20 Years Later: Pluto Is Still A Planet, and the IAU Is Still Wrong

 

One of the many pro-Pluto websites that sprang up in the immediate aftermath of the disastrous IAU planet definition vote back in 2006 featured the image of a counter depicting Pluto’s time taken away since the vote. That counter updated itself every day, updating the days, months, and eventually years since the travesty.

That website has long since gone down—though various others remain—but many are still counting Pluto’s time taken away in terms of planetary recognition. For those of us doing this, today is an especially difficult anniversary, one we long hoped we would never see.

Twenty years. Two decades of a very flawed, very biased planet definition wrongly portrayed by many in the mainstream media as the final answer it most certainly never was. Two decades of sloppy reporting that completely obscured the reality of the ongoing debate over planet definition, feeding the public a one-sided view that is more politics than science.

This should never have gone on for so long. The reason it has is because of the effective stalemate in the fields of astronomy and planetary science. Given the backlash against their definition, which was adopted by just 4% of the IAU in a vote that violated its own bylaws, it is no wonder the organization has shown no interest in reopening the discussion, even in the wake of new exoplanet discoveries.

The IAU planet definition is largely ignored by most planetary scientists, many of whom prefer the geophysical definition instead. These scientists, the chief dissenters to the IAU definition, want nothing to do with the IAU and have no intention of ever approaching the organization on this issue. The result is the current stalemate, with two camps that essentially operate independently of one another, with little dialogue between them.

Significantly, the IAU planet definition applies only to our own solar system, as it requires a planet to orbit the Sun rather than a star. At the 2006 session where the vote took place, IAU leaders promised to separately address exoplanets, but over 20 years, this has not been done.

In 2006, the number of known exoplanets was around 300. Today, NASA’s Exoplanet Archive counts a total of 6,354 while the European Extrasolar Planets Encyclopedia counts 8,284.

Small or dwarf exoplanets have not yet been found, but their discovery remains a matter of time.

There will be those who hail this anniversary as a triumph of science, who claim it is proof the world has “moved on” from the planet debate. Some continue to claim that opposition to the IAU definition is based solely on emotion. Both of these views are false.

Many incorrect and/or questionable theories in science have persisted for far more than 20 years. The geocentric view of the solar system dominated astronomy for millennia. Harmful medical practices such as bloodletting prevailed for centuries. Germ theory and the importance of hand washing by doctors treating patients were long ridiculed before finally become recognized practice.

The passage of time does not magically make bad science good. It does not make incorrect theories correct. And it does not make a flawed decision made by a tiny group of scientists with their own inherent bias any more true or accurate.

IAU leadership cited the discovery of Eris and other Kuiper Belt small planets as the compelling reason for enacting the group’s planet definition. Yet, nine years later, they never considered New Horizons’ findings at Pluto, which clearly revealed it to be a geologically active world, as an equally compelling reason to reopen the discussion.

Anyone can cherry pick the facts and developments they choose to support the position they want to advocate.

Meanwhile, the IAU has made no attempt to incorporate exoplanets into its definition or to address the question of rogue planets, which float freely in space without orbiting any star. Neither have they addressed the issue of brown dwarfs, which fall between the largest planets and the smallest, least massive stars.

An organization that cannot admit it may have made a mistake, that cannot incorporate new information into its scientific understanding for such a long period of time, has no business claiming itself to be an “authority” on such matters or justifying its claim that it alone has the right to adopt “official” scientific definitions.

Science is a process done by consensus based on accurate and replicable data. It is not something decreed by a self-proclaimed “authority.” That went out with Galileo over 400 years ago.

Today, people’s trust in science is at an all-time low, regarding a host of topics ranging from climate change to the effectiveness of vaccines. The actions of the IAU not just in 2006 but over the past 20 years only aggravate this situation, promoting the false view that an edict by a tiny, unaccountable group of people can determine what something is.

Dwarf planets are planets, just like dwarf stars are stars, and dwarf galaxies are galaxies. The two dwarf planets explored by spacecraft, Ceres and Pluto, have revealed to have the same complexity and processes as their terrestrial counterparts. The only difference is that they are smaller.

