Pluto should be reclassified as a planet, scientists claim

A team of scientists will have Pluto classified as a planet again – along with dozens of similar bodies in the solar system and any found around distant stars.

The call goes against a controversial decision from 2006 of the International Astronomical Union, which decided that Pluto is only a “dwarf planet” – but scientists say a reconsideration will put science back on track.

Pluto had been considered the ninth planet since its discovery in 1930, but the IAU – which names astronomical objects – decided in 2006 that a planet should be spherical, orbit the sun and have gravitationally “cleared” its orbit for other objects.

Pluto meets two of these requirements – it is round and it orbits the sun. But because it shares its trajectory with objects called “plutinos”, it did not qualify 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 — and Pluto was expelled from the list.

But a study announced in December by a team of researchers in the journal Icarus now claims that the IAU’s definition was based on astrology – a type of folklore, not science – and that it harms both scientific research and the popular understanding of the solar system.

