 "dwarf planets" out in what is known as the "Kuiper Belt" outside Neptune's orbit (Pluto is arrowed in the photo, with it's orbit in blue). It was thought that Pluto was merely the largest of this class of object, but a few years ago, Eris was discovered, which is about 25% larger. There are many others nearly as large as Pluto, so it is obvious that it was only the first discovered Kuiper Belt Object (KPO).

He points out that if all the solar system objects are examined carefully, they can be categorized into 4 groups. They consist of the inner 4 rocky planets (which includes Mercury, Venus, Earth and Mars - the upper 4 circles in the photo), the outer gas giants (Jupiter, Saturn, Uranus and Neptune - the large 4 circles), the asteroids, mostly falling between Mars and Jupiter (those bigger than 1 pixel at this scale representation are drawn below the red disk of Mars), and the Kuiper Belt Objects outside Neptune (also drawn to scale below the blue disk of Neptune if larger than 1 pixel). Eris and Pluto are so small as to be insignificant in comparison to the 8 major planets. Dr. Brown did a lot better job than my feeble attempt - you should have been there! It was also like a homecoming with all the Tucson Amateur Astronomers there - many whom I had not seen in years. It was a great evening!

1 comment:

Laurel Kornfeld said...

I would have been there, but Tucson is a bit too far from NJ for one lecture. Brown is probably glad I wasn't there because I would have taken issue with nearly everything he said.

Pluto is not dead. Significantly, the definition that demoted it was adopted by only four percent of the IAU, most of whom are not planetary scientists. No absentee voting was allowed. It was done so in a highly controversial process that violated the IAU’s own bylaws, and it was immediately opposed by a petition of 300 professional astronomers saying they will not use the new definition, which they described accurately as “sloppy.” Also significant is the fact that many planetary scientists are not IAU members and therefore had no say in this matter at all.

I take issue with Brown's classification of solar system objects and offer an alternative, broader definition of planet supported by many planetary scientists, including Dr. Alan Stern, Principal Investigator of NASA's New Horizons, whose article appeared today in "Science News." You can find it here:

I urge Melinda to read Stern's article; they might strengthen her conviction in Pluto's planet status.

Many believe we should keep the term planet broad to encompass any non-self-luminous spheroidal object orbiting a star.
We can distinguish different types of planets with subcategories such as terrestrial planets, gas giants, ice giants, dwarf planets, super Earths, hot Jupiters, etc.

Pluto IS a planet because unlike most objects in the Kuiper Belt, it has attained hydrostatic equilibrium, meaning it has enough self-gravity to have pulled itself into a round shape. When an object is large enough for this to happen, it becomes differentiated with core, mantle, and crust, just like Earth and the larger planets, and develops the same geological processes as the larger planets, processes that inert asteroids and most KBOs do not have.
Not distinguishing between shapeless asteroids and objects whose composition clearly makes them planets is a disservice and is sloppy science.
As of now, there are three other KBOs that meet this criterion and therefore should be classified as planets--Haumea, Makemake, and Eris. Only one KBO has been found to be larger than Pluto, and that is Eris.

I gave a co-presentation on this topic just a week ago at Amateur Astronomers, Inc. in Cranford, NJ, and attended the Great Planet Debate, which took place in August 2008. There was a strong consensus at that national conference there that a broader, more encompassing planet definition is needed. I encourage anyone interested to listen to and view the conference proceedings at You can also read more about this issue on my blog at

In short, don't mourn; Pluto is not in any way dead.