Eccentric orbits are not unique to Pluto or dwarf planets. Giant exoplanets have been found, some with orbits around their stars far more eccentric than Pluto’s orbit around the Sun. If these giant exoplanets aren’t planets, what are they?

Meanwhile, Pluto’s story continues to inspire people worldwide, including children born long after the IAU vote.

As noted in an article published last year in Morning Overview, “Pluto is unique in many ways. Its size and composition are comparable to other planets in the solar system, albeit smaller. It has a rocky core surrounded by a mantle of water ice, and its surface is covered in nitrogen ice, with traces of methane and carbon monoxide. This composition is not unlike that of terrestrial planets, further blurring the lines between Pluto and its larger counterparts.”

That same article acknowledged the worldwide influence by those of us around the world who have been publicly rejecting the flawed IAU definition for two decades.

“Popular culture, through media and educational systems, has maintained Pluto’s image as a planet…Grassroots movements and public interest can significantly influence scientific decision-making, as seen in past scientific debates and reclassifications. As discussions continue, the potential reclassification of Pluto as a planet remains an intriguing possibility that could reshape our understanding of the solar system.”

 Our efforts have never been futile. They have been and are making a difference. The fight for Pluto and for a better planet definition continues. This is far from over.

 

 

New Study: Pluto's Atmosphere: Hazy with Signs of Thinning

In this latest study published in the Planetary Science Journal, researchers measure a thinning of Pluto's atmosphere as it recedes from the Sun. Whether the atmosphere completely freezes out as Pluto approaches its aphelion (furthest point from the Sun) in 2114 remains unknown.

Alan Stern on the Colin McEnroe Show: What's Done Is Done...Or Is It?

Just days after the 11th anniversary of New Horizons' 2015 Pluto flyby, mission Principal Investigator Alan Stern was interviewed on NPR's Colin McEnroe show on the subject of "making the case for undoing." Stern is the third of three people interviewed--the other two addressed different topics. You can listen to Stern's interview at https://www.ctpublic.org/show/the-colin-mcenroe-show/2026-07-20/whats-done-is-done-or-is-it-we-make-the-case-for-undoing starting at 37:07.

"Go undo something that's wrong!" McEnroe directs his listeners. You can do that by joining the Pluto resistance and spreading the word: Pluto--and all dwarf planets--are planets!

Will Pluto Become A Planet Again? Today, 1 pm Eastern time, Alan Stern and Mike Brown

https://www.youtube.com/live/tIr7RO4qAvU?si=NtpjeX78v_ME1xjb

The Tide Is Turning

When longtime supporters of the IAU planet definition publicly come out rethinking their positions on this debate, we can regard their change of view as a sign that the tide is turning in favor of a more inclusive planet definition.

Tonight, I could hardly believe what I was reading when a friend and fellow Pluto supporter tweeted me the link to an article by Ethan Siegel, to whom I have previously written responses, titled, "The Science Case for Why Pluto Should Become A Planet (Again)."

Published on the site "Big Think," the article notes the significant progress in our understanding of the planetary processes on solar system bodies other than Mercury through Neptune while also taking into account the many exoplanet discoveries made since 2006.

Siegel also acknowledges that only a tiny fraction of the IAU took part in the 2006 vote and that over the past 19 years, many scientists who study planets have not used the IAU definition.

This article is well worth reading and hopefully a sign that in 2026, we can finally correct the error made in 2006.

Opposing Pluto's Demotion Does Not Equal "Whining"

    The distant Planet Pluto

Adam Frank’s April 28 Forbes column, “Quit Whining: Pluto’s Not a Planet” is not just written in a very condescending, patronizing tone. It also includes outdated information and lacks important data based on which many planetary scientists consider Pluto and all dwarf planets to be a subclass of full planets.

Words matter, and the words Frank uses are unprofessional, disrespectful, and downright insulting to those who don’t share his view and support a geophysical planet definition over the IAU’s dynamical one.

Using terms like “wailing, “whining,” and “crying” to describe the reaction of those who oppose the IAU vote sets the wrong tone for the article and amounts to putting down those with whom he disagrees with defamatory language.

Frank invokes the strawman argument that accuses Pluto supporters of being motivated by emotion. This completely dismisses the genuine objections by some of the world’s leading planetary scientists to the IAU definition that excludes Pluto, adopted back in 2006.

