This article by Jacqueline Hale of Jacqueline's Space Blog does a great job discussing the weaknesses of the IAU decision and the scientific arguments for the geophysical planet definition. I encourage all Pluto supporters to read it.
Fifteen years have passed since the debacle perpetrated by four percent of the IAU when the group violated its own bylaws and adopted a controversial planet definition that it then attempted to impose on the world.
The event was not just, as New Horizons principal investigator Alan Stern accurately described, “an embarrassment to astronomy.” It was also an embarrassment to journalism because the media chose to report that vote as a seismic change to the solar system rather than what it actually is—one position taken by one very limited group of people, rejected by an equal number of people in the field.
The anniversary, especially as one that ends with a “0” or “5,” means it’s time for the usual media reports that present the IAU definition as the only “truth,” with the usual insulting adjectives for those who reject it, referring to us as “disappointed,” “sore,” “dismayed,” etc. Those words are derogatory because they characterize dissenters as people motivated by emotion. The reality is that many, not just a few people, including most of the world’s leading planetary scientists, dissent with the IAU position and reject it in favor of the geophysical definition presented by Kirby Runyon at the 2017 Lunar and Planetary Sciences Conference or other alternative definitions that do not require orbit clearing for planethood.
A kids’ article in the Washington Post even went as far as claiming August 24 is celebrated as an “international holiday,” Pluto Demotion Day, by scientists worldwide. This could not be further from the truth. Unfortunately, after a decade and a half, the media continues to misrepresent this issue, doing a tremendous disservice to readers.
This year, Science News does a better job in discussing the milestone, quoting Kirby Runyon on the geophysical definition and doing so not just in one sentence tagged on to the end. Writer Lisa Grossman even admits she may be coming around to supporting a more inclusive rather than exclusionary definition.
Of course, there is the usual line by Mike Brown, who claims, “Fifteen years ago, we finally got it right. Pluto had been wrong all along,” and “If you dropped me in the solar system for the first time, and I looked around and saw what was there, nobody would say anything other than, ‘Wow, there are these eight — choose your word — and a lot of other little things.’ ”
Brown is wrong on both counts. There is nothing “final” about the 2006 IAU vote, which violated the organization’s own bylaws. Numerous discoveries of additional solar system planets and exoplanets have been made since then. New Horizons revealed Pluto to the world and showed it to have complex planetary processes, some seen elsewhere in the solar system only on Earth and Mars.
No one got anything “right” in 2006. Pluto was never “wrong”; instead, it represented a new subclass of planets that had just started to be discovered. Unfortunately, some could not wait for the spacecraft to reveal this and didn’t care to use its revelations to revise their flawed conclusion.
It is interesting that Brown presumes to speak for aliens who might be “dropped into the solar system” and in doing so, projects his view onto them. What any alien would see would depend on their level of technological advancement and what they are looking for. Some might see only Jupiter and Saturn and think the system has no habitable planets. Others might focus on small planets that could serve as outposts of intelligent civilizations and specifically look for worlds like Pluto and Eris.
Anyone from an advanced civilization would clearly discern that solar system planets come in many types. Rather than see just “eight big things,” they likely would see rocky planets, gaseous planets, and smaller dwarf planets. If they looked at Pluto up close, they very likely would see its round shape, and think planet, as per what Alan Stern describes as the “Star Trek test.”
Actually, if they looked at the Pluto system up close, they would see a binary or double planet system, the only one in the solar system. Rather than orbit Pluto, the four small moons orbit the center of gravity, known as the barycenter, between Pluto and its large moon/fellow planet Charon.
Former IAU president Catherine Cesarsky is quoted stating the very unscientific argument that the solar system cannot have “too many planets” and that it would be difficult to keep changing the number of solar system planets with each new discovery.
Why is the number even important? Finding new solar system planets is exciting! And almost every textbook has a web page that can be instantly updated to reflect new discoveries.
Cesarsky didn’t have New Horizons data back in 2006, but in 2021, referring to Pluto as “a new class of solar system objects” instead of “a new class of solar system planets” has little merit. The spacecraft clearly revealed Pluto to be a planet though the first in a new subclass of planets.
Unfortunately, most science reporting in the mainstream media does not involve a lot of research. Most of the journalists who write these stories are generalists who are not well informed about their subjects and do just cursory research before writing their stories.
In the planetary science world, things are very different, as the Science News article indicates. The IAU definition there has clearly been a failure. It is seldom used by planetary scientists, and new developments have made its logic significantly more tenuous over the last decade and a half.
Take one exoplanet discovery: the L 98-59 system, a red dwarf star with as many as five Earthlike planets, just 35 light years away. Two of those five, which still need to be confirmed, orbit in a different plane than the other three and therefore cannot be detected by the transit method, which searches for the dimming of a star’s light as a planet passes in front of it. These same two are also the star’s largest planets, and one of them may be in the star’s habitable zone.
If planethood requires all objects in a system to orbit on the same plane, then what are these two objects?
The science that has been done in the last 15 years, the discoveries made, are what weaken the IAU definition the most!
Where do we go from here? Over a decade and a half, the IAU has shown complete intransigence in making clear it never wants to revisit this issue, discoveries and new science be damned.
It’s time to forget the IAU and find another path forward.
At the same time, Alan Stern and the New Horizons team have published the textbook of the generation, The Pluto System After New Horizons, which will serve as the textbook for college and graduate studies on the subject.
