The Whole Story of Pluto: Responding to Elizabeth Stanway of the University of Warwick

Dr. Elizabeth Stanway’s article, “The Story of Pluto” is highly problematic on several levels, starting with the fact that instead of actually telling the story of Pluto, it tells one side of that story, a story that remains a matter of ongoing debate to this day.

 Notably, Stanway is not a planetary scientist but, according to her web page at the University of Warwick, a specialist in observational cosmology. In spite of this, the University of Warwick had her write an article about Pluto, a subject she does not study. Could they not find a planetary scientist to write this?

Not once in her article does Stanway ever mention the fact that Pluto’s status and the question of how to define the term planet remains a controversy with two legitimate sides. She begins by saying that adults and books more than 10 years old portray the solar system as having nine planets, in contrast with newer books that portray it as having eight. This is already untrue, as many books have been published over the last decade that teach the controversy rather than present one view of it as gospel truth.

She fails to mention that the controversial IAU planet definition was adopted by just four percent of its members, most of whom were not planetary scientists but other types of astronomers, and that their decision was immediately opposed by an equal number of planetary scientists in a formal petition led by New Horizons principal investigator Alan Stern, one of the leading Pluto scholars in the world.

Neither does she acknowledge the alternate geophysical planet definition, which was presented to the Lunar and Planetary Sciences conferences in 2017 and 2018, garnering significant media attention.

Stanway goes on to say that “the planets we knew about in the outer solar system” are composed of cold gas and ice, in contrast to Pluto, which is made up mostly of rock. What she doesn’t say here is that this makes Pluto similar to the terrestrial planets, which are also made of rock. There is no scientific rationale for the implied claim that rocky planets cannot exist in the outer solar system. Because she wants to make the point that Pluto is different, she contrasts it only with one class of planets, all but ignoring the other classes.

Her claim that Pluto is not much larger than the asteroids Ceres and Pallas is quite a stretch. Pluto has a diameter of 1,473 miles. In contrast, Ceres has a diameter of 580 miles, and Pallas has a diameter of 320 miles. These are not small differences.

The actual division here is between Ceres and Pluto on the one hand and Pallas on the other. Ceres and Pluto are spherical, rounded by their own gravity, while Pallas comes close to this threshold but is not quite there. According to the geophysical definition, this makes Ceres and Pluto planets of the dwarf planet subcategory. Pallas remains a protoplanet, or a planet that never completely formed.

Next, Stanway mentions Pluto’s “weird orbit,” which she describes as “tilted compared to all the other planets,” somehow failing to mention that Mercury is also tilted compared to most solar system planets. Furthermore, several exoplanet systems have been discovered in which multiple giant planets all orbit in different planes. If tilted orbits preclude objects from being planets, what then are these large exoplanets?

In discussing the discovery of the largest Kuiper Belt Objects starting around the year 2000, she describes Quaoar, Sedna, Orcus, Makemake, and Eris as “big space rocks,” with the subjective claim that “some of them looked uncomfortably similar to Pluto.” Why does she use the word “uncomfortably?” It seems like she is organizing the information to make a case that they cannot all be planets. In other words, she is using the same old argument that the solar system cannot have “too many planets,” an argument that has no scientific merit whatsoever.

Her description of the situation facing the IAU in 2006 is so subjective that it is laughable. She says, “Generations of people had grown up thinking Pluto was a planet and they didn’t want to let it go but the evidence was overwhelming: whatever the IAU decided would change our picture of the Solar System forever.”

Yet there was no such overwhelming evidence. The only “evidence” here showed that the solar system has more small planets than anyone previously thought. There was never an overwhelming consensus at the 2006 General Assembly that Pluto should be reclassified. In fact, the IAU’s own Planet Definition Committee, which had met for months in preparation for addressing this issue at the General Assembly, actually presented a resolution that included Ceres, Pluto, and Eris in the roster of planets. This was voted down, at which time proper procedure according to IAU bylaws called for the issue to be sent back to the proper committee.

Of course, we know that didn’t happen. Instead, the IAU violated its own bylaws, which require resolutions to first be vetted by the appropriate IAU committee before being put to the General Assembly, and several dynamicists who had their own agenda of wanting Pluto out hastily threw together a second resolution on the last day of the conference. By then, the chair of the Planet Definition Committee, Owen Gingerich, had gone home, as had most participants, assuming the issue would not be addressed again until the next General Assembly. Since no absentee voting was allowed, out of 10,000 members, just 424 took part in the vote. Of those 424, 333 decided that dwarf planets, a term first coined by Alan Stern in 1991, are not planets at all but a different type of object entirely.

This statement was not borne out by New Horizons, which revealed Pluto to have the same planetary processes and structures seen on the terrestrial planets, including some processes seen elsewhere in the solar system only on Earth and Mars.

Additionally, science is not done by voting but by consensus that develops over time based on new discoveries. Somehow, Stanway has no problem with throwing out the scientific method, instead appealing to “authority.”

Written in overly simplistic and somewhat condescending language, Stanway’s story concludes by saying Pluto “couldn’t be a planet anymore” because it doesn’t clear its orbit and because the IAU issued a ruling. She belittles opponents for “not wanting to let Pluto go,” completely ignoring the science-based objections to the IAU definition.

Never does she address the fact that orbit clearing is a poor, questionable requirement for planethood, as it is based solely on an object’s location and is biased against planets further from their parent star, which have larger orbits to “clear.” Planetary scientists have determined that if Earth were placed in Pluto’s orbit, it would not clear that orbit either. Therefore, the IAU definition results in the absurd situation of the same object being classed as a planet in one location and not as a planet in another location.

Nor does Stanway say even one word about the findings of the New Horizons mission. It seems she is fine with science by decree of “authority,” which went out 400 years ago with Galileo, but has no problem ignoring the data, which is what science is supposed to be about. Her article literally says nothing about Pluto’s intrinsic properties because for the IAU definition, those properties don’t matter—only location does.

Dwarf planets are not “a whole new type of object.” They are simply small planets. Ceres and Pluto both have subsurface oceans that could potentially host microbial life, and this could be the case for the other Kuiper Belt planets as well. We won’t know until we send probes to all of them.

Stanway’s story is an oversimplification that completely glosses over the ongoing debate and subsequent discoveries. It amounts to, the solar system only has eight planets because the IAU says so. This is a disservice to readers, and it is disappointing to see the University of Warwick publish such a one-sided article.