Just four percent of the IAU voted on that controversial resolution, and most weren’t planetary scientists but other types of astronomers. Their decision was immediately opposed in a formal petition signed by an equal number of planetary scientists, led by New Horizons principal investigator Alan Stern.

Unfortunately, the media has barely covered this side of the ongoing debate.

Opposition to the IAU definition is centered on preference for a geophysical planet definition over a dynamical one. The geophysical definition puts primacy on a celestial object’s intrinsic properties; in contrast, the IAU definition, which is a dynamical definition, puts primacy on an object’s location.

Frank completely ignores the fact that the IAU vote was held in violation of the IAU’s own bylaws, which require any resolution put to the floor of the General Assembly to first be vetted by the appropriate IAU committee.

This was not done in 2006. The resolution adopted was thrown together the last day of the two-week conference, after most attendees, including the chair of the organization’s Planet Definition Committee, Owen Gingerich, had already gone home. No absentee voting was permitted.

In addition to objecting to a very flawed planet definition, opponents of the IAU decision also object to the problematic way in which the vote was conducted.

Alongside his article, Frank includes an image of the largest known Kuiper Belt Objects that is outdated by 15 years. That image includes a caption that says, “Pluto isn’t even the largest Kuiper Belt Object,” and portrays Eris, aka Xena, as bigger.

While Eris was initially thought to be bigger than Pluto, a team led by scientist Bruno Sicardy in 2010 measured Eris’s size when it occulted a star and determined it is marginally smaller than Pluto though 27 percent more massive. In 2015, New Horizons confirmed that Pluto is bigger.  Why is Frank making an argument using outdated information?

The biggest problem with Frank’s article is his blurring of the distinction between tiny, shapeless Kuiper Belt Objects and those large enough to be rounded by their own gravity. The latter threshold matters because this is when active geology begins.

Frank lumps all worlds beyond Neptune as “construction debris left over from the assembly of the solar system.” This fails to acknowledge the very important distinction between objects large enough to be rounded by their own gravity and the majority of tiny KBOs that are shaped only by their chemical bonds.

When celestial objects are rounded by their gravity, they begin to develop complex geology, which New Horizons found on Pluto. Most KBOs are tiny and shaped only by their chemical bonds. In contrast, Pluto is likely layered into core, mantle, and crust, and has weather, varied terrains, active geology, and a likely subsurface ocean. Frank never mentions any of this. In fact, he never says anything about Pluto’s intrinsic properties.

Pluto and Ceres are actually two of a growing number of solar system ocean worlds, worlds that have subsurface oceans that could potentially host microbial life. Other such worlds include Europa, Ganymede, Enceladus, Titan, and possibly Triton.

Pluto being smaller than Earth’s moon does not negate its planet status according to the geophysical definition, which considers dwarf planets to be a subclass of full planets, and does not require orbit clearing as a criterion for planethood. Any celestial object that meets the critical threshold of being rounded by its own gravity is a planet according to this definition.

This is very similar to the situation with stars, where variety in size and mass is even greater than it is among planets. The Sun is tiny compared to massive, giant stars. Red dwarf stars, whose planets may be a top target in the search for extraterrestrial life, are even smaller and less massive than the Sun. Yet no one claims they aren’t stars, just like no one claims dwarf stars are not stars, or dwarf galaxies are not galaxies.

By saying Pluto is not a planet but a Kuiper Belt Object, Frank presents an incomplete view of both Pluto and the solar system. There is no reason objects cannot be dually classed. Pluto is both a planet and a Kuiper Belt Object. The first tells us what it is; the second tells us where it is. Haumea, Makemake, and Eris are similarly dually classed.

The problem is not with the term dwarf planet but with the claim that dwarf planets aren’t planets at all but another type of object. This claim is contradicted by the Dawn mission’s findings at Ceres and the New Horizons mission’s findings at Pluto, both of which of which found these worlds to contain complex planetary processes.

Why should Earth and Jupiter be put in the same class but Pluto excluded? Earth has more in common with Pluto than it does with Jupiter. Jupiter has no solid surface, is composed of mostly hydrogen and helium like the Sun, and has its own mini-solar system of moons. In contrast, both Earth and Pluto have solid surfaces, active geology, nitrogen atmospheres, and interaction between those atmospheres and their surfaces.

