Space.com is usually a very credible information site about all matters space and astronomy, which is why its October 23, 2022 article, “Why is Pluto Not a Planet?” is so disappointing in its blatant one-sidedness.
More than 16 years after the controversial IAU vote on a highly flawed planet definition, it is difficult to understand why this site would publish an article that is extremely selective in every one of the sources it uses as references. From the IAU web page to Mike Brown to Ethan Siegel, formerly of Forbes, to a Neil Tyson video, the article refers almost solely to supporters of the IAU definition and completely excludes the many essays and articles written from the opposing point of view.
The article is problematic starting with its title, which, instead of acknowledging the ongoing debate over planet definition and Pluto’s status, simply states the IAU view as fact. This misleading title essentially presents the IAU view as “the truth” rather than as one view in an ongoing debate, and this notion is repeatedly assumed in the article, in spite of very sparse references to dissenting views.
Yet, there is absolutely NO reason to give the IAU position privilege as somehow being the “official” one in use. It is simply one of several definitions currently used by planetary scientists.
We need to address the factual errors in this article. Like many writings on this subject over the last 16 years, the writers cite Ceres’s discovery as a planet and subsequent demotion to asteroid in the mid-19th century as an earlier example of what happened to Pluto. However, they fail to acknowledge the other, crucial part of Ceres’s history.
Because it is very small, Ceres could not be resolved into a disk by 19th century telescopes. At the time, it therefore made sense to demote it to one of many asteroids in the belt between Mars and Jupiter. However, since then, Ceres has been found to be spherical, meaning unlike nearly every asteroid in that belt, it is squeezed into a round or nearly round shape by its own gravity. Most asteroids are simply rubble piles shaped by chemical bonds. The threshold for an object being spherical, in a state known as hydrostatic equilibrium, is crucial because this is when objects begin to experience geology and the complex processes seen on rocky planets.
In 2015, NASA’s Dawn mission revealed Ceres to have these complex features, including cryo-volcanoes and a possible subsurface ocean that could potentially host microbial life. These findings indicate the 19th century demotion was in error and that Ceres is more like the larger rocky planets than almost all objects in the asteroid belt, with the exception of Vesta and Pallas, which appear to have once been spherical only to have had a large portion knocked off during impacts with other objects.
While some, including the writers of this article, invoke Pluto’s eccentric orbit and 17-degree tilt to the ecliptic as reasons for demoting it from planethood, the fact is several exoplanet systems contain multiple giant planets all orbiting in different planes. At least two systems have giant planets that cross one another’s orbits. And the ecliptic, often wrongly depicted as the path of the Sun, is actually the plane of the Earth in its solar orbit. Requiring objects to orbit in the same plane as Earth is a violation of the Copernican principle, which essentially states that Earth is just another planet, not the center of anything. And if giant objects that cross the orbits of other giant planets in their systems are not planets, what then are they?
The next inaccuracy in the Space.com article is its attribution of Eris’s discovery solely to Mike Brown. While Brown often presents himself as Eris’s sole discoverer, the fact is, Eris was discovered by a team of three scientists, the other two being Chad Trujillo and David Rabinowitz. Significantly, both Trujillo and Rabinowitz reject the IAU planet definition. Rabinowitz even signed a petition with hundreds of planetary scientists back in 2006 in response to the IAU definition, saying they would not use it.
The reference to the late Brian Marsden is also problematic, as Marsden, who apparently had a long feud with the late Pluto discoverer Clyde Tombaugh, was obsessed for decades with demoting Pluto so it could be put under his auspices at the IAU Minor Planet Center. Like Brown, he is a scientist who appears to have had his own agenda from the start.
