Scientists claim Pluto and Moon should be classified as planets. This is amid concern that current definitions of planets are rooted in folklore, astrology, and other factors. 

Experts studied how a planet’s definition has changed since the time of Galileo in the 17th century to the decision by the International Astronomical Union (IAU) to create a new definition in 2006, which meant Pluto was no longer classed as a planet.

The IAU’s controversial definition of a planet requires it to ‘clear’ its orbit – in other words, be the largest gravitational force in its orbit.

Pluto was made a planet because Neptune is affected by Pluto’s gravity. Pluto orbits with the Kuiper Belt and frozen gas objects.   

However, authors of this new study argue that the IAU’s current definition was rushed ‘before vital issues were sorted’ and should now be rescinded. 

They claim that a better way to define a planet is whether or not it was ever geologically active. However, this would require moons, asteroids, and other objects.

This would potentially mean there would be several hundred objects classified as planets in our Solar System – one of the arguments for keeping the IAU’s classification. 

Pluto (pictured) is not a planet, but a dwarf planet. In 2006, the International Astronomical Union, a global group of astronomy experts, established a definition of a planet that required it to 'clear' its orbit – in other words, be the largest gravitational force in its orbit

Pluto (pictured above) is not considered a planet. It’s a dwarf planet. In 2006, the International Astronomical Union, a global group of astronomy experts, established a definition of a planet that required it to ‘clear’ its orbit – in other words, be the largest gravitational force in its orbit


Pluto is not a planet, but a dwarf planet.

The International Astronomical Union (a worldwide group of experts in astronomy) established in 2006 a definition for a planet. It had to “clear” its orbit. This means it must have the greatest gravitational force within its orbit..

Pluto was made a less important planet because Neptune’s gravity affects Pluto. Pluto also shares an orbit with Kuiper-belted objects and frozen gases. 

According to the researchers, however, moons can’t be considered planets because of 1800s astrology. 

Charlene E. Delich (a researcher and geologist at Johns Hopkins University Applied Physics Laboratory) stated that “for the term planet,” she and most planetary scientists believe round icy moons are planets. 

They all possess active geologic processes driven by internal processes. 

“As a geologist it’s much easier to separate planets by their intrinsic attributes than by orbital dynamics.

Detelich was 10 years old at the time that the IAU took the 2006 decision. He had just begun learning about planets and this was his first exposure to them.

She said, “I have always been troubled by the argument that the 8-planet Solar System model should be preserved for easy memorization for schoolchildren,”

Imagine how they would see the world if they understood the complexity of the universe, and where we fit in it. We’re not just one of the eight planets; we’re one of more than 200. 

In a five-year review of the last 400 years of literature on the subject of planets, published in the journal Icarus, the researchers found that the geophysical definition of a planet established by Galileo – that a planet is a geologically active body in space – had been ‘eroded’. 

Galileo Galilei, who was born in Pisa in 1564, is considered one of the fathers of modern science

Galileo Galilei is considered to be one of the fathers in modern science. Galileo Galilei was a Pisa native in 1564.


International Astronomical Union (IAU) adopted the latest definition of planet in 2006. 

It states that a planet must accomplish three things. 

It should orbit a star in our galaxy, which is the Sun.

It should be large enough that it has enough gravitation to cause it to take on a spherical form. 

– It should be so large that it’s gravity has wiped out all other similar objects in its vicinity around the Sun. 

From the time Galileo proposed it in 1600s based on his observations of the Moon’s mountains, to the beginning of the 1900s, this geophysical definition was widely used in scientific literature.

Phillip Metzger (a University of Central Florida planetary scientist) said that Galileo’s proposal for planets to revolve around the Sun and reconceive Earth as a globe got him imprisoned. 

“When scientists agreed with his view, he was in some sense released from prison. Then, in the early 1900s we sent him back to jail when we adopted this popular concept of an ordered number of planets. 

“So in a way, Galileo was rejailed.” In a way, we want to get Galileo released from jail so his deep insights will become crystal clear. 

The study found that there was an abrupt decline in papers about planet science from the 1920s to 1950s.

‘We’ve shown through bibliometrics that there was a period of neglect when astronomers were not paying as much attention to planets,’ said Metzger.

“And it was during this period of neglect, that the transmissions of pragmatic taxonomy from Galileo were interrupted.

The researchers also blame almanacs – annual books that contain information such as weather predictions that rely on astrological factors, such as planetary position.

Popular almanacs of the middle 18th century ‘disseminated a view of the natural world infused with religious significance’, the study authors say – in other words, they clouded the work of Galileo with pseudoscience.

The popular almanacs of the middle 18th century 'disseminated a view of the natural world infused with religious significance', the study authors say

Study authors claim that popular 18th-century almanacs ‘disseminated the view of the natural environment infused with religious significance.


