Proposed Planet Definition Means Solar System Has 110 Planets

Proposed Planet Definition Means Solar System Has 110 Planets
NASA/CXC/M.Weiss
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The International Astronomical Union currently defines a planet as:

"a celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (c) has cleared the neighborhood around its orbit."

A newly suggested definition takes a different approach:

"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."

To the layperson, the differences between the two might seem of little consequence. Astronomers, however, will immediately recognize a significant gulf. The first definition delineates eight planets in our solar system: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. The second definition expands the field to 110! The dwarf planet Ceres would be a full-fledged planet. Earth's Moon would be a planet. And yes, Pluto would once again be a planet.

Which definition do you prefer? The first is undoubtedly more conventional. Haggled over mightily more than a decade ago, it has the backing of the majority of astronomers. But don't discount the second. In a proposal set to be shared at the upcoming Lunar and Planetary Science Conference, six astronomers make a strong case on its behalf.

For starters, they insist that the present definition is inherently 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... Finally, and most severely, by requiring zone clearing the mathematics of the definition are distance-dependent..."

This means that an Earth-sized object in the Kuiper belt could not be considered a planet, they say.

Most importantly, the authors argue that the current definition relegates a number of fascinating worlds to second-class status, some without proper classification. For example, Ganymede and Titan, though each larger than the planet Mercury, remain relatively forgotten amongst the general public, simply because they respectively orbit Jupiter and Saturn as moons. Meanwhile, at the edge of the solar system, the icy world Sedna defies definition.

Indeed, one could argue that the present wording is almost too solar system-centric, defining a planet based upon tired tradition and the skewed dynamics of our own cosmic neighborhood.

This leads into the best argument for the new definition. It "emphasizes a body’s intrinsic physical properties over its extrinsic orbital properties," the authors say. While the current definition is fit for the solar system, the newly suggested version is fit for the Universe.

Will it gain traction in the astronomical community? We'll have to wait and see.

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