In 2006, the International Astronomical Union (IAU) voted on the definition of a planet. Famously, Pluto no longer met the criteria and was demoted to a dwarf planet. Things have been a bit of a mess ...

The debate over the definition of planethood has been simmering for years – and it bubbled up again this week, thanks to new research into free-floating planemos, or planetary-mass objects. It turns ...

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Dear Mr. Tyson, I think Pluto is a planet. Why do you think Pluto is no longer a planet? I do not like your anser!!! Pluto is my faveret planet!!! You are going to have to take all of the books away ...