Discovered ByClyde W. Tombaugh

DiscoveryGet it hereThe date: February 18, 1930

Temperature of the Surface: -387°F (-232°C)

Orbit period: 247.92065 Earth Years

Orbit distance5874,000,000 km (39.26AU) 

All known moons: 5 (Charon, Nix, Hydra, Kerberos and Styx)

Diameter: 1,471 miles (2,368 km)

Mass: 13,050,000,000,000 billion kg (0.00218 x Earth) 

Many almanacs published today publish valid astronomical information, but they also feature recipes, folklore, gardening and home remedies.  

Although almanac popularity declined during the period of neglect in planetary science from the 1910s to the 1950s their influence remained. 

According to researchers, this is how astrological views such as that moons don’t belong on planets began to creep into scientific literature. 

Metzger stated that while it might not seem much, this change undermined Galileo’s central notion about the planets. 

“Planets” were not defined anymore by their complexity, active geology, and potential for civilisation. 

“Rather, they were defined as simple and followed certain idealised routes around the Sun.

Space missions were a hot topic in the 1960s.

Some scientists used the Galileo-derived geophysical description again during this time period.

However, the dismissal of moons and many other planetary objects as being less than planets was the belief that prevailed when the IAU decided to vote on the definition in 2006, the researchers say. 

And to justify that belief, the IAU proposed an additional requirement for a planet – that it must clear its own orbit.


New research has shown that Pluto’s atmosphere is receding with its distance from the Sun.  

Pluto requires 248 Earth-years to orbit the Sun. It is also quite far from its nearest point, which at 30 AU (2.77 billion miles) and 50 AU (4.76 billion miles).    

To observe the distant planet as it passes in front of a star in Mexico, researchers set up telescopes. This briefly illuminated the dwarf world and exposed its small nitrogen rich atmosphere.

They observed that Pluto’s orbit is becoming longer and farther from the Sun, causing it to get colder.

Read more: Pluto’s atmosphere is disappearing, study finds

Metzger stated that scientists had tried to devise a mathematical method to justify the existence of a limited number planets. This was to prove that each planet must clear its orbit.

“And this was actually developed post-facto in order to maintain an ordered, limited number of planets.” 

Clearing an orbit describes a planet’s trajectory at the moment, but it does not reveal its intrinsic nature. This is what the team claims. 

It was also shown that scientists never used it as a criteria for classifying the planets.

Metzger says, “It’s an actual description of the current status of things.” Metzger says that if a star crosses our Solar System and causes disruption, planets won’t have their orbits clear anymore.

‘It’s like defining mammals – they are mammals whether they live on the land or in the sea. 

It’s not their physical location. It is about their intrinsic qualities that make them who they are. 

Planetary scientist Paul Byrne at Washington State University, who was not involved with this new study, also believes in the geologically active planetary definition.

Byrne stated that the 2006 vote on the classification of the new planets was overwhelmingly cast by astronomers, rather than scientists in Science Focus. 

The IAU also recommended that children not be surrounded by posters of the hundreds of planets found in the solar system. 

Pluto’s ice-covered 'heart' is clearly visible in this false-colour image from NASA’s New Horizons spacecraft. The left, roughly oval lobe on Pluto's surface is the basin informally named Sputnik Planitia. Pluto's largest moon, Charon appears top left

Pluto’s ice-covered ‘heart’ is clearly visible in this false-colour image from NASA’s New Horizons spacecraft. This is Pluto’s basin, officially called Sputnik planitia. Charon is Pluto’s biggest moon. It appears at the top of the page.

“If Pluto is recognized as a world, should the Moon be considered a planet?” Or Ganymede. Jupiter’s largest moon and larger than Mercury, but less massive. Geologist: My opinion is, “Yes. Why not?” Byrne said.

He said that children could’recite their name with ease as they list dinosaurs with the same ease they would have if the definition had been changed. Several hundred Solar System objects are now officially classified as planets.

‘Whenever I give a public talk and mention Pluto, the first question I’m asked is whether Pluto is still a planet – not, say, why its surface looks so damned weird.

The Fifteenth Year on’ [from the IAU’s reclassification]Pluto’s dismissal gets more media attention than what we have learned about Pluto, which is an unfortunate science communication failure. 

As new technology such as James Webb Space Telescope, which launched this month, makes it possible to discover more planets outside our Solar System (exoplanets), the issue becomes increasingly important. 


In November, NASA revealed a grainy photo of Pluto’s dark side, six years after it was taken by its New Horizons spacecraft. 

The image – taken in July 2015 when Pluto was 3 billion miles from Earth – shows the portion of the dwarf planet’s landscape that wasn’t directly illuminated by sunlight. 

Researchers used 360 images from New Horizons’ fly-by to create the image.  

To capture the shot, you had to leverage light from Charon (the largest of Pluto’s five moons), in order to get it. 

The image reveals a large and ‘conspicuously bright’ region midway between Pluto’s south pole and its equator, which may be a deposit of nitrogen or methane ice, similar to Pluto’s icy ‘heart’ on its opposite side. 

Learn more: Researchers present Pluto’s dark side in new photos