Should Pluto and Earth's Moon Be Called Planets? The Debate Heats Up

The solar system just got a lot more complicated. While most of us learned that there are eight planets orbiting our Sun, a growing group of scientists is challenging this cosmic classification system. They're arguing that not only should Pluto regain its planetary status, but that Earth's Moon—yes, our very own lunar companion—should also be considered a planet. This isn't just academic nitpicking; it's a fundamental question about how we understand and categorize the celestial bodies that make up our cosmic neighborhood.

The controversy stems from the International Astronomical Union's (IAU) 2006 decision to reclassify Pluto as a "dwarf planet." This decision wasn't based on Pluto's size or composition, but rather on its failure to meet one specific criterion: it hasn't "cleared the neighborhood" around its orbit of other objects.

The current IAU definition requires a celestial body to meet three criteria to be considered a planet: it must orbit the Sun, have sufficient mass to assume a roughly round shape, and have cleared the neighborhood around its orbit. It's this third requirement that has sparked the most debate among planetary scientists.

Many researchers argue that this definition is overly restrictive and doesn't reflect the true nature of what makes an object planetarily significant. They contend that focusing on orbital dynamics ignores the fascinating geological and atmospheric processes that make these worlds unique.

Leading the charge for a new definition is a team of scientists who propose what they call the "geophysical planet definition." This approach focuses on the intrinsic properties of celestial bodies rather than their orbital characteristics.

Under this definition, a planet would be any sub-stellar mass body that has never undergone nuclear fusion and has sufficient self-gravitation to assume a roughly round shape. This seemingly simple change would dramatically expand our solar system's planetary family.

The geophysical definition emphasizes what these scientists consider the most important aspect of planetary science: understanding the geological processes, atmospheric dynamics, and potential for habitability that these worlds possess.

Perhaps the most surprising candidate for planetary status is Earth's Moon. Under the geophysical definition, our lunar companion would qualify as a planet because it's large enough to maintain a spherical shape and shows evidence of geological activity.

The Moon is larger than Pluto and possesses many characteristics we associate with planets: a differentiated interior with a core, mantle, and crust; evidence of past volcanic activity; and a thin atmosphere. Some scientists argue that the Earth-Moon system should be considered a "double planet" rather than a planet-satellite system.

This reclassification would make our Moon the fifth-largest planet in the solar system, larger than Mercury and rivaling Mars in size. It would also mean that Earth isn't alone in its orbit—we'd be sharing our orbital space with another planet.

Pluto's demotion in 2006 was controversial from the start, and new discoveries have only strengthened the case for its planetary status. The New Horizons mission revealed Pluto to be a geologically active world with nitrogen glaciers, methane dunes, and a complex atmosphere.

Pluto has five known moons, with its largest moon Charon being so large relative to Pluto that they form what's essentially a binary planet system. The world shows evidence of subsurface oceans, seasonal atmospheric changes, and active geology—all hallmarks of planetary complexity.

Perhaps most importantly, Pluto represents an entire class of objects in the outer solar system. By excluding it from planetary status, we're potentially ignoring some of the most interesting and diverse worlds in our cosmic neighborhood.

If the geophysical definition were adopted, our solar system would suddenly have over 100 planets. This would include not just Pluto and the Moon, but also other large moons like Europa, Ganymede, Titan, and Enceladus—many of which are targets in the search for extraterrestrial life.

This expansion would fundamentally change how we think about planetary science and exploration priorities. Instead of focusing primarily on eight worlds, we'd be studying a diverse collection of over 100 planetary bodies, each with unique characteristics and scientific value.

The debate also highlights the evolving nature of scientific classification. As our understanding of the solar system grows, our definitions and categories must evolve to reflect new knowledge and discoveries.

The International Astronomical Union has not indicated any plans to revisit the 2006 planet definition, despite growing calls from planetary scientists for reconsideration. The debate continues in scientific journals, conferences, and classrooms around the world.

Regardless of official classifications, the scientific community continues to study these worlds with the same rigor and enthusiasm. Pluto remains a target for future missions, and our Moon continues to be a stepping stone for human space exploration.

Perhaps the most important outcome of this debate is the reminder that science is not static. Our understanding of the universe continues to evolve, and our classification systems should evolve with it.