While the article does acknowledge that only 424 IAU members voted on the controversial planet definition in 2006, it fails to note that this vote was held in violation of IAU bylaws, which prohibit putting a resolution to the floor of a General Assembly without first vetting it by the proper IAU committee. It also fails to mention that these 424 people were largely not planetary scientists but other types of astronomers, that they represented only four percent of the IAU’s membership, that no electronic or absentee voting was permitted, that 91 of the 424 voted for dwarf planets to be classed as a subclass of planets, and that an equal number of professional planetary scientists signed the afore-mentioned petition rejecting the IAU definition within days of its adoption.
Additionally, the writers fail to mention that when initially coined by Alan Stern in 1991, the term “dwarf planet” was intended to designate an additional class of small planets, not to designate a class of non-planets. In its unauthorized vote, the IAU essentially misused the term “dwarf planet” based on the fiction that dwarf planets are fundamentally different compositionally than their larger counterparts, a position proved false by the findings of the Dawn and New Horizons missions.
Even Neil Tyson is on record saying he has no problem with dwarf planets being considered a subclass of full planets!
As the article writers note, New Horizons found Pluto to have active geology; diverse features including windswept dunes, varied terrains, a layered atmosphere, cryo-volcanism, interaction between atmosphere and surface, and a likely subsurface ocean. Some complex features discovered on Pluto exist elsewhere in the solar system only on Earth and Mars!
To make matters worse, the article never acknowledges the alternative geophysical planet definition, which is preferred by most planetary scientists, beyond a vague mention of a 2017 proposal that “defined a planet as a round object in space that’s smaller than a star.” This is a major disservice to the proposal, presented to the Lunar and Planetary Science Conference that year, which centers planet definition on an object’s intrinsic properties rather than on its location, which the IAU definition does.
Most problematic is the article’s inherent assumption that only the IAU has the ability or power to determine what a planet is. Conveyed throughout this article, that sentiment can be seen in the statement noting Alan Stern and David Grinspoon’s 2018 Washington Post article urging reconsideration of planet definition “have fallen on deaf ears so far, and it seems unlikely theIAU will revisit the controversy any time soon.”
If such pleas to the IAU fall on deaf ears, and the organization refuses to
address new data returned by the Dawn
and New Horizons missions, then it is
time to consider other venues for this discussion. The notion of science being
decided by any type of “authority” went out with Galileo’s discovery of Jupiter’s
moons over 400 years ago. A science publication like Space.com should know better than to appeal to any “authority”
regarding scientific matters.
Finally, the article falls back on attributing opposition to the IAU decision to emotion, as seen in its Mike Brown quote, “Nostalgia for Pluto is not a very good argument, but that’s basically all there is. Now, let’s get on with reality.”
This comment is not only demeaning; it is also blatantly false. Scientists, amateur astronomers, and members of the public oppose the IAU definition not because of nostalgia or emotion but because the geophysical definition, which classifies objects first and foremost by their intrinsic properties, simply makes more sense based on what we have discovered about these objects. Brown does not get to unilaterally end the debate and get the last word simply because he wants to do so.
There are so many scientists and publications that articulately present the other side of this debate. I urge Space.com to do a better job in presenting this fascinating controversy to give readers a comprehensive understanding of both positions, allowing them to decide for themselves where they stand.
The authors of this article are right on one thing: This debate will continue for the foreseeable future.
Sources in response to those presented
in the Space.com article:
Note: I wrote the response to the IAU statement and to Siegel’s Forbes article and embrace my role in
this debate on Twitter with the handle @plutosavior
.
I have covered the New Horizons mission for the website Spaceflight Insider since 2014 though the opinion expressed here is solely my own and not necessarily that of the site or its editors.
“Responding to the IAU: Pluto and the Developing Landscape of the Solar System,” a point-by-point rebuttal to the IAU statement:
http://laurelsplutoblog.blogspot.com/2013/02/responding-to-iau-pluto-and-developing.html
https://www.hou.usra.edu/meetings/lpsc2017/eposter/1448.pdf
“An Organically Grown Planet Definition,” by Alan Stern and Kirby Runyon, https://astronomy.com/magazine/2018/05/an-organically-grown-planet-definition#.Wv3x0d97CCI.blogger
