New Horizons Redefines Definition of "Planet" and "Moon"


New Horizons

Based on stories in the New York Times (and elsewhere) it would seem that the NASA New Horizons mission team is redefining what it means to be a "planet" and a "moon". Do NASA and the IAU agree?

NASA Spacecraft Get a Closer Look at Dwarf Planets Pluto and Ceres, NY Times

"Marc W. Buie, a planetary scientist at the Southwest Research Institute in Boulder, Colo., and a member of the New Horizons team, agrees with Dr. Stern but wishes the issue would go away. Years ago, people would be fascinated to hear the scientific puzzles about Pluto. Now, conversations usually start with "Is Pluto a planet?" "It's a very annoying, distracting issue," Dr. Buie said. "You have to get past this wall of this nonscientific issue before you get to the good things."

"The moon is such a planet I can't even stand it," [Fashion designer and QVC co-host] Mizrahi says, exasperated. "Well, what else is it if it's not a planet?" Under Dr. Stern's definition, Mr. Mizrahi would win the argument. "I am happy to defend him," Dr. Stern said via email Sunday. "I see no logical reason why large moons that are in hydrostatic equilibrium should not be considered planets too, and I call them that." Dr. Stern's classification system distinguishes moons as "secondary planets," while "primary planets" directly orbit around the sun -- pushing the number of planets in the solar system to more than 20."

Keith's note: So ... let's see if I understand the New Horizons mission's revised solar system nomenclature: Planets orbit the sun. Planets also orbit other Planets. Moons orbit Planets but Moons do not orbit the sun otherwise they'd be Planets which also orbit Planets and the sun. But wait - there's more: now we need to add Primary Planets and Secondary Planets into the mix. So when does a Moon become a Secondary Planet? Is it still a Moon also? Can Planets be Moons and Moons be Planets?

Iapetus is not in hydrostatic equilibrium so it is not a Planet (right?). But it is a Moon (right?). But Iapetus is larger than Ceres which is .. a Planet (right?) Pluto's Moon Charon is smaller than Iapetus but Pluto fans refer to it as a Planet. Alas, Pluto fans always love to use the "Titan is larger than Mercury" argument to justify Titan as a Planet.

I can't wait to see how all NASA education materials are adjusted for the New Horizons mission so as to tackle this issue. Textbooks will clearly need to be revised to reflect NASA's latest discoveries. Who determines how these revisions will be made? Will other missions be required to adapt accordingly or is NASA going to be talking about more than one system of planetary nomenclature? What will happen at press events - will NASA be required to issue press releases in both nomenclatures (as well as English/metric)? WiIl this IAU Vs Pluto fans thing just drag on and on?

The oddest thing of all is how the Pluto fans rant about how some small group of people at IAU made this decision about what a Planet is without consulting everyone else - yet the Pluto fans have gone out and proclaimed this new nomenclature for Planets and Moons without consulting anyone else. Pot-Kettle-Back.

Why make things more complex? Our solar system has lots of worlds. Ice worlds. Rock worlds. Gas worlds. Some worlds are big others are small. Some worlds orbit the sun and are "planets". Some worlds orbit planets and are called "moons". This simply defines the location of a world - not its inherent physical nature. #OcamsRazor

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