Alan Stern is a Planetary Scientist at NASA and has led many planetary and lunar missions, including the New Horizons Pluto-Kuiper Belt mission, a robotic spacecraft on its way to explore the dwarf planet Pluto. He talks about what qualifies an object in space as a planet.
NOVA: The Pluto Files
NOVA: 10th Planet
Comment on This Article
Support for WGBH is provided by:Become a WGBH sponsor