06 December 2015

UCLA: “UCLA professor proposes simpler way to define what makes a planet”

The new approach would require only estimates of the star’s mass and the planet’s mass and orbital period — all of which can be easily obtained with Earth — or space-based telescopes. According to Margot’s criteria, all eight planets in our solar system and all classifiable exoplanets — the large bodies that orbit stars other than our sun — would be confirmed as planets.

When applied to our own solar system, the test clearly places the eight planets into one distinct category and the dwarf planets — Ceres, Pluto and Eris — into another. The disparity between planets and non-planets is striking, Margot said. The sharp distinction suggests that there is a fundamental difference in how these bodies formed, and the mere act of classifying them reveals something profound about nature.

Stuart Wolpert

Interesting approach to a decade-long debate about the definition of planets – and the subsequent demotion of Pluto from this category. It’s so straightforward that I am frankly surprised nobody attempted it before. Curiously, when applied to the Moon, the planet test is above the threshold (although much lower than for the other rocky planets), so if the Moon would orbit the Sun instead of Earth, it should be classified as a planet rather than a dwarf.

Mass required to clear an orbital zone in Solar System
Mass required to clear an orbital zone as a function of semi-major axis for a host star of mass 1 M. From the paper A Quantitative Criterion for Defining Planets by Jean-Luc Margot

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