Make Pluto a Planet Again: NASA's Chief Reopens Astronomy's Most Divisive Classification Fight

On April 28, 2026, NASA Administrator Jared Isaacman sat before the Senate Committee on Appropriations to discuss the agency's Fiscal Year 2027 budget request. The hearing covered launch schedules, the Roman Space Telescope, and deep-space exploration funding. Then a Kansas senator asked about Pluto, and the hearing became international news.

"I am very much in the camp of 'Make Pluto A Planet Again,'" Isaacman told the committee [1]. He went further: "We are doing some papers right now on, I think, a position that we would love to escalate through the scientific community to revisit this discussion and ensure that Clyde Tombaugh gets the credit he received once and rightfully deserves to receive again" [2].

The statement was not casual. The head of the United States' space agency publicly committed NASA to producing formal scientific arguments against a classification that has stood for twenty years. Whether those arguments can survive peer review — and whether the International Astronomical Union will entertain them — is a different question entirely.

The 424 Who Decided

The classification that Isaacman wants overturned was adopted on August 24, 2006, at the IAU's XXVIth General Assembly in Prague. Under Resolution 5A, the IAU defined a "planet" as a celestial body that (a) orbits the Sun, (b) has sufficient mass for self-gravity to pull it into hydrostatic equilibrium — roughly spherical — and (c) has "cleared the neighbourhood around its orbit" [3]. Pluto meets the first two criteria but fails the third: it shares the Kuiper Belt with thousands of similar icy objects.

The legitimacy of this vote has been questioned since the day it happened. Of approximately 10,000 professional astronomers who were IAU members at the time, only 2,411 registered their attendance at the Prague assembly. Of those, just 424 cast votes on the decisive resolution — roughly 4.2% of the total membership [4]. Resolution 5A passed with 237 votes in favor, 157 against, and 30 abstentions [4].

Source: IAU Press Release

Data as of Aug 24, 2006CSV

Critics, including New Horizons principal investigator Alan Stern, have called the vote unrepresentative. Many planetary scientists had already left Prague by the final day of the two-week assembly, when the vote was held during the closing session [5]. The voting method itself was rudimentary: members raised yellow ballots, and counters estimated the totals by sight [4]. There was no electronic polling, no absentee mechanism, and no quorum requirement for changing a definition that would reshape science education worldwide.

Defenders of the process note that the IAU's Working Rules govern how resolutions advance, requiring review by the Resolutions Committee and Executive Committee before a plenary vote [3]. The process, they argue, followed established procedures, and the result reflected the informed judgment of those who participated in the full two weeks of scientific discussion preceding the vote.

What Isaacman Is Proposing — and What It Would Mean

Isaacman did not specify the scientific criteria NASA's forthcoming papers would advocate, but the most prominent alternative to the IAU definition is the Geophysical Planet Definition (GPD), championed by Stern and formalized in a 2017 paper presented at the Lunar and Planetary Science Conference [6]. Under the GPD, a planet is "a sub-stellar mass body that has never undergone nuclear fusion and that has sufficient self-gravitation to assume a spheroidal shape adequately described by a triaxial ellipsoid regardless of its orbital parameters" [6].

The critical difference: the GPD drops the "clearing the neighbourhood" requirement entirely. It also drops the requirement that a planet orbit the Sun — or any star — meaning that free-floating objects and large moons would qualify.

The consequences for solar system taxonomy would be dramatic.

Source: Planetary Science Research

Data as of Apr 30, 2026CSV

Under the current IAU definition, the solar system has eight planets. Adding the five IAU-recognized dwarf planets (Ceres, Pluto, Eris, Makemake, and Haumea) brings the count to thirteen. Including all known trans-Neptunian objects large enough to be in hydrostatic equilibrium — Gonggong, Quaoar, Orcus, Sedna, and Charon (Pluto's largest moon) — reaches approximately seventeen [7]. And if the GPD is applied without an orbital restriction, counting all spheroidal moons in the solar system, the total reaches roughly 110 objects that would qualify as "planets" [8].

