This week marked the 90th anniversary of the discovery of Pluto.

It's considered a dwarf planet — a designation that continues to spur vigorous debate.

Pluto also continues to fascinate astronomers because of its small size, distance from Earth, unusual orbit, and synchronous moons.

NASA's New Horizons mission launched in 2016 to explore Pluto and other icy objects in the Kuiper Belt.

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Ninety years ago this week, a young astronomer named Clyde Tombaugh discovered a small, icy object beyond the reaches of the known solar system. It was Pluto.

Astronomers had suspected Pluto's existence for about 15 years before that — astronomer Percival Lowell had observed deviations in Neptune and Uranus' orbits that suggested the gravitational pull of another body.

Lowell ultimately died without finding that mysterious "Planet X," as he called it. Tombaugh continued his work.

When he discovered Pluto, Planet X became real — for a time. But in 2003, a world larger than Pluto was found in the Kuiper Belt — the region beyond Neptune that's thought to contain trillions of other icy bodies. That eventually prompted the International Astronomical Union to reclassify Pluto as a dwarf planet.

Many planetary scientists still think it's a planet, though.

"How can an adjective in front of a noun not describe the noun?" Alan Stern, the principal investigator of NASA's New Horizons probe, said in an interview in 2017. "There are dwarf stars but they're still considered stars."

In 2016, the New Horizons mission launched to explore Pluto, its moons, and the Kuiper Belt. The probe has sent back unprecedented images of Pluto and other distant space objects — here are some of our best images of the dwarf planet, and what its discovery taught us about the solar system.