When Was Pluto Discovered and Why Was It Demoted From Being a Planet?

It has now been 15 years since ex-planet Pluto was re-classified as a dwarf planet by the International Astronomical Union (IAU).

On August 24, 2006, the IAU convened at its 26th general assembly meeting to discuss a number of topics, including planet classification.

That day, members agreed that in order to be a "planet" a world has to have a certain mass and shape, along with other characteristics involving its orbit and how it clears debris within its orbit.

In simple terms, it meant that tiny, icy Pluto suddenly did not meet the characteristics needed to be a planet as it has not cleared its debris field.

So the number of planets in the Solar System was reduced to eight: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

The IAU said in a press release at the time that "the 'dwarf planet' Pluto is recognized as an important prototype of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects."

The decision has not been without controversy. Some—including former NASA administrator Jim Bridenstine—have contested that Pluto is still a planet in their view.

Regardless, the little world is still a dwarf planet today as far as the IAU is concerned. Below and some facts about what was once the ninth planet to orbit the sun.

Cold, dark, and distant

Pluto was discovered in 1930 by astronomer Clyde W. Tombaugh at the Lowell Observatory in Flagstaff, Arizona.

The dwarf planet's name was given by an 11-year-old girl, Venetia Burney, who lived in Oxford, England at the time.

On hearing about the planet's discovery, she told her grandfather the distant world should be named after Pluto, the Roman god of the underworld. Burney's grandfather sent the suggestion to the Lowell Observatory and it stuck, according to NASA.

Pluto really is tiny. At around 1,400 miles wide, Pluto isn't even as big as the Earth's moon and is only half as wide as the U.S. Pluto's biggest moon, Charon, is roughly half as big as Pluto itself.

It's also distant. On average, it is located around 3.6 billion miles away from the sun—around 39 times as far away from the sun as Earth is. It takes 5.5 hours for sunlight to get there.

Because of its distance, Pluto is also very dark. If someone were to stand on Pluto at noon and look up, they would find that the sun is only 1/900th as bright as it would be on Earth.

Without a spacesuit, nobody would be standing on Pluto for long. Its thin atmosphere of nitrogen, methane and carbon monoxide would not be breathable and its average temperature is -387 F.

It takes 248 Earth years to orbit the sun and each day—the time it takes to fully rotate—takes around 153 hours.

Photos previously taken of Pluto show that planet is home to huge mountains and glaciers.

This image shows Pluto photographed by a camera on board NASA's New Horizons spacecraft in July 2015. The planet is tiny compared to the Earth. Getty / NASA / APL / SwRI