The International Astronomical Union made a controversial 2006 decision to redefine what it means to be a planet and demoted Pluto to the status of dwarf planet. However, members of the New Horizon team believe that this may have been premature and have outlined a possible new definition based on the physics and intrinsic properties of each world in an abstract of a paper that was presented at the 47th Lunar and Planetary Science Conference. The authors of the abstract included Principle Investigator Alan Stern, Geology and Geophysics Imaging team members Kirby Runyon and Tod Lauer, and Co-investigators Will Grundy, Michael Summers and Kelsi Singer.
At the time of the 2006 vote that demoted Pluto, New Horizons was still in transit to Pluto. Images of Pluto that had been captured did not have enough resolution to capture features like the now-famous “heart” on Mars. When New Horizons flew past Pluto in December 2015, it captured data and images that provided images that Pluto is actually a complex world that might even have an underground H2O ocean.
These team members especially questioned the IAU’s third criterion for what constitutes a planet: that it has cleared its orbit around its parent star. They say that this would disqualify all of the recognized planets in our solar system because other objects orbiting our sun may still cross the planets’ paths and shed dust and debris in the planets’ orbits in the process. This problem increases with distance from the host star. Pluto would have to be many times larger than Earth to clear its orbit, for instance, because it has a longer path to clear. “[E]ven an Earth-sized object in the Kuiper Belt would not clear its zone,” the abstract’s authors noted.
The scientists who wrote the abstract proposed to replace the IAU’s definition with, “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.” For now, brown dwarfs that are larger than conventional planets but too small to ignite nuclear fusion are left out of the equation.
The proposed definition for a planet need not complicate things for teachers because, even then, students would not need to memorize very many more worlds than they do anyway when covering astronomy. Even under the current official definition, planets are classified in categories such as “terrestrial,” “gas giants” and “ice giants.” This change to the definition of a planet would add a couple of new categories such as “moon planets.” This category would cover objects such as the Galilean moons orbiting Jupiter. Ganymede and Titan are both bigger in diameter than Mercury, and Callisto is only a little smaller! “Dwarf planets” would still be used to describe worlds like Pluto and Makemake.
Although the New Horizons team scientists did not mention plans to present this proposed definition to the IAU at a future General Assembly, only five percent of astronomers voted on the resolution that demoted Pluto in 2006. Most astronomers already use the term “planet” in the way that the New Horizons scientists have proposed.