A group of scientists wants Pluto to be classified as a planet again, along with similar objects in the Solar System and those found around distant stars.
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.
New debate
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.
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.
Space telescopes are getting more sophisticated
This definition is becoming increasingly important as space telescope techniques and technology improve, such as the James Webb Space Telescope which will find more exoplanets around distant stars.
Metzger says most star systems are not like ours. Instead of several planets orbiting at great distances, they often have several very large planets, perhaps in orbit by large moons, that revolve very close to their star. That means any definition based on our Solar System will be irrelevant to most other definitions.
"Because of the diversity of planetary architectures that we discovered, we thought it was important to improve this at this time," Metzger said.
Considered a science error
But apparently, there is no push in the IAU to change its definition, and the campaign to make Pluto a planet again was not welcomed by supporters of the 2006 resolution.
Caltech astronomer Michael Brown, author of the memoir "How I Killed Pluto and Why It Had It Coming", said the IAU made the right decision by correctly classifying it as a dwarf planet.
"I think the IAU corrects a shameful mistake that has been perpetuated for generations. The solar system now makes a lot of sense," he said.
Jean-Luc Margot, a professor and astronomer at the University of California, Los Angeles, added in an email that the IAU definition helps the study of exoplanets by classifying them properly, as it is usually impossible to determine whether an exoplanet is geologically active or not.
Another recent study looked at a strange feature seen in New Horizons photos, in the form of polygonal patches visible on Pluto's surface.
Lead author Adrien Morison, a physicist at the University of Exeter in the UK, said the polygon pattern was caused by the sublimation of nitrogen ice. The remaining ice cools and becomes denser than before, so it sinks and is replaced by ice from below. The result is a landscape likened to a "lava lamp".
The polygons show Pluto changing from a low-temperature geological process. But explanations are needed for other features, such as ridges and surface faults. "We still know a little bit about all the processes that could happen there," he said.
Both Morison and Byrne agree the IAU classification has scientific implications, and think Pluto and similar objects should be classified as planets.
"But it doesn't really matter whether the IAU agrees or not, it doesn't prevent us as scientists from using a more convenient definition for our purposes."
