Textbooks had to be rewritten. Members of the public were outraged. Our understanding of the solar system itself was forever changed on Aug. 24, 2006, when researchers at the International Astronomical Union (IAU) voted to reclassify Pluto, changing its status from a planet to a dwarf planet — a relegation that was largely seen as a demotion and which continues to have reverberations to this day.
Several years later, many still don't quite understand all the fuss, nor why Pluto was knocked from its planetary position. But the solar system's transformation from nine planets to eight was a long time in the making and helps encapsulate one of the greatest strengths of science — the ability to alter seemingly steadfast definitions in light of new evidence.
What is a planet, anyway?
The word planet stretches back to antiquity, deriving from the Greek word "planetes," which means wandering star. The five classical planets — Mercury, Venus, Mars, Jupiter and Saturn — are visible to the naked eye and can be seen shifting in strange pathways across the sky compared with the more distant background stars. After the advent of telescopes, astronomers discovered two new planets, Uranus and Neptune, which are too faint to spot with the naked eye.
