Could Pluto Once Again Be Considered a Planet? New Remarks From NASA’s Administrator Highlight an Enduring Debate Among Scientists

Two decades ago, the International Astronomical Union—which defines and names celestial bodies—redefined the criteria for being a planet, putting Pluto into the new category of dwarf planet

May 5, 2026 11:56 a.m.

Pluto on a black background — Pluto was considered the solar system's ninth planet for more than 70 years. NASA / JHU-APL / SwRI

For decades, scientists and the general public have argued over the astronomical status of beloved dwarf planet Pluto. In the latest salvo, NASA Administrator Jared Isaacman said that he was “very much in the camp of ‘make Pluto a planet again,’” during a Senate hearing on April 28.

Isaacman’s statement highlights the longstanding debate over Pluto’s classification. His remarks were made during a hearing over the Trump Administration’s proposed budget cuts to NASA.

Pluto was discovered by American astronomer Clyde Tombaugh in 1930 at an observatory in Flagstaff, Arizona. Tombaugh, born in Illinois and raised in Kansas in a farming family, had a childhood fascination with space and was about to embark on career building telescopes when a magazine article about Mars alerted him to the scientists working at Lowell Observatory in Flagstaff. Within months after arriving, according to the Observatory, Tombaugh took over the search for “Planet X” and found Pluto by examining photographic plates with a machine called a blink comparator.

For more than 70 years, it was considered the solar system’s ninth, most distant planet. In 2006, however, the International Astronomical Union (IAU)—which defines and names celestial bodies—announced a new definition for “planet” that saw Pluto booted from the classification.

“I would say 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,” Isaacman said during the hearing.

Davide Castelvecchi at Nature reports that it’s unclear which papers he was referencing, and NASA did not respond to the outlet’s request for clarification. In a March interview with the Daily Mail’s Charlie Spiering, Isaacman cited Pluto’s uniquely American origins, saying, “I think we owe it to everyone from Kansas and all their great contributions to astronomy and aerospace to rightfully restore that discovery to a planet.”

According to the IAU’s 2006 decision, a celestial body must fulfill three criteria to be considered a planet: orbit the sun, have enough mass and gravity to pull itself into a round shape and clear the area surrounding its orbit from other debris.

Pluto doesn’t meet the last requirement, therefore putting it into the category of dwarf planet, a new class the organization established in 2006. Part of the ruling stemmed from the 2005 discovery of Eris, an astronomical body with more mass than Pluto, once poised to become the solar system’s tenth planet. With the updated guidance, Eris was also dubbed a dwarf planet.

Did you know? NASA’s Pluto flyby mission

Earlier in 2006—before the demotion—NASA launched the New Horizons spacecraft to fly by Pluto. It reached the distant celestial body nearly ten years later and collected a trove of data about the icy world. It’s the only probe ever to study the dwarf planet, and it’s now hurtling toward interstellar space.

The change was meant to reflect a modern understanding of planetary systems, but the IAU’s decision was—and still is—contentious.

“Many of us feel that the word ‘planet’ should be defined by the intrinsic properties of a body, not by its dynamical environment,” David Grinspoon, an astrobiologist at the Planetary Science Institute, tells Nature. “If, suddenly, the Earth was surrounded by a swarm of small objects—as it was for the first several hundred million years of its existence—would it no longer be a planet? This seems silly.”

The definition of a planet could also be considered problematic in ways unrelated to Pluto. For example, the criterion of orbiting the sun—which refers to just our solar system’s star—means the only official planets in the universe are Earth and its seven neighbors. All the others are technically exoplanets.

“It’s a sloppy definition,” said Philip Metzger, a planetary scientist at the University of Central Florida, in a 2018 statement. “They didn’t say what they meant by clearing their orbit. If you take that literally, then there are no planets, because no planet clears its orbit.”

Metzger led a review of scientific literature published over the past 200 years and found just one publication that included the orbit-clearing requirement when categorizing planets. He and his co-authors argue that the new definition is not supported by past research.

In 2024, three planetary scientists proposed making the definition of “planet” more quantifiable, arguing that a body is a planet if it orbits one or more stars, stellar remnants or brown dwarfs and has a mass between 10²³ kilograms, enough to clear its orbit of debris, and 2.5 x 10²⁸ kilograms, or 13 times the mass of Jupiter. Notably, Pluto is still not massive enough to meet these criteria.

“I believe that the decision taken [by the IAU] was the correct one,” Catherine Cesarsky, an astronomer and IAU president from 2006 to 2009, told Science News’ Lisa Grossman in 2021. “Pluto is very different from the eight solar system planets, and it would have been very difficult to keep changing the number of solar system planets as more massive [objects beyond Neptune] were being discovered.”

You Might Also Like

Get the latest stories in your inbox every weekday.