Monday, February 18, 2013
Responding to the IAU: Pluto and the Developing Landscape of the Solar System
Today is the 83rd anniversary of Planet Pluto’s discovery by 24-year-old Clyde William Tombaugh at the Lowell Observatory in Flagstaff, Arizona, on February 18, 1930. As such, it is an ideal time to write an entry I have been intending to do for a while: a point-by-point response to the IAU “official” statement, “Pluto and the Developing Landscape of Our Solar System,” which can be found at http://www.iau.org/public/pluto/
The Discovery of Pluto
The description of Tombaugh’s discovery is accurate. However, one important detail is missing. The IAU statement says, “Tombaugh was searching for an elusive planet—planet X—that (Percival) Lowell had believed (incorrectly) to be responsible for perturbing the orbits of Uranus and Neptune.”
Not mentioned is that Lowell was far from the only astronomer to believe that an unknown gas giant planet was perturbing the orbits of Uranus and Neptune. Many astronomers of the late 19th and early 20th centuries believed this as well because the two giant planets’ actual orbits didn’t match the orbits astronomers had calculated they should have. But the reason for this was not the existence of another gas giant beyond Neptune. It was simply human error, erroneous calculations. This fact was not recognized until 1986 and 1989, when Voyager 2 flew by both these planets, confirming that nothing was perturbing them.
There is some irony in that the source given as a reference several times in the IAU statement is the book Pluto and Charon: Icy Worlds on the Ragged Edge of the Solar System, by Dr. Alan Stern and Dr. Jacqueline Mitton. Stern, Principal Investigator of the New Horizons mission, is one of the most vocal opponents of the IAU planet definition. It seems the IAU has no problem relying on his work as a reference when doing so is expedient, while at the same time repeatedly ignoring his input on this issue for six-and-a-half years.
The Changing Landscape of the Solar System
While the IAU accepted the name Pluto for Tombaugh’s discovery, it was the Lowell Observatory that named it Pluto, partly at the suggestion of young British girl Venetia Burney. By the time the IAU “officially adopted” the name, it was already a done deal. Why does this matter? The answer is, the chain of events leading up to Pluto being named was not driven or controlled by the IAU. At that time, the IAU did not even concern itself with planets and planetary science. The IAU did not have the type of power its leadership wants people to believe it has always had, and there is no reason for it to have the final say on planetary matters.
The statement goes on to say that, “The view of our solar system’s landscape began to change on August 30, 1992, with the discovery by David Jewitt and Jane Luu from the University of Hawaii of the first of more than 1,000 now-known objects orbiting beyond Neptune in what is often referred to as the trans-Neptunian region.”
Yes, the first trans-Neptunian Object other than Pluto was discovered in 1992, but many astronomers long suspected the existence of such objects, so the discovery was not that much of a surprise. The famous astronomer Dr. Harlow Shapley, in 1930, only months after Pluto’s discovery, questioned whether Pluto was the first in a new class of solar system objects and wondering whether similar bodies remained undiscovered in the outer solar system. In 1943, Dr. Kenneth Edgeworth suggested the existence of a reservoir of comets beyond the orbit of Neptune. Eight years later, Dr. Gerard Kuiper suggested a large region of icy bodies existed out to 120 astronomical units (AU) beyond Neptune. While the existence of this region was not confirmed until 1992, it certainly was anticipated long before then.
The IAU statement goes on to say, “With so many Trans-Neptunian Objects being found, it seemed inevitable one or more might be found to rival Pluto in size.” What is not explained is why the existence of an object of similar size would influence Pluto’s classification. How does it make sense to classify one body by the characteristics of another? Pluto’s classification should be based on its own characteristics, not those of Eris or Ceres, or any other celestial body. Would the existence of other “Earths” change the designation of our own planet?
The discovery of Eris is accurately credited to the team of Mike Brown, Chad Trujillo, and David Rabinowitz, yet the references at the bottom of the page cite only Brown, and nowhere in the article is acknowledgement that Rabinowitz opposed the IAU decision, as evidenced in his signing Alan Stern’s petition of 300 astronomers on August 30, 2006. Additionally, the article does not acknowledge that the initial view of Eris being larger than Pluto has been proven incorrect. Eris is described as being “around 2500 km across.” In November 2010, when Eris occulted a star, that measurement was revised downward to approximately 2340 km., in comparison with Pluto, whose size is estimated at 2344 km. Why was this information never updated on the IAU page? Eris is believed to be about 27 percent more massive than Pluto, which only indicates it is more rocky and therefore, more like the terrestrial planets. Pluto is estimated to be 70 percent rock, a fact often lost by people describing it as an “iceball.”
The IAU article notes that Eris, too, was found to have a satellite. Yet Pluto is now known to have five satellites, or moons (four if you count Pluto-Charon as a binary planet system), yet the IAU leadership insists this changes nothing and refuses to reopen discussion on the issue. Does having moons make a difference in an object’s classification? Based on this article and subsequent reactions to new discoveries, one can only conclude that it matters when the IAU leadership decides it matters but means nothing when that same leadership wants it to be irrelevant.
