- The IAU definition bars the Moon from being a planet because it orbits Earth.
- A 2017 geophysical definition would count the Moon as a planet based on its shape alone.
- The Moon is unusually large — more than one-quarter the width of Earth.
Skylar Grayson gets this question more than most astronomers. The Arizona State University astrophysicist studies galaxies, but when she talks to the public, the conversation often drifts closer to home: Is our Moon a planet?
"Whether or not the moon is a planet actually depends on who you ask," Grayson told Live Science.
The official answer is no. The International Astronomical Union requires a planet to orbit the Sun, be massive enough to become round, and have cleared its orbital neighborhood.
Our Moon fails on two counts. It orbits Earth, not the Sun directly. And by definition, a planet cannot be a satellite of another object.
But not everyone accepts that definition.
A Different Way to Define Planets
In 2017, planetary scientist Kirby Runyon led a team including Alan Stern, the NASA scientist who directed the New Horizons mission to Pluto, in proposing an alternative. Their geophysical planet definition ignores orbital properties entirely.
What is the geophysical planet definition?
A proposal that defines planets by physical properties alone: any body massive enough to be round but too small for nuclear fusion counts as a planet, regardless of what it orbits. Under this definition, our Moon, Pluto, and roughly 110 other objects in the Solar System qualify as planets.
The team's logic is straightforward. If an object has enough mass for gravity to pull it into a sphere, it behaves like a world. What it happens to orbit shouldn't change that.
Under this framework, dwarf planets like Ceres and Pluto rejoin the planetary family. So do large moons like Europa, Titan, and Earth's Moon.
Why the Moon Complicates Everything
Our Moon is unusually large. At roughly 3,500 kilometers in diameter, it measures more than one-quarter the width of Earth. No other inner planet has a moon anywhere close to that ratio. Mercury and Mars have nothing comparable. Venus has no moon at all.
Key figure
3,500 kilometers
The approximate diameter of the Moon.
This size creates an odd gravitational situation. Earth and Moon orbit a shared center of mass (their barycenter) located about 4,671 kilometers from Earth's center. That point currently sits inside our planet, about 1,700 kilometers below the surface.
But the Moon is drifting away at roughly 38 millimeters per year. Eventually, the barycenter will migrate into space between them.
When that happens, neither body will technically orbit the other. They will orbit each other.
Pluto and its moon Charon already work this way. Their barycenter floats in empty space, leading some astronomers to call them a binary dwarf planet system.
Where the Debate Stands
The IAU has not revisited its 2006 definition. Most astronomers use it in formal contexts. Many planetary geologists quietly prefer the geophysical approach.
Grayson sees this as revealing something about how science works. "Most things in the universe exist not in neat boxes but rather on a spectrum," she said. "We have to decide where we want to draw the lines between categories."
The Moon has quakes, ancient volcanism, and a crust that formed from a global magma ocean. It behaves like a world.
Whether it deserves the label "planet" depends on which question matters more: what it orbits, or what it is.
Sources
- Research Paper: Runyon et al., 2017 – "A Geophysical Planet Definition" (Lunar and Planetary Science Conference) https://www.hou.usra.edu/meetings/lpsc2017/pdf/1448.pdf
- Additional Context:
- Grayson, S. – Interview with Live Science, "Is the moon a planet?" https://www.livescience.com/space/the-moon/is-the-moon-a-planet
- A geophysical planet definition - phys.org
- Earth–Moon Relations - ebsco.com
Fact Check: Claim-by-Claim Verification Verified
The recap closely matches the Live Science article and the cited geophysical planet proposal, with only minor simplifications and no substantial distortions.
Commentary
- The recap adds quantitative details about the Earth–Moon barycenter location and the Moon’s recession rate that are not in the Live Science article but are consistent with standard astronomical references.
Sources used for verification
Academic/Peer-reviewed:
- A Geophysical Planet Definition - hou.usra.edu
- IAU 2006 Resolution 5A: Definition of a Planet in the Solar System - iau.org
- Tidal dissipation in the Earth–Moon system and lunar recession - adsabs.harvard.edu
- The early evolution of the Moon and its magma ocean - lpi.usra.edu
Other reliable sources:
- Is the moon a planet? - livescience.com
- A geophysical planet definition - phys.org
- Earth–Moon Relations - ebsco.com
- Lunar Laser Ranging: The Moon is moving away - nasa.gov
- The definition of a planet - iau.org
Fact-checked by Perplexity Sonar Pro on 2026-01-09
