Is Pluto a Planet?

**SPEAKER_1** (0:00)
This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales. Using automation, analytics, and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (0:31)
This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome, that's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up required compatibility and availability varies 18 plus.

**Matt O'Dowd** (0:56)
You know what a planet is, right? A big round thing that orbits a star?
Not so fast. The surprisingly vicious debate over the planetary status of Pluto has given us a fascinating glimpse into what a scientific definition really is. And that perhaps the word planet is too vague to be used as a scientific definition at all.
We love to classify things. Labels help us keep stuff organized in our heads. In science, categorization provides a fast and easy way to know the properties of a member of a group just by knowing what group it belongs to. Chemists group elements on the periodic table. Those groups exhibit similar chemical properties that reflect outer shell electron number. Biologists group organisms by similar physical characteristics, and this taxonomy reflects genetic relationships. Astronomers are all about space taxonomy. We classify galaxies based on their shape, black holes based on how they feed and how they're oriented, stars based on their color and brightness, and planets by a set of criteria that has caused more tension and heartbreak than any made up grouping scheme really should. Because a change in that scheme demoted Pluto from planet to not planet. Today, we're going to settle whether this was reasonable and whether we should keep the word planet at all. The definition of planet has changed a lot. If you were an ancient astronomer, like say Ptolemy, the planets were the Asteris Planitai, the wandering stars. These included Mercury, Venus, Mars, Jupiter and Saturn, but also the Sun and the Moon, basically anything that moved relative to the background stars. Note that they did not include the Earth. This definition of planet was the most sensible classification for thousands of years, based on our observations and understanding of the universe.
But understanding improves. In 1543, Nicholas Copernicus published On the Revolutions of the Heavenly Spheres, which cast the Sun and not the Earth as the center of the universe. And the Earth in its proper place among the planets. This picture was cleaned up by the observations of Galileo and Brahe and the mathematical models of Kepler and Newton. Newton's law of gravity led to consistent predictability of the motion of the heavenly spheres.
The solar system finally made observation and theoretical sense. There were now six planets orbiting the Sun in perfect mathematical harmony.
But new discoveries were about to mess up this beautifully simple picture. See, that's the thing about science. No so-called fact is ever perfectly safe. Everything is subject to revision if evidence turns against it.

**SPEAKER_1** (3:57)
This episode is brought to you by Accenture. When you're advertising operations fall out of sync, everything else follows. Spotify and Accenture are working together to reinvent the rhythm of ad sales, using automation, analytics and smarter workflows to simplify campaign delivery and access better data across the business. The result? Less time spent on operations, more time connecting brands with the moments and fandoms that matter most. Learn more at accenture.com/spotify.

**SPEAKER_2** (4:27)
This episode is brought to you by Google Chrome. You think you know a browser, but Gemini and Chrome? That's new. It can help you with practically anything on the web, like restoring a vintage motorcycle from a 50-page restoration block, or finally break down that long article you've had open for weeks. Gemini and Chrome is here for it. Ready to make anything online make sense? There's no place like Chrome. Check responses set up require compatibility and availability varies 18 plus.

**Matt O'Dowd** (4:53)
That's the beauty of the scientific process. Keep this in mind when we get to Pluto. New wandering stars were discovered in the centuries following Newton. Uranus had been spotted many times throughout history, but was only identified as a planet after William Herschel recorded its movement in 1781 That same motion almost perfectly reflected the clockwork predictions of Newtonian mechanics.