“With an object larger and more massive than Pluto now beyond Neptune and ever more of these Trans-Neptunian Objects being discovered, astronomers were beginning to ask: ‘Just what constitutes a planet?’” Over two years have passed since the discovery that Eris is not larger than Pluto, yet the IAU has never updated this information.
And astronomers have been asking, “just what constitutes a planet” ever since Galileo in 1610 discovered four “planets” orbiting Jupiter. Inherent in the IAU statement is the bias that “we cannot have too many planets.” More and more galaxies have been discovered as the Hubble Telescope has peered further back into the universe, yet no one has argued a need to determine what constitutes a galaxy because we now have too many. Never mind that to this day, no object larger than Pluto has been discovered in the Kuiper Belt. The sentence clearly turns away from scientific analysis to a subtle favoring of one interpretation.
A New Class of Objects and How to Define A Planet
Dr. Owen Gingerich, who served as chair of the IAU Planet Definition Committee, is quoted saying, “On the scientific side, we wanted to avoid arbitrary cut-offs simply based on distances, periods, magnitudes, or neighboring objects.”
But the IAU definition does everything Gingerich urges against! Its decision is clearly based on distances because the further an object orbits from its parent star, the larger an orbit it will have to “clear.” If Earth were in Pluto’s orbit, it would not “clear” that orbit of Kuiper Belt Objects either.
Similarly, the IAU decision is definitely based on neighboring objects! Pluto is classified not by its characteristics but by the existence of other objects—tiny Kuiper Belt Objects—in its orbit. The debate over Pluto’s status came about because of another object—Eris.
And Gingerich himself was kept out of the loop, never told that his own committee’s resolution would be rejected in favor of another one hastily thrown together. At the Great Planet Debate, he admitted that had he known this was going to happen, he would have canceled his early flight and stayed to the end of the conference.
While the article says the IAU has been responsible for the nomenclature of planetary bodies since the early 1900s, we already know this has not exactly been the case. The Lowell Observatory sent the suggestion of the name Pluto to the American Astronomical Society and the British Royal Astronomical Society, but not to the IAU.
Subsequent discussion describes the IAU as having created a committee representing a broad range of scientific interests to create a new definition of planet. What is not stated is that at the 2006 General Assembly, the IAU rejected the draft resolution created by that very committee and instead threw together an alternate resolution at the last minute, completely bypassing that committee and violating IAU bylaws, which require all resolutions to first be vetted by the appropriate committee before being placed on the floor of the General Assembly.
Former IAU president Ron Ekers, who opposed the 2006 decision, is quoted as having said, “Such decisions and recommendations are not enforceable by any national or international law; rather they establish conventions that are meant to help our understanding of astronomical objects and processes. Hence, IAU recommendations should rest on well-established scientific facts and have a broad consensus in the community concerned.”
The IAU leadership accurately claims its decision is not enforceable but has spent more than six years insisting that educators, textbooks, and the media use its definition as the sole “official” planet definition.
What about the need for decisions “to rest on well-established scientific facts and have a broad consensus in the community concerned?” Well, the IAU document has erroneous, outdated statements when it comes to scientific facts, and over more than six years, the organization has never allowed the discovery of new facts to influence its decision. As for a broad consensus in the astronomy community, that never happened. When 300 professional astronomers sign a petition rejecting a decision, when those same astronomers ask for the discussion to be re-opened three years later and are denied, when the leading expert on Pluto in the world and the head of the only space mission two years from rendezvous with Pluto repeatedly describes that decision as “an embarrassment to astronomy,” it is clear no such consensus exists today or ever existed.
The Final Resolution
The chaotic and flawed process by which the resolution demoting Pluto was crafted is whitewashed with the words, “a new version slowly took shape,” which was “more acceptable to the majority…” In what universe does slowly taking shape equate to something thrown together over a few hours? Many astronomers at the Closing Ceremony, where the vote took place, did not see the text of the resolution until it came to them for a vote that day. As for a majority, well, the majority of the 2,500 attendees had already gone home assuming the resolution recommended by the IAU committee would be voted on. Those not attending the conference weren’t eligible to vote. In whose math is 424 equal to a majority of 2,500? And of those 424 (really 423 since one astronomer has since admitted to having been warned of “consequences” to his/her career if he/she did not vote against Pluto), 91 voted in favor of Resolution 5b, which would have established dwarf planets as a subclass of planets.
There was no majority. A total of 333 IAU members, most of whom came with the agenda of creating a definition that excluded Pluto, determined that dwarf planets are not planets at all, misusing the term first coined by Dr. Stern in 1991.
