New Definition Would Make the Moon and Pluto Planets

A suggested update to the International Astronomical Union criteria would add over 100 planets to the solar system

Pluto Charon
Pluto and its moon Charon NASA/JHUAPL/SwRI

Anyone fuming over Pluto’s demotion from full-fledged planet to dwarf planet in 2006 will be happy to hear that there is still hope. Pro-Plutonians recently suggested a new definition of a planet that would add the celestial sphere back to the solar system’s list of heavy hitters. The only catch? It also reclassifies Earth’s moon and 100 other bodies orbiting the sun as planets as well.

The reason Pluto was given the cold shoulder was because it did not fulfill one of the three criteria set by the International Astronomical Union that define a full-fledged planet. The first two criteria—that it orbit around the sun and have sufficient mass to have a round shape—Pluto passed with flying colors. But it failed the housekeeping test—after millions of years, its gravity had not “cleared its neighborhood,” or become the gravitationally dominant object in its orbit, since Pluto shares its neighborhood with several “plutinos” that are affected by the gravity of Neptune.

Alan Stern, the principal investigator of the New Horizons mission to Pluto, and his colleagues argue in a paper, published in the journal Lunar and Planetary Science, that the definition should be changed. They suggest this mouthful: “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.”

They write that this simplifies more or less into a grade-school-friendly definition of  “round objects in space that are smaller than stars.”

Science Alert reports that the astronomers aren’t just plumping for Pluto, but level three main criticisms at the current definition of a planet. First, the current definition only applies to objects in our solar system, meaning that technically, any of the many exoplanets, including the seven Earth-sized planets circling the star TRAPPIST-1 announced last week, aren’t technically planets since they don’t orbit our sun.

Second, they argue that none of the planets in the solar system actually satisfy the “neighborhood clearing” criteria since every planet, including Earth, has many objects like trojans, quasi-satellites and mini-moons that are popping in and out of planetary orbits all the time.

And the last argument is that the current definition of a planet does not ever define the neighborhood or zone that a planet’s gravity is expected to clear, meaning the dividing line between planets and non-planetary objects is arbitrary.

For Stern and his colleagues, there is also a public-relations element to the redefinition. Between the time the New Horizons probe launched in January 2006 and the time it reached Pluto in July 2015, the sphere went from being a planet to a dwarf planet. “In the decade following the supposed 'demotion' of Pluto by the International Astronomical Union (IAU), many members of the public, in our experience, assume that alleged 'non-planets' cease to be interesting enough to warrant scientific exploration, though the IAU did not intend this consequence,” the team writes in their paper. “To wit: a common question we receive is, 'Why did you send New Horizons to Pluto if it’s not a planet anymore.'”

Calling something "planet," they argue, gives it a little extra status when it comes to exploration.

There is no word on whether the IAU has any plans to reassess its definition of a planet, but there seems to be scientific and public interest in hashing out the issue. In 2014, a debate at the Harvard-Smithsonian Center for Astrophysics about Pluto stirred up the controversy once more, with the audience of academics and astrogeeks voting that Pluto is a planet based on their preferred definition that “A planet is the smallest spherical lump of matter that formed around stars or stellar remnants.”

There hasn't been much debate on whether the moon should get an upgrade as well. Stephen Pumfrey, a historian at Lancaster University, writes at The Conversation that Greek and medieval astronomers considered the moon a planet. It wasn’t until Copernicus pointed out that the moon orbits the Earth and not the sun that it became simply a satellite. Reclassifying the moon as a planet, he writes, would take astronomy full circle.

If the definition does change and Pluto resumes its spot in the celestial lineup, it may have to find a different nickname other than the Ninth Planet. Astronomers are hopeful they will soon discover another planet orbiting the Sun beyond Pluto tentatively called Planet 9.