Why is Pluto not a planet anymore?

In the early 1900s, American mathematician and astronomer Percival Lowell pushed forth the theory that the gravitational pull of an unknown planetary body was responsible for the wobbles in the orbits of Uranus and Neptune. In 1905, he began a search for that planet with the help of his observatory’s staff. Though he died without successfully finding it, the search continued and culminated in the discovery of Pluto in 1929 by fellow astronomer Clyde W. Tombaugh, who used a 13-inch telescope and a blink comparator to locate it.

For the next several decades, Pluto — which was given the Roman name of the Greek god of the underworld — was considered the most distant planetary member of the solar system. Located nearly 6,000,000,000 kilometers away from the sun and surrounded by five moons, it remained largely a mystery because of its small size. As technology evolved, scientists, were able to determine that Pluto’s radius is less than half of Mercury’s and that its atmosphere consists of mostly nitrogen, methane, carbon monoxide and hydrogen cyanide.

In August 2006, however, researchers at the International Astronomical Union (IAU), however, voted to reclassify Pluto as a dwarf planet based on a new set of standards. In order for a planet to be defined as one, it had to be in orbit around the sun, must be round and must clear the neighborhood around its orbit. With its eccentric orbit, Pluto, unfortunately, did not meet the last requirement.

There were also other reasons for not giving Pluto planetary status. Over a decade earlier, in 1992, scientists had discovered the Kuiper Belt, a doughnut-shaped ring that comprised of not only Pluto but also other comets, asteroids and small bodies of ice that extended beyond Neptune’s orbit. During the discovery, scientists discussed whether to give some of the other objects in the belt planetary status — one controversial proposal would have listed 12 planets, which would have included an asteroid and Pluto’s moon Charon, in the solar system. The problem was that doing so would have, at some point, resulted in an endless number of yet-to-be-discovered objects also being designated as planets.

Ultimately, the 424 astronomers at the IAU voted to create three main categories in the solar system: planets, dwarf planets and small solar system bodies. As a result, there are now only eight globally recognized planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. While Pluto is known as a dwarf planet today, it still generates heavy interest — just recently, astronomers noticed a heavy drop in Pluto’s atmospheric pressure, which is largely regulated by an ice-covered basin called Sputnik Planitia.

To find out more, check out the above episode of “Space Down to Earth.” If you enjoyed this story, you might want to check out the best photos of Earth taken from space.

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