While many people can point to a picture of Jupiter or Saturn and call it a "planet," the definition of this word is much more subtle and has changed over time. Many astronomers decided on a new definition in 2006 after the discovery of several worlds at the fringes of the solar system — a decision that remains controversial.
The International Astronomical Union defined a planet as an object that:
orbits the sun
has sufficient mass to be round, or nearly round
is not a satellite (moon) of another object
has removed debris and small objects from the area around its orbit
The IAU also created a newer classification, "dwarf planet," which is an object that meets planetary criteria except that it has not cleared debris from its orbital neighborhood. This definition meant that Pluto — considered a planet at the time — was demoted and reclassified as a dwarf planet.
But not all scientists agree with this classification, particularly after the New Horizons spacecraft flew by Pluto in 2015. The spacecraft revealed a complex world full of geological change. This included mountains reaching as high as 11,000 feet (3,500 meters), a heart-shaped region nicknamed Tombaugh Regio that contains methane ice and other substance, and weird ice-ridged terrain that looks like snakeskin, among many other features Since then, members of the New Horizons team have made scientific presentations arguing Pluto is indeed a planet. The new scientific findings continue to amaze the public, Alan Stern said in 2018.
I think my two biggest surprises were first, just how utterly amazing Pluto turned out to be — how many different kinds of features were on the surface and even in the atmosphere," Stern said in a NASA podcast interview. "There was something for everyone. And the second amazing finding was how many members of the public really wanted to participate in it and just be a part of this exploration. We expected it would be a big response, but it was much bigger than we thought."