What Happened to Pluto? Why Is It No Longer a Planet but Has Become a World?

On February 18, 1930, Pluto was discovered by Clyde Tombaugh at the Lowell Observatory in Flagstaff, Arizona. For several decades, the most distant planet from the Sun has long been considered the ninth planet in our Solar System until its status was downgraded into a dwarf planet.

What Happened to Pluto?

Pluto is located in the Kuiper Belt, a region beyond the orbit of Neptune which contains hundreds of thousands of icy, rocky bodies. For a long time, astronomers used to believe that Pluto is a unique entity in the Kuiper Belt. However, as scientists learned more about this zone, they discovered that there are lots of celestial bodies which resemble Pluto.

On August 24, 2006, Pluto was demoted by the International Astronomy Union (IAU) during their general assembly in Prague, Czech Republic. Before the resolution, the term "planet" had no working definition and was only based on significant discoveries in outer space. As new objects were found in the Kuiper Belt, it has become necessary for experts to revisit the definition of a "planet."

For instance, an object named 2003 UB313 or Eris was found in the fringes of the Kuiper Belt. This object is just slightly smaller than Pluto, and it was thought it would officially get a planet status at the IAU meeting. However, if IAU would classify Eris as a planet, the same decision should be given to other Pluto-size bodies such as Makemake, Sedna, and Haumea.

The IAU came up with three rules to establish a planet's definition. First, the object must be orbiting the Sun. Second, its mass must be enough to make it roughly spherical. Lastly, the object must have an orbit free from other bodies with a group comparable to its own.

Pluto satisfies the first two conditions but not the third. While Pluto is large enough to become spherical, it is not big enough to exercise gravitational dominance in its orbit and clear the neighborhood that surrounds its orbit.

As a result, Pluto is now considered a dwarf planet along with the other objects in the same category, such as Ceres, Eris, and Makemake. It has been assigned as Dwarf Planet 134340.

A Complex and Mysterious World

Recently, astronomers used the term "world" in describing places in the Solar System. When talking about dwarf planets or moons, they use the titles "icy world," "volcanic world," or "ocean world."

On July 14, 2015, the New Horizons interplanetary space probe flew past Pluto, becoming the first spacecraft to explore the dwarf planet up close. The mission took 9.5 years before achieving the epic flyby. The Dawn spacecraft orbited and explored Ceres that same year, providing thousands of images of the celestial body.

The New Horizon and Dawn missions have revealed much more than astronomers expected. From these data, the two new dwarf planets were revealed to be candidates for "ocean worlds," just like Jupiter's moon Europa and Saturn's moons Enceladus and Titan.