The scientist who first coined the term dwarf planet, Alan Stern, intended it to designate a new subclass of small planets, not to designate a class of non-planets. The four percent of the IAU who voted in 2006 misused his term, stating in a vote of 333-91 that dwarf planets are not planets. They never revisited the issue in spite of the Dawn and New Horizons findings.

Yes, letters from children may be emotional, but as someone who has written and spoken publicly on this issue, I believe they reflect an inherent recognition of Pluto’s nature. Children look at its roundness and complexity and see a planet. They are not wrong.

Frank’s support of a dynamical planet definition is a legitimate scientific position. However, his mischaracterization of those who oppose it and why they oppose it, as well as his denial of the ongoing debate, detract from the credibility of his article.

Opposition to the IAU definition is a scientifically legitimate position, not “whining.” It will continue until a better definition is reached. If NASA Administrator Jared Isaacman wants to lend his support to this view, more power to him!

After all, it is NASA, through the New Horizons mission, not the IAU, to whom we owe practically all the knowledge we have about the Pluto system.

 

To NASA Administrator Jared Isaacman: Please Grant Kaela's Request

 

Twitter is currently buzzing with news of a 10-year-old girl named Kaela who has written to NASA Administrator Jared Isaacman urging Pluto’s planetary reinstatement.

Kaela, who was born 10 years after the controversial and wrongful IAU demotion of Pluto, is correct, and I wholeheartedly second her request. I have tweeted Isaacman myself asking his support for this.

Her request is noteworthy not just because it reveals the critical thinking of a precocious child, but also because it reveals the error of those who, nearly 20 years ago, claimed that younger generations would accept Pluto’s status as a non-planet and look askance at older people who refused to accept the change.

This clearly did not happen. To this day, children visiting observatories and planetaria regularly ask about Pluto and express their preference for it to be classified as a planet. Clearly, there are inherent, compelling reasons that transcend age and nostalgia, that compel people who look at this tiny, complex world to see a planet.

In this case, a child really does understand the situation better than the four percent of the IAU who voted in 2006 and those who support their decision.

Sometimes, claims are presented as scientific when they are instead political. An example of this is when oil companies conduct studies that conclude that fossil fuels are not causing global warming. In decades past, tobacco companies conducted similar studies claiming cigarette smoking does not cause cancer.

Coming up with a desired conclusion and then twisting the evidence to support that conclusion is not science but propaganda.

This is essentially what four percent of the IAU did in 2006. A majority of those who voted wanted Pluto out as a planet. They then contrived the reason of orbit clearing, which had never been used in planet definition, to get the result they wanted and exclude Pluto.

This is not to say that orbit clearing isn’t significant. That does not mean it should be a requirement for planethood. A 2000 study distinguished between “uber planets,” which clear their orbits, and “unter planets,” which do not. However, this study, which was cited by IAU members who voted in 2006, never said that “unter planets” should not be considered planets at all.

There is no reason why we cannot say that some planets gravitationally dominate or clear their orbits while others do not. This puts them in different subcategories but still under the general umbrella of the broad category of planets.

Evidence for the contrived nature of the IAU decision can be seen in the fact of who remained on the last day of the two-week General Assembly to vote on the resolution that “reclassified” Pluto. The chair of the IAU’s own Planet Definition Committee left before the vote because he had no idea there would be a vote! The same is true for the majority of IAU members who attended the conference.

IAU rules require any resolution put to the General Assembly to first be vetted by the appropriate IAU committee. This was not done in 2006, meaning the vote violated the IAU’s own bylaws! An earlier resolution that did go before the Planet Definition Committee was defeated at the General Assembly. At that point, the appropriate thing would have been to wait until the next General Assembly in 2009 and go through the proper channels.

But this was not done! A resolution hastily thrown together the night before the last day of the conference was put to a vote on that day, before most attendees had even had a chance to read its contents and consider its merits.

The IAU cited “new information,” namely the discovery of Eris, as a reason for the reclassification. However, they have been very selective in deciding which new information counts as a reason to reconsider the issue.

The New Horizons flyby clearly revealed Pluto to be a complex world with active geology and planetary processes, some seen elsewhere in the solar system only on Earth and Mars, yet this new information was never considered a reason to reopen the discussion of planet definition, as it should have been.

