This Day In Space (August 24, 2006): Pluto loses its planetary status (but not our hearts)

On this day 15 years ago, children across the world had to come up with a new way to remember the names of the planets in the solar system. No more pizzas were coming from our educated mothers, instead it would be… nachos?

Discovery of Pluto as Planet Nine

After the discovery of the 7th and 8th planets, Uranus and Neptune, astronomers believed that a ninth planet was out there based on its possible effects on the new planets, called Planet X. While they had the best information they could find at the time, later data would show us that their numbers were just off.

In a task to photograph the entire night sky, Clyde Tombaugh recognized a faint moving object. After confirming the discovery with further photos, the discovery was dispatched to the Harvard College Observatory and announced to the world in 1930. Teams at the Lowell Observatory where it was found, quickly voted on a name for the new discovery. After shooting down several choices the team voted on Pluto, Roman god of the underworld (sorry not after Micky Mouse’s dog). The name was suggested to them by Venetia Burney, an 11-year-old girl from Oxford, England.

Original plates that shows the discovery of Pluto.

The downfall of the beloved planet

After its discovery, it was thought to be the solution to astronomers’ Planet X theory but soon after the projected mass of Pluto did not meet expectations. Throughout the 20th century, the size of Pluto went from being close to Earth to only 1% of Earth’s size. With more and more experts in the field growing wary of its continued classification as a planet.

Since its discovery astronomers continued to discover more objects with similar orbits to Pluto’s. Eventually discovering that Pluto was the same size and made of the same material, mostly ice, as these other objects. This drew similarities to another unknown object that use to be called a planet, Ceres.

A series of photos by the Hubble Space Telescope showing the rotation of the Pluto.

Ceres was discovered in 1801 between the orbits of Mars and Jupiter. Soon other objects of the same size were found around it. This caused Ceres to be removed from the list of planets and asteroids were added to the classification list. This area is now known os the “Asteroid Belt”.

With this knowledge, the International Astronomical Union held a debate to hash out a final definition of the term “planet”. Within that debate came out the three qualifications to be recognized as one:

  1. The object must be in orbit around the Sun.
  2. The object must be massive enough to be rounded by its own gravity. (AKA being a sphere)
  3. It must have cleared the neighborhood around its orbit. (Basically not surrounded by similar sized objects)

With this decision, Pluto was reclassified as a minor planet. Bringing the number of planets in our Solar System back down to 8.

Pluto’s glow up just to show us it belonged

Photos of Pluto have been sparse, with the small size and far-off distance all the photos have been pixelated disks. That was until NASA’s New Horizons flew past the icy object, launched 8th months before Pluto’s reclassification. There New Horizon captured a pink/orange surface with a massive heart-shaped feature. A real-world analogy that for those of us who grew up with Pluto as a planet will always keep it in our hearts as one.

High resolution photo of the surface of Pluto taken during New Horizons’ flyby.

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