This is not a hypothetical exercise. The GPD's authors have explicitly argued that "dwarf planets are planets, too" and that large moons like Europa, Titan, and Ganymede should be classified as planetary bodies [9]. Whether Isaacman's NASA papers will adopt the full GPD or propose a narrower revision remains unknown.

The Scientific Fault Lines

The Pluto debate is often presented as a binary: "planet" or "not a planet." The actual scientific disagreement is more specific and more technical.

The case against "clearing the neighbourhood": Stern and his allies argue that the orbital-clearing criterion is physically ambiguous. Worked out mathematically, the mass required to "clear" an orbital zone increases with distance from the Sun. An object massive enough to clear Mercury's orbit might not clear Earth's; an object that clears Earth's orbit would fail at Pluto's distance [10]. No planet in our solar system has a perfectly cleared orbit — Jupiter has thousands of Trojan asteroids, and Earth shares its orbital space with near-Earth objects [10]. The criterion, critics say, is a continuum dressed up as a binary threshold.

The case for orbital dynamics: Astronomer Mike Brown of Caltech — co-discoverer of Eris, the trans-Neptunian object whose discovery triggered the 2006 debate — has been one of the most visible defenders of the reclassification. Brown argues that the eight classical planets are fundamentally different from Kuiper Belt objects: they gravitationally dominate their orbital zones by orders of magnitude, while objects like Pluto exist in resonant orbits controlled by Neptune's gravity [11]. Ron Ekers, past president of the IAU, has similarly defended the orbital criterion on practical grounds: the orbits of large planets are not significantly perturbed by smaller objects around them, while Kuiper Belt objects are shepherded into resonance by the large planets [12].

Brown has gone further, suggesting that the word "planet" itself may be the problem. Rather than forcing a single term to cover objects with fundamentally different physical histories and dynamical roles, he has argued for adopting more precise scientific vocabulary [11].

The Exoplanet Complication

One of the strongest technical objections to reclassification concerns its implications beyond our solar system. The IAU's 2006 definition explicitly applies only to objects orbiting the Sun. Exoplanets — planets around other stars — operate under a separate IAU working definition, last amended in 2018, which uses a mass-based threshold (below roughly 13 Jupiter masses, the limit for deuterium fusion) rather than orbital dynamics [13].

If the solar system definition were changed to a geophysical standard, the two definitions would need to be reconciled. With current telescopes, it is impossible to determine whether a distant exoplanet has cleared its orbital zone, making the IAU's solar system criterion effectively inapplicable to exoplanet science already [13]. But a mass-and-shape definition applied universally would classify unknown numbers of exo-moons and rogue objects as planets, creating cataloging and communication problems across the field.

Proponents of the GPD counter that this inconsistency already exists: planetary scientists routinely refer to Pluto, Titan, and Europa as "planets" in conference presentations and papers, while the IAU definition says otherwise [10]. Standardizing on geophysical criteria, they argue, would resolve this incoherence rather than create new problems.

Follow the Money and the Missions

Isaacman's remarks did not occur in a vacuum. They came during a budget hearing, and the institutional context matters.

NASA's New Horizons mission, which performed its historic flyby of Pluto on July 14, 2015, cost approximately $780 million through its primary mission [14]. The spacecraft revealed a world of startling geological complexity: nitrogen-ice glaciers, mountain ranges of water ice, a thin atmosphere, and evidence of possible subsurface oceans [15]. NASA's own public communications described Pluto as "geologically alive" — framing that planetary scientists have used to argue for its planetary status ever since [15].

The mission's extended operations cost roughly $10 million per year, and the FY2026 White House budget proposal attempted to cut New Horizons funding before Congress intervened [16]. A Pluto that is a "planet" rather than a "dwarf planet" carries different institutional weight when competing for mission funding in a constrained budget environment.

The 2006 reclassification also had measurable downstream effects on education. Officials in the multibillion-dollar school publishing industry estimated it would take one to three years to revise textbooks across grade levels [17]. States were forced to review science assessments and standards. California faced an immediate textbook-adoption cycle; Texas would not adopt new science textbooks until 2010 [17]. Online curriculum providers like OdysseyWare updated their 349 references to Pluto within 72 hours, but print publishers absorbed significant costs [17].