When the Internet first became a thing, people hoped it would serve as a democratizing influence, giving everyone a voice in issues important to them. While this has not always happened, the Internet has played a major role in opposition to the demotion of Pluto, and Kaela’s letter represents only the latest in nearly two decades of these efforts by people around the world.

I not only second Kaela’s motion, but genuinely urge our NASA Administrator to consider the facts surrounding the IAU vote and the reality of Pluto as discovered by New Horizons, to consult with New Horizons principal investigator Alan Stern, and to grant Kaela’s request.

Pluto Discovered 96 Years Ago Today

 

Today is International Pluto Day, marking the discovery of planet Pluto by Clyde Tombaugh at the Lowell Observatory 96 years ago, on February 18, 1930.

It is a day to celebrate all things Pluto, a planet whose understanding has eluded people for nearly a century.

When first discovered, Pluto’s planethood was questioned because telescopes of the time could not resolve it into a disk. Later, when it was revealed to be spherical, meaning rounded by its own gravity, many planetary scientists assumed it to be a dead world with no active geology or planetary processes.

The New Horizons flyby proved that wrong, revealing Pluto to be a surprisingly active world with complex processes, some of which are seen elsewhere in the solar system only on Earth and Mars.

Incorrect assumptions have often been made about Pluto because it is so small and so far away, making it a challenge to study and understand.

One of those incorrect assumptions, also a premature one, was the controversial decision by four percent of the IAU to label Pluto before the first reconnaissance of this world. Had the IAU waited for the New Horizons flyby, it would have become clear that Pluto, though small, has the same geology and processes seen on larger planets.

While Pluto was discovered in 1930, one could legitimately say people have been “discovering” its true nature for close to 100 years now, as Pluto continues to surprise us.

At last summer’s conference, “Progress in Understanding the Pluto System: Ten Years After Flyby,” possible next steps in the discovery of Pluto’s nature, namely hypothetical orbiter missions, were presented and discussed. As of now, there are no formal plans for any of these missions, but they are still worth being given serious consideration.

Planetary scientist Carly Howett, in her presentation titled, “Persephone: A Pluto System Orbiter and Kuiper Belt Explorer,” outlined a NASA-funded concept study involving returning to the Pluto system with an orbiter.

Orbiters are generally the next step in planetary exploration after the first reconnaissance of a world in a flyby, like that New Horizons did at Pluto in 2015.

The goal of the Persephone mission would be understanding Pluto’s composition, including its possible subsurface ocean, determining how Pluto’s surfaces and atmosphere evolved, and studying the diversity of the Kuiper Belt.

According to the proposal, the spacecraft would orbit Pluto for 1.3 years, then leave on an extended mission to explore the Kuiper Belt.

The proposal recommends a payload of 11 science instruments and notes that ice-penetrating radar would have to be powered by a radioisotope thermoelectric generator (RTG), the nuclear battery that powers New Horizons.

The spacecraft would conduct high and low flybys of Pluto, use Charon to change its orbit, and image all of Pluto at high resolution.

Drawbacks include the $3 billion cost, excluding launch vehicles, and the fact that travel time from Earth to Pluto would take 27.3 years.

Another proposal, “The Gold Standard: A Combined Pluto Orbiter and KBO Explorer Mission” was presented by Alan Stern. This spacecraft would be powered by an electric propulsion system using RTGs but have a smaller payload than Persephone.

The spacecraft would orbit Pluto for two years, then go on to visit other KBOs.

Stern noted that braking the spacecraft to enter Pluto orbit would double the travel time to the planetary system.

Including launch vehicles, this mission would cost $2.7 billion. Using three RTGs, existing launch systems could launch this spacecraft with current technology.

Returning to Pluto is a worthwhile endeavor for many reasons. Pluto joins a growing number of ocean worlds like Europa and Enceladus, which could potentially host microbial life in their subsurface oceans.

Embarking on such a mission would also give humanity another chance to study the Kuiper Belt in situ rather than from a great distance.

Discovery is an ongoing process not limited to one person or one time period. When Clyde Tombaugh first found Pluto, no one really understood what it was. Today, New Horizons has given us more knowledge and understanding of this small world than ever, but that is only a beginning.

