Pluto could be set to regain its planetary status after 11 years in exile, if NASA scientists have their way.
A new definition of planets would add over 100 to our solar system, with even Earth’s moon due a promotion.
The International Astronomical Union (IAU) currently requires an object to be orbiting the Sun to be classified as a planet.
But the NASA team wants the IAU to drop that requirement, insisting that a world’s physical properties are more important than their interactions with stars.
“In keeping with both sound scientific classification and peoples’ intuition, we propose a geophysically-based definition of ‘planet’ that importantly emphasises a body’s intrinsic physical properties over its extrinsic orbital properties,” the researchers explain.
The proposal was made by a team of NASA scientists led by Alan Stern, principal investigator of the space agency’s New Horizons mission to Pluto.
Stern’s team suggests a 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 parameters.”
That might sound pretty broad, but it rules out a number of celestial objects, including white dwarfs, neutron stars and black holes.
Stern has previously spoken out about the IAU’s decision to demote Pluto to a dwarf planet in August 2006.
The decision was made after astronomer Mike Brown from the California Institute of Technology proposed a new definition of planets which required such worlds to clear the neighbourhood around their orbit.
Pluto fell foul of this rule.
In an interview with Business Insider in 2015, Stern said the move was “bullshit”.
“Why would you listen to an astronomer about a planet,” Stern, a planetary scientist, asked.
Astronomers focus on a wide variety of celestial objects and cosmic phenomena, while planetary scientists specialises in planets, moons and planetary system.
Stern compared asking the advice of an astronomer over a planetary scientist was like going to a podiatrist for brain surgery.
“Even though they’re both doctors, they have different expertise,” Stern said. “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.”
Under the new definition, our moon, and other moons such as Titan, Enceladus, Europa and Ganymede would all be promoted to planetary status.
The proposal is at least partly motivated by the public’s perception of the importance of non-planetary worlds within our solar system.
The researchers write: “A common question we receive is, ‘Why did you send New Horizons to Pluto if it’s not a planet anymore?’”
There’s no guarantee the IAU will accept the new definition, and even if they do, it’s set to be some time before it becomes official.
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