Thursday, February 23, 2017

Pluto is still a planet, and geophysical planet definition proposal is not a "non-starter"

In his Feb. 22 New Scientist comment, "Pluto is Still an Ex-Planet," Mike Brown attempts to impose one view in an ongoing debate as fact.

The community of planetary scientists who support Pluto's planet status is not, as he describes, "a small but vocal group" and is not limited to those on the New Horizons mission. Neither did a "majority of astronomers" reject the notion of Pluto retaining its planet status in 2006.

With these statements, Brown presumes a consensus in the science community that never existed. Planetary scientists and astronomers were just as divided about Pluto's status and about how to define the term "planet" 10 years ago as they are today.

Only four percent of the IAU voted on the controversial 2006 definition, and most were not planetary scientists but other types of astronomers. Their decision was immediately rejected by an equal number of professional planetary scientists in a formal petition released just days later.

Brown's claim that nothing about Pluto has changed since 2006 ignores the extensive findings of the New Horizons mission, all of which show Pluto to be a geologically living object with wind-blown dunes like those on Earth and Europa; flowing glaciers not seen anywhere else in the solar system besides Earth and Mars; tectonic forces; an internal heat source; cryovolcanoes, and even a possible subsurface ocean. All these are features characteristic of planets.

His citation of the discovery of Ceres as a precedent actually works against the argument he makes. Nineteenth-century telescopes were not powerful enough to resolve Ceres into a disk, so astronomers of the day had no way of knowing that unlike the asteroids discovered after it, Ceres is in hydrostatic equilibrium. As shown by NASA's Dawn mission, Ceres is very different from the majority of asteroids, which are largely rubble piles. Like Pluto, it experiences complex geological processes and may harbor an underground ocean.

It is misleading to conflate the new proposal's inclusion of Earth's Moon as a satellite planet with the geocentric view of the universe held 500 years ago. Compositionally, spherical moons are much like the terrestrial planets except for the fact that they orbit other planets instead of orbiting the Sun directly. Designating them as "moon planets," as the proposal does, sufficiently distinguishes them from planets in primary orbits around the Sun.

We can have a scientifically consistent definition of planet that includes numerous subcategories to account for both orbital dynamics and intrinsic properties.

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