We do know that Pluto is both a small planet and a Kuiper Belt Object. Neither of these categories precludes the other. The first tells us what it is while the second tells us where it is.

One legacy of Tombaugh’s find is not just that there are more planets out there than previously thought, but there are more types of planets out there than we recognize, even today.

Exploring the frontiers of our solar system is just a first step in looking even further beyond to the amazing variety of worlds and phenomena waiting to be discovered.

I Heart Pluto Festival Returns to Lowell Observatory this Weekend

This weekend kicks off Lowell Observatory’s annual celebration of Pluto’s 1930 discovery by Clyde Tombaugh with its annual I Heart Pluto celebration.

The celebration actually began last night, with a Beer Unveiling and Astronomy on Tap where this year’s Pluto-themed beer was unveiled.

Tonight, there will be a Pluto Pub Crawl throughout Flagstaff, Arizona, where participants can enjoy custom-made Pluto-themed drinks and interact with astronomers and science educators.

The keynote of this year’s celebration will take place tomorrow night, February 14, with the Night of Discovery, whose theme is “Mother Road to the Stars,” honoring both Pluto and the centennial of Route 66.

It will be followed by several talks on Sunday, February 15, including “Fantastic Worlds and How to Find Them” with Dr. Alex Polanski, “Tour of the Solar System from the Sun to Pluto and Beyond” with Dean Regas, “Defending Earth: The Science of Planetary Defense” with Dr. Nick Moskovitz, “How Low Temperatures Enable Pluto’s Exotic Geology” with Dr. Will Grundy, and “Iron Rain to Supersonic Winds: Our View of Exo-Jupiters” with Dr. James Sikora.

Pluto’s Birthday Bash will take place on Monday, February 16, featuring a birthday cake, vendors, and crafts. There will also be activities at Lowell including a talk about Pluto, a special Pluto tour, a beer garden, and a celebration of the New Horizons mission at the Astrolab.

I don’t know whether any of the talks will be streamed online or recorded and put online later, but if they are, I will post the links to them.

For more details, visit Lowell Observatory’s I Heart Pluto page.

New Horizons Saved as Congress Reverses NASA Cuts

 

The New Year begins with some good news for New Horizons and for many other NASA missions. The US House and Senate have passed a bill that rejects the cuts to NASA first proposed in June by the White House and instead approved $24.4 billion for NASA this year, of which $7.25 billion will go to the agency’s Science Mission Directorate.

New Horizons will continue to be funded, as will all but one other NASA mission.
While the total NASA budget is slightly below that of 2025, both houses of Congress overwhelmingly rejected the controversial proposal to cut 24% of NASA’s total funding of which 47% would have been cut from NASA science missions.

This reversal did not just happen on its own. Credit goes to the Planetary Society for its tireless efforts in lobbying against these cuts. According to an announcement of the good news sent by the Planetary Society, these efforts included close to 100,000 messages sent to Congress, outreach to Senators and Congress members in every state and district, and two days of lobbying action on Capitol Hill in October in which 346 people participated.

We owe a debt of gratitude to the Planetary Society and all who took part in these efforts for saving so many valuable space missions, including readers of this blog.

No current mission can do what New Horizons is currently doing because it is the only spacecraft with operational science instruments deep in the Kuiper Belt. New Horizons continues to observe this region up close while also studying the solar wind as well as structures and processes in the Milky Way.

Efforts to find a third flyby target, another KBO in the spacecraft’s path, are ongoing.
Sometimes, it’s hard to remember that this overwhelmingly successful mission endured multiple cancellations before it even launched. It is a mission that happened because people believed in it and fought for it over years and even decades.

When the NASA cuts were proposed last summer, New Horizons principal investigator Alan Stern joked, “We’ve been dead before.” That statement held an underlying truth—that this particular mission has survived numerous attempts to cancel it. If it were as “dead” as it were those times, things might not have been as bad as they seemed.

And sure enough, New Horizons is once again back from the brink of death.

Advocacy and action by large numbers of people can and does make a difference. Democracy is not a spectator sport. The tireless efforts of many brought this and various other crucial NASA missions back from near death.

A third New Horizons flyby may not be a given, but I certainly wouldn’t count it out just yet.