How the definition of a planet has changed over 250 years — and why the count may not be final
By
Samantha Mathewson
Summary
This article traces the evolving definition of a planet over the past 250 years, from the six known planets in 1776 to today's eight. It explores how discoveries of asteroids, Pluto, and trans-Neptunian objects forced astronomers to repeatedly revise what counts as a planet, culminating in the 2006 IAU reclassification that demoted Pluto to dwarf planet status. The piece examines whether the current tally is final or if future discoveries could change it again, reflecting on how scientific classification systems evolve with new knowledge.
Source
Twitter / XHow the definition of a planet has changed over 250 years — and why the count may not be finalspace.comKey quotes
· 3 pulledOver the past 250 years, the number of 'planets' in our solar system has ranged from six to nine — and, briefly, even 11 — depending on what astronomers knew at the time and how they defined a planet.
As the United States prepares to celebrate its 250th anniversary, that changing tally offers a unique lens on humanity's evolving understanding of the cosmos since 1776.
The 2006 reclassification of Pluto from planet to dwarf planet remains one of the most controversial decisions in modern astronomy.
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