The call runs counter to a controversial 2006 resolution by the International Astronomical Union (IAU) that decided Pluto was just a "dwarf planet". But researchers argue and say rethinking will get science back on track.
Pluto has been considered the ninth planet since its discovery in 1930, but the IAU, as the agency that names astronomical objects, decided in 2006 that a planet should be spherical, orbit the Sun and gravitationally "clear" its orbit of other objects.
Pluto satisfies two of those requirements, it is spherical in shape and orbits the Sun. But because it shares its orbit with objects called "plutinos", Pluto is argued to be ineligible under the new definition.
As a result, the IAU decided that the Solar System had only eight major planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Pluto was also removed from the list.
But a study published in December from a team of researchers in the journal Icarus now claims the IAU"s definition is based on astrology, not science. According to them, it is detrimental to scientific research and popular understanding of the Solar System.
The researchers say Pluto should be classified as a planet under the definition used by scientists since the 16th century: that a "planet" is a geologically active object in space.
Apart from Pluto, the definition includes many other objects, such as the asteroid Ceres and the moons Europa, Enceladus and Titan. But researchers say the more the merrier.
"We think there may be more than 150 planets in our Solar System," said Philip Metzger, lead author of the study and a planetary physicist at the University of Central Florida.
The study comes amid research based on data from NASA"s New Horizons probe that flew over Pluto in 2015. The investigation"s revelations have reignited debate about Pluto"s status , said planetary geologist Paul Byrne of North Carolina State University.
"There is such interest from flying over New Horizons," said Byrne, who was not involved in the study. "But every time I give a lecture and I put up a picture of Pluto, the first question isn"t about the geology of the planet, but why was it taken down? That"s what sticks with people, and it"s a real shame."
"Researchers argue that the IAU"s definition contradicts the centuries-old definition of a planet. Objects similar to Pluto, such as Eris and Makemake, were discovered in 2006, so the IAU engineered its definition to exclude them," Metzger said.
That led to the IAU, and the public, adopting the "astrological" concept that Earth and the other planets were few and special, instead of a better classification that would greatly increase the number of planets. The result is that most planetary scientists now ignore the IAU definition.
"We keep calling Pluto a planet in our papers, we keep calling Titan and Triton and several other moons "planets". Basically, we ignore the IAU," explains Metzger.