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The "United Nations" of astronomers has announced a new definition of what a planet is, slightly revising the description preferred by an international panel including an MIT professor that was tasked with the challenge.
Members of the International Astronomical Union (IAU) voted on August 24 to define a planet as an object that is in orbit around the sun, is large enough for its own gravity to pull it into a nearly spherical shape, and has cleared the neighborhood around its orbit -- in other words, it has no other large bodies crossing its path.
The third condition was added to the draft definition of a planet submitted to the IAU about a week ago by MIT's Richard Binzel and colleagues. Binzel, a professor of planetary science in the Department of Earth, Atmospheric and Planetary Sciences, took responsibility for presenting the final version of the resolution at the time of the final vote.
This means that while Pluto will still be considered a planet, it will technically be a dwarf planet because it is smaller than Mercury. It will be joined in that category by Ceres and 2003 UB313 (a temporary name for an object discovered only three years ago). More "dwarf planets" are expected to be announced by the IAU in the coming months and years.
As a result of the new definition, our solar system now contains eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune, plus the three dwarf planets led by Pluto.