The Tombaugh Factor and American Pride

Senator Jerry Moran, a Kansas Republican, was the one who raised the Pluto question during the hearing — noting that Pluto's discoverer, Clyde Tombaugh, was from Moran's home state [1]. Isaacman's response leaned into the nationalistic framing: Pluto is the only planet discovered by an American, and restoring its status would "ensure that Clyde Tombaugh gets the credit he received once and rightfully deserves to receive again" [2].

This rhetorical choice has drawn scrutiny. Scientific classification is not typically determined by the nationality of a discoverer, and critics have noted that framing the debate in terms of American pride risks politicizing what should be a scientific question [2]. The IAU, headquartered in Paris with members from 82 countries, sets standards through international scientific consensus, not national advocacy [3].

At the same time, public sentiment has consistently favored Pluto's planetary status. The 2015 New Horizons flyby generated enormous public interest, and "Make Pluto a Planet Again" has been a durable cultural slogan for nearly two decades [1]. Whether public enthusiasm should influence scientific taxonomy is itself a contested question.

The Academic Dimension

The debate has sustained a significant body of academic literature. According to OpenAlex data, nearly 4,000 research papers touching on "Pluto planet definition" have been published since 2011, with a peak of 490 papers in 2023 [18]. Publication volume spiked notably around the 2015 New Horizons flyby and again in 2023, when several formal proposals for revised definitions circulated in planetary science journals.

Source: OpenAlex

Data as of Jan 1, 2026CSV

The academic divide broadly tracks disciplinary lines. Planetary scientists — who study the physical properties of celestial bodies — tend to favor geophysical definitions that treat roundness and geological activity as the markers of planethood. Dynamicists — who study orbital mechanics and the gravitational architecture of planetary systems — tend to favor definitions that account for how objects interact with their orbital environment [12]. The two communities attend different conferences, publish in different journals, and use the word "planet" in functionally different ways.

How the Definition Could Change

The IAU remains the sole internationally recognized authority for astronomical nomenclature [2]. Any change to the planet definition would need to follow the same process used in 2006: a resolution proposed through the Resolutions Committee, reviewed by the Executive Committee, debated at a General Assembly, and passed by a plenary vote of attending members.

The next IAU General Assembly is scheduled for August 10–19, 2027, in Rome [19]. For a revised planet definition to reach the floor, a formal resolution would need to be submitted through IAU channels well in advance. As of May 2026, no such resolution has been publicly announced, though Isaacman's statement that NASA is preparing papers suggests the groundwork is being laid.

The vote threshold for passing a resolution is a simple majority of those present and voting [3]. Given the controversy of 2006, any new resolution on planet definition would likely draw significantly higher participation than the 424 members who voted two decades ago — though the IAU has no mechanism for remote or electronic voting, meaning the outcome would again depend on who physically attends the assembly.

What Comes Next

Isaacman's statement has moved the debate from academic journals to institutional policy. If NASA publishes formal position papers advocating reclassification, it would represent the first time a national space agency has formally challenged the IAU's planet definition. The IAU has not yet responded publicly to Isaacman's remarks.

The underlying scientific questions remain genuinely unresolved. The 2006 definition has known weaknesses — its inapplicability to exoplanets, the physical ambiguity of "clearing the neighbourhood," and the small number of voters who adopted it. But the alternatives carry their own costs: a geophysical definition that potentially creates 110 planets in our solar system, blurs the line between planets and moons, and requires renegotiating international standards that have functioned, however imperfectly, for twenty years.

What has changed is the political weight behind one side of the argument. Whether a NASA administrator's advocacy can translate into a revised IAU resolution — and whether it should — will test the boundary between scientific consensus and institutional pressure. The Rome General Assembly in August 2027 is the earliest realistic venue for a formal vote. The papers Isaacman promised will need to be published, reviewed, and debated before then.

Pluto, meanwhile, continues its 248-year orbit around the Sun, indifferent to what anyone calls it.