Pluto, which stepped down from its place as the ninth planet in the solar system in 2006, is back at the center of debate. That is because the head of the National Aeronautics and Space Administration (NASA) publicly said the agency should reexamine Pluto's planetary status.
On the 28th of last month (local time), Jared Isaacman, NASA Director General, said at a U.S. Senate Appropriations Committee hearing, "I am fully in favor of making Pluto a planet again," adding, "NASA is preparing a paper proposing that the scientific community discuss Pluto's status again."
However, Pluto will not immediately regain planetary status just because NASA wants it. The body that sets the official classification standards for solar system objects is the International Astronomical Union (IAU). Still, Director General Isaacman's remarks show the debate over Pluto is not over.
◇ From the ninth planet to a dwarf planet
Pluto was discovered in 1930 by American astronomer Clyde Tombaugh at Lowell Observatory in Arizona. For the next 76 years, it was called the ninth planet of the solar system after Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. In school textbooks, science books, and popular culture, the notion that "the solar system has nine planets" took hold.
But after the 1990s, as objects similar to Pluto were discovered one after another beyond Neptune, the question arose: "If Pluto is a planet, shouldn't similar objects also be called planets?" Pluto is smaller than the moon, and its orbit is more elongated than those of other planets. It also belongs to the Kuiper Belt, where small objects beyond Neptune are clustered, sharing its orbital neighborhood with many bodies.
The decisive trigger came with the 2005 discovery of "Eris." At the time, Eris was regarded as an object larger than Pluto, prompting claims that if Pluto was a planet, Eris should be one too. For a time, Eris was even mentioned as a candidate for the tenth planet.
In the following year, the IAU set a new definition of a planet at its General Assembly in Prague, Czech Republic. The three conditions were that it orbits the sun, has sufficient mass for its self-gravity to overcome rigid body forces so it assumes a nearly round shape, and is gravitationally dominant in its orbital neighborhood.
Pluto met the first two conditions, but failed the third because it shares its orbital zone with many Kuiper Belt objects. In the end, Pluto was reclassified not as a planet but as a "dwarf planet" and a representative example of a trans-Neptunian object.
◇ The debate isn't over yet… "A matter tied to American pride"
The Pluto debate did not stop there. When NASA's New Horizons spacecraft made a close flyby of Pluto in 2015, it revealed that Pluto is far more complex and dynamic than expected. The probe found ice mountain ranges and plains, a tenuous atmosphere, haze layers and signs of geological activity on Pluto.
Since then, some planetary scientists have argued that the criteria for a planet should be sought not only in orbital dominance, but in the object's own physical and geological characteristics. The logic is that a body like Pluto—with a rounded shape from self-gravity, complex surface terrain and atmosphere, and traces of geological activity—can be regarded as a planet.
The Pluto debate has stood out particularly in the United States. Moon Hong-kyu, principal researcher at the Korea Astronomy and Space Science Institute (KASI), said, "Pluto is the only planet discovered by an American and has been accepted as a symbolic discovery that American astronomy left in world history—in other words, a matter of American pride," adding, "Some scientists' push to view Pluto as a planet is tied to 'national pride.'"
Director General Isaacman's remarks align with this sentiment. At the hearing, he bolstered the position of making Pluto a planet again, saying in effect that "Clyde Tombaugh, who discovered Pluto, should regain the recognition he once received and rightly deserves."
So, can Pluto actually regain planetary status? Moon said, "Because U.S. astronomers are an overwhelming majority, if more than a certain quorum demands that it be put to a vote, it cannot be refused," not ruling out the possibility that the IAU will revisit the issue.
However, Moon added, "There are too many dwarf planets with similar orbits and comparable stature to Pluto," and "If Pluto is recognized as a planet, even outer objects that have not yet been identified could fall into the planetary category. If that happens, the very criterion for a planet could become ambiguous."