Dwarf Planets, Plutoids, and the Solar System Today
We already know that it was never “agreed” that planets and dwarf planets should be two distinct classes of objects although one could accurately say it was imposed. Even fewer astronomers had input into the creation of the term “plutoids,” which was done in a classic backroom deal. Planetary scientists interviewed by the media, including Brown, who supported the IAU decision, all said they knew nothing of how that decision was reached or that it had been under consideration. Just who did come up with this term? Why is such a term even necessary when these objects can be characterized as trans-Neptunian dwarf planets?
Does the term “plutoids” help our understanding of astronomical objects? We have a separate class of dwarf planets that does not include Ceres. Some dwarf planets are “plutoids”; others are not. Objects that share Pluto’s orbital resonance with Neptune are called “plutinos,” but these are tiny, non-spherical objects, not dwarf planets. Are you confused yet?
Planets, Dwarf Planets, and Small Solar System Bodies
Get ready to be more confused. Here we have a big mess. Dwarf planets are supposed to be a class separate from Small Solar System Bodies, the new name for asteroids and comets, often referred to as “minor planets.” However, the IAU assigned all dwarf planets minor planet numbers! If these objects are not minor planets, why give them minor planet designations, completely blurring the distinction between tiny, shapeless asteroids, comets, and Kuiper Belt Objects on one hand and the complex worlds rounded by their own gravity that dwarf planets are on the other hand.
There are clearly more objects beyond Neptune large enough to be in hydrostatic equilibrium, but to this day, none has been given the designation of dwarf planet by the IAU. The IAU statement describes how Ceres was considered an asteroid for more than a century, but never clarifies that it no longer should be classed as such because it is now known to be spherical. Pluto is described as residing “within a zone of other, similarly-sized objects known as the Trans-Neptunian region,” yet the overwhelming majority of Kuiper Belt Objects are nowhere near Pluto’s size, and the other trans-Neptunian dwarf planets do not orbit anywhere near Pluto. Eris orbits in the Scattered Disk, the furthest region of the Kuiper Belt.
New data from the Dawn mission that calls into question Vesta’s status as an asteroid is not even mentioned.
Should Charon be considered a dwarf planet? With the discovery of Pluto’s fourth and fifth moons, both of which are in resonance with Charon, the answer is likely yes. The IAU statement says this may be considered later, but don’t hold your breath. Discussing the dwarf planethood of Charon inevitably brings up the subject the IAU leadership is doing everything to avoid discussing—the planetary status of Pluto and all dwarf planets.
As of yet, there have been no discoveries of objects larger than Pluto in the outer solar system. Some researchers believe there could be a gas giant orbiting the Sun far beyond Pluto’s orbit, yet to be discovered. According to the IAU definition, that object likely would not be considered a planet either because it would inevitably have tiny Kuiper Belt Objects in its path.
In a Q and A, the IAU poses the question of how new discoveries will be classified, whether as planets, dwarf planets, or small solar system bodies. The answer: "The decision on how to classify newly discovered objects will be made by a review committee within the IAU. The review process will be an evaluation, based on the best available data, of whether or not the physical properties of the object satisfy the definitions. It is likely that for many objects, several years may be required to gather sufficient data."
If the IAU isn’t even using the “best available data” and seeking input from leading planetary scientists now, how can we trust they will do so in the future?
Why can a spherical satellite of a dwarf planet not be considered a dwarf planet itself—because Resolution 5b says so? That doesn’t sound like scientific reasoning to me.
In plain language, an equally legitimate scientific definition of planet is, a planet is an object orbiting a star and large enough to be rounded by its own gravity. Some planets clear their paths around the Sun; some do not. The latter are dwarf planets. An object may not even need an orbit or a host star to be a planet, as astronomers have now seen “rogue” planets wandering in space not orbiting any star.
According to this, the geophysical planet definition, our solar system has 13+ planets: Mercury, Venus, Earth, Mars, Ceres, Jupiter, Saturn, Uranus, Neptune, Pluto, Haumea, Makemake, and Eris. Charon could very well be considered a dwarf planet, and so should the other known spherical Trans-Neptunian Objects. Spherical moons of planets should be considered “satellite planets.”
None of this information is new, and all of it has appeared in other blog entries. But with educators directing students to the IAU site for a discussion of the Pluto question, these responses need to be reiterated, if only to bring fairness and balance to the debate.
Happy 83rd anniversary of Planet Pluto’s discovery!
Boyle, A., 2009, The Case for Pluto
Petition of 300 Astronomers Rejecting IAU Decision, August 2006: http://www.ipetitions.com/petition/planetprotest/
Stern, S.A., and Levison, H.F., 2000, Regarding the Criteria for Planethood and Proposed Planetary Classification Scheme, Transactions of IAU
The Great Planet Debate, Johns Hopkins University Applied Physics Lab, August 2008: http://gpd.jhuapl.edu/
Tombaugh, C.W., and Moore, P., 1980, Out of the Darkness: The Planet Pluto
Weintraub, D., 2007, Is Pluto A Planet