Is Pluto a planet? This Week in Astronomy with Dave Eicher

New Horizons captured this color-enhanced view of Pluto from a distance of 280,000 miles (450,000 kilometers) as it flew by the dwarf planet in 2015. Credit: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute.

Hi folks, tune in every week of 2023 for the best in astronomy from Astronomy Editor Dave Eicher, brought to you by Celestron. Dave’s weekly video series will cover all the latest sky events, scientific results, overviews of cosmic mysteries, and more!

This week, we’re talking about one of the most controversial topics in astronomy and planetary science: Is Pluto a planet?

Of course, according to the International Astronomical Union (IAU) — the body of astronomers that is recognized as the authority on such matters — Pluto officially is not a planet. In 2006, the IAU voted to define a planet as a body that orbits the Sun (and is not itself a star or a moon), is large enough to be round, and has cleared its orbit of other small bodies. That third requirement effectively demoted Pluto, as it is just one of hundreds of thousands of objects that orbit in the Kuiper Belt.

The IAU recategorized Pluto as a dwarf planet, which somewhat confusingly is not a subcategory of planet, resulting in the awkward situation that a dwarf planet is not a planet. The chair of the IAU’s own planetary definition committee, the late, great Owen Gingerich, astronomer and historian at Harvard University, memorably labelled this a “linguistic catastrophe.”

While the IAU has not shown any desire to reconsider its decision, many planetary scientists have simply ignored it. Gingerich eventually concluded that the IAU should never have tried to formally define a planet at all, arguing it is as much a cultural definition as a scientific one. So if you want, you can have it both ways.

For more on Pluto’s planetary status, read our interview with Alan Stern, leader of NASA’s New Horizons mission to Pluto and the Kuiper Belt.