More than 15 years after the International Astronomical Union demoted Pluto from planet to "dwarf planet," some scientists are saying the distant world should be reinstated as a planet—and joined by more than 100 other objects in our solar system. In a study published in the journal Icarus, researchers argue that the definition of planet has shifted over the centuries, and the IAU's reclassification of Pluto pushed the definition closer to the nonscientific beliefs of astrologers and further away from the definition used in the time of Copernicus. Pluto was demoted because it doesn't "clear its own orbit" of other objects, but researchers say a more useful definition of planet would include Pluto because it is geologically active.
"The folk taxonomy of our Solar System, which sees planets as an orderly system of just a few culturally significant objects, reflects the values and cosmological theories of the non-scientific public during the early 1800s when it developed," researchers wrote. Planetary geologist Paul Byrne says many scientists are already ignoring the IAU's definition, NBC reports. "We are continuing to call Pluto a planet in our papers, we are continuing to call Titan and Triton and some other moons by the term 'planet,'" he says.
Researchers say classifying geologically active bodies in space as planets would include the asteroid Ceres and numerous moons in the definition, as well as Pluto and other dwarf planets. "We think there's probably over 150 planets in our solar system," says lead study author Philip Metzger, a planetary physicist. With the launch of the James Webb Space Telescope, Metzger says it's more important than ever to define a planet accurately. He says scientists have already discovered that most solar systems do not resemble our own, but have large planets orbiting very close to their stars. "Because of the diversity of planetary architectures that we’re discovering, we think it’s important to get it right at this time," he says. (Read more Pluto stories.)