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The new definition required a planet to do three things. First, it must orbit the sun. Second, it must have enough gravity to mold itself into a round shape. Third, it must have enough gravity to clear its orbit of other objects. Pluto didn’t meet the third requirement. It orbits the sun in a ring of icy bodies beyond Neptune called the Kuiper Belt. So, Pluto was demoted to “dwarf planet.”
Planet or not, one thing is certain about Pluto: It is just as fascinating a place as Mars, Jupiter or any other member of the planetary club. And it’s very different from those other orbs. Pluto’s orbit, for instance, isn’t circular, and it doesn’t occur on the same plane as the big planets. Its tilted, oval-shaped path takes it above and below the orbits of the other planets. And while Pluto sits an average 5.8 billion kilometers (3.6 billion miles) from the sun, sometimes it approaches closer than Neptune.
Pluto is tiny. At about 2,380 kilometers (1,400 miles) wide, it is only half the width of the United States. But the dwarf planet’s surface features were largely a mystery until NASA’s New Horizons mission flew by in 2015. That spacecraft was the first to view Pluto up close. The spacecraft’s observations unveiled Pluto’s famous heart-shaped glacier. New Horizons also revealed Pluto’s dunes of methane ice and a gash in the planet twice as deep as the Grand Canyon. These features and others make up Pluto’s richly varied topography — another reason some scientists say Pluto should count as a planet.
Even as the debate over Pluto’s planetary status rages on, some sky watchers seek a new ninth planet. This would be a planet about 10 times as massive as Earth, lurking on the fringes of the solar system.
The motions of some icy bodies in the Kuiper belt have hinted at the existence of such a planet, known as Planet Nine or Planet X. Other recent evidence suggests it does not exist. But for many researchers and citizen scientists, the hunt is on.
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