A group of scientists have come together to launch a campaign to bring Pluto back into the club of planets. They argued that it deserves to be a full planet, along with more than 110 other bodies in the solar system, including Earth's moon.

The six scientists, from institutions across the United States, presented a paper at an international planetary science conference, explaining that geological properties, such as shape and surface features, should determine what constitutes a planet.

The International Astronomical Union (IAU) in 2006 adopted a definition for a planet based on characteristics that include clearing other objects from its orbital path which led to the reclassification of Pluto as a dwarf planet. The decision left the solar system with eight planets.

But, the paper's lead author, Kirby Runyon, a doctoral candidate at Johns Hopkins University, and his colleagues argue that the IAU does not have the authority to set the definition of a planet. "There's a teachable moment here for the public in terms of scientific literacy and in terms of how scientists do science," Runyon added. "And that is not by saying, 'Let's agree on one thing.' That's not science at all."

Runyon's group advocates for a sub-classification system, similar to biology's hierarchal method. This approach would categorise Earth's moon as a type of planet too.