A group of NASA scientists are proposing a new definition of the word planet that would reinstate Pluto’s status – and add tens of other bodies including Earth’s Moon and moons like Titan.
The proposal, led by Alan Stern, the principal investigator of the New Horizons mission to Pluto, claims that the current definition accepted by the International Astronomical Union (IAU) is too narrow and doesn’t jibe with what people understand planets to be.
Stern has previously spoken out about the demotion of Pluto, telling Business Insider in 2015 that the move was “bull—-”.
“Why would you listen to an astronomer about a planet?” he said, complaining that the new definition came from CalTech astronomer Mike Brown rather than a planetary scientist.
“You really should listen to planetary scientists that know something about this subject. When we look at an object like Pluto, we don’t know what else to call it.”
The group proposes the following new definition:
“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 parameter.”
They claim that their definition already fits with the common usage of the word planet, even in peer-reviewed planetary science publications. And the proposal states that the IAU definition of a planet is “technically flawed”.
“First, it recognizes as planets only those objects orbiting our Sun, not those orbiting other stars or orbiting freely in the galaxy as “rogue planets.” Second, it requires zone clearing, which no planet in our solar system can satisfy since new small bodies are constantly injected into planet-crossing orbits, like NEOs near Earth. Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent, requiring progressively larger objects in each successive zone. For example, even an Earth sized object in the Kuiper Belt would not clear its zone,” the proposal explains.
Stern and his colleagues want to help the public and policymakers see the value in exploring beyond the main planets of our Solar System.
“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 proposal says. “To wit: a common question we receive is, “Why did you send New Horizons to Pluto if it’s not a planet anymore?””
The new definition would have historical precedence, sound scientific classification and match up with peoples’ intuition, the proposal states.
“In our numerous talks with the public, we find they resonate happily with the geophysical definition we offer, especially as it is a definition reflecting a body’s intrinsic physical properties, not its location, and is a definition that leverages their intuition. This definition highlights to the general public and policymakers the many fascinating worlds in our Solar System that remain unexplored and are worthy of our exploration, along with the necessary budgets.”
Even if the IAU decides to accept the new definition of planets, it would take some time to become official, so for now, Pluto will have to wait.