Alan Stern has never been happy with Pluto’s demotion to dwarf planet. Principal investigator of NASA’s New Horizons mission, he told Gizmodo the International Astronomical Union’s new definition of a planet, which excludes Pluto, is “bullshit.” So he and a team of other NASA scientists have submitted a proposal to the IAU to once again refine the definition of what makes a planet, which would not only include Pluto, but also pretty much any round object in space that is smaller than a star.
In the introduction of the new proposal, the scientists express concern that Pluto’s demotion to dwarf planet diminishes its standing in the public perception. Apparently a lot of people want to know why NASA sent New Horizons to Pluto if it’s not a planet anymore. To mitigate the public’s concern, they propose a “geophysically-based definition of “planet” that importantly emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties.”
In this case, a planet would be “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.”
As Gizmodo points out, such a definition would make a lot more objects in space planets, including Earth’s moon, but the existing definition excludes a lot of space bodies that deserve new consideration. Here are a few of NASA’s concerns with the existing definition of planets, as broken down by Science Alert:
“First, it recognises as planets only those objects orbiting our Sun, not those orbiting other stars or orbiting freely in the galaxy as ‘rogue planets’,” they explain.
Second, the fact that it requires zone-clearing means “no planet in our Solar System” can satisfy the criteria, since a number of small cosmic bodies are constantly flying through planetary orbits – including Earth’s.
Finally, and “most severely”, they say, this zone-clearing stipulation means the mathematics used to confirm if a cosmic body is actually a planet must be distance-dependent, because a “zone” must be clarified. This would require progressively larger objects in each successive zone, and “even an Earth-sized object in the Kuiper Belt would not clear its zone.”
While Stern formerly expressed concern that astronomers, not planetary scientists, have control over this definition, the final decision rests with the IAU. Pluto fans stay tuned.
Images via NASA