What we need now is the formation of a planetary science group that will distill the findings in this book and come up with K-12-level discussions of Pluto and its place in the solar system, with which it can approach textbook publishers and educators. These can and should be incorporated into the textbooks’ depictions of the solar system.
A formal organization grants a level of legitimacy that is badly needed to represent the view opposing the IAU definition. Such a group, ideally composed of both professionals and amateurs, can publish articles, newsletters, a web site, and even books that teach the controversy rather than promote one side.
Ultimately, the geophysical definition is not so much about Pluto but about a much more expansive, inclusive, exciting view of the universe, one in which there is always more to discover, more to understand, more to question.
Since the IAU vote, the world has added nearly another billion people to its population. It is time to do right by them and give all eight billion people on Earth the opportunity to know the whole story of the solar system and worlds beyond.
Just four percent of the IAU voted on the controversial 2006 demotion of Pluto, and their decision was immediately rejected in a formal petition by an equal number of professional planetary scientists. Yet nowhere is this mentioned in the article. Instead, the writer simply makes the biased statement, "the ninth planet was no more" regarding the aftermath of the IAU 2006 vote, completely ignoring the fact that the majority of planetary scientists reject that decision to this day.
In this Science Direct paper, planetary scientist Phil Metzger discusses the history of planetary classification and the fact that planetary scientists do not use the IAU definition in their peer-reviewed published papers.
Those who reject Pluto's planet status completely ignore the stunning revelations of active geology and planetary processes on Pluto by the New Horizons probe in 2015.
Nowhere does the article mention the alternative geophysical planet definition, presented by planetary scientist Kirby Runyon at the Lunar and Planetary Sciences conference in 2017 and in a 2018 article in Astronomy magazine. This definition views dwarf planets as a subclass of planets and therefore keeps Pluto, Ceres, Haumea, Makemake, and Eris in the planet list. It is the definition preferred and used by most planetary scientists.
Also problematic is the repeated quoting of Mike Brown, a controversial figure who represents just one side in this debate. Brown unprofessionally promotes himself as the man who "killed" Pluto, and it is disappointing to see the writer repeat this without question and even promote his twitter account. Brown was one of a team of three who discovered Eris, but he did not "kill" planet Pluto, and journalists should not be promoting this nonsense. Notably, the other two members of the discovery team reject the IAU planet definition in favor of the geophysical one.
Brown controversially named the hypothetical planet in question "Planet Nine" to further promote the self-serving notion that he "killed" planet Pluto. In 2018, a group of planetary scientists formally objected to this term in their publication Planetary Science Exploration Newsletter because of its inherent bias. There, they noted that the appropriate professional term for a hypothesized but undiscovered planet is "Planet X," with "X" referring to the unknown, not the number 10.
It is unfortunate that some scientists continue to mislead the public by promulgating their own biased positions as fact, without even acknowledging the existence of a debate or of other legitimate scientific positions. I urge the BBC to be more vigilant in the future when it comes to the planet definition debate to use neutral language that acknowledges the fact that this issue is not settled but remains an ongoing debate in the science community.
Today is the 91st anniversary of Pluto’s discovery by Clyde Tombaugh on February 18, 1930, and in honor of the occasion, the Lowell Observatory held an amazing six-night virtual “I Heart Pluto” festival that just concluded.
For those who may have missed the live events, all programs and presentations area available for viewing on the Lowell Observatory’s YouTube channel.
This was not the observatory’s first “I Heart Pluto” celebration, but it was the first virtual one and therefore the first I attended. The fact that people around the world could attend is a silver lining to a very difficult time, and I hope that even once live events resume, Lowell will do a livestream virtual event alongside the in-person one.
Over the last six nights, there was a lot of discussion of “Pluto culture,” of Pluto’s far-reaching impact, not just in science but in the arts and humanities as well. Pluto artwork is on display and for sale at the I Heart Pluto website.
And in a fortuitous coincidence, NASA’s Perseverance rover successfully touched down on Mars on this 91st anniversary of Pluto’s discovery.
Yes, Pluto culture is a thing. It is a beautiful, inspiring thing, centered on a tiny planet that touches both minds and hearts. Pluto has a pull on so many people; it fascinates and inspires in an intangible way that is difficult to put into words. That type of inspiration is what produces not just great science but also amazing art, music, writing, etc. Once it’s in your blood, it stays forever.
As noted in the event’s closing discussion, the ridiculous IAU vote in 2006, followed by New Horizons’ flyby in 2015, only served to make Pluto more beloved and more popular.
What makes Pluto have lasting power as an astronomical rock star? Is it the fact that it, its discoverer, and the spacecraft that revealed it to the world are plucky underdogs who cannot be kept down? Is it the beauty of its many exotic terrains revealed by New Horizons? Is it that an object that was supposed to be a dead rock turned out to be so geologically active?
Or is it all of these things and more, waiting for us at the frontiers of the solar system?
Ninety-one years ago, in the depths of the Great Depression, people found hope and promise in the unlikely discovery of a new planet. In many ways, today’s pandemic and economic crisis are a reflection of the despair and deprivation so many experienced at that time. On this day of discovery, the success of a very complex maneuver that landed a robotic probe on the Red Planet, and the continued inspiration Pluto is to so many once more offer hope and promise even in the darkest of times.