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Scientists against the demotion of objects like Pluto, Eris, Sedna, etc. to "dwarf planet" status have crafted a new definition:
It's no secret that Alan Stern and other scientists who led the New Horizons mission were extremely displeased by Pluto's demotion from planet status in 2006 during a general assembly of the International Astronomical Union. They felt the IAU decision undermined the scientific and public value of their dramatic flyby mission to the former ninth planet of the Solar System.
But now the positively peeved Pluto people have a plan. Stern and several colleagues have proposed a new definition for planethood. In technical terms, the proposal redefines planethood by saying, "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." More simply, the definition can be stated as, "round objects in space that are smaller than stars."
From the proposal:
The eight planets recognized by the IAU are often modified by the adjectives "terrestrial," "giant," and "ice giant," yet no one would state that a giant planet is not a planet. Yet, the IAU does not consider dwarf planets to be planets. We eschew this inconsistency. Thus, dwarf planets and moon planets such as Ceres, Pluto, Charon, and Earth's Moon are "fullfledged" planets. This seems especially true in light of these planets' complex geology and geophysics. While the degree of internal differentiation of a given world is geologically interesting, we do not use it as a criterion for planethood in the spirit of having an expansive rather than a narrow definition.
Here's another article about the significance of the New Horizons mission. New Horizons will fly by 2014 MU69 on January 1, 2019.
(Score: 3, Insightful) by khallow on Wednesday February 22 2017, @04:19PM
You end up with the solar system being Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, Ceres, Pluto, Haumea, Makemake, and Eris now, which isn't too bad, but there are an estimated 200 objects with similar characteristics to Pluto, and no one is going to remember that list.
Why is that supposed to matter? The real problem here is that the current definition only works for the Solar System. In other star systems, you may have either lack of information to determine the "clearing of orbit" of a body or weird orbital dynamics that prevents us from even defining "clearing the orbit" (such as in a binary star system where multiple large exoplanets have the same resonance dynamics with respect to the smaller star as Pluto and the other current trans-Neptunian dwarf planets have with respect to Neptune, alternately hang out in the L4/L5 positions of the binary system, or orbiting inside a ring of material like Saturn's "shepherd moons").
It's hard to determine orbital dynamics and what other bodies may be precluding the potential planet from being a planet, but fairly easy to determine mass.