Is Pluto a Planet? Scientists Battle Over the Answer

Pluto has a complicated past. Once it was a planet, but then, it was downgraded. Now, it's a "dwarf planet," and again, at the center of a scientific feud. A team of scientists argues that Pluto should once again be called a planet. They say dozens of other similar objects in the solar system should also be classified planets.

Pluto was discovered in 1930. And it had been classified a planet until 2006. But then the International Astronomical Union (IAU) changed the definition of a planet. It said a planet must be round, orbit the sun, and have the largest gravitational force in its orbit.

Pluto meets two of those conditions: It’s round and orbits the sun. But Pluto is surrounded by objects called “plutinos.” They share its orbit. That caused the IAU to drop Pluto from the list of planets. So, that leaves our solar system with just eight planets. They are Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

Now, researchers argue in a new study that the IAU definition is flawed. They say it's based more on astrology than planetary science. (Astrology is a pseudoscience. It connects human behavior to the movements of objects in space.) The researchers based their conclusion on data from NASA’s New Horizons probe. It visited Pluto’s orbit in 2015.

“We think there’s probably over 150 planets in our solar system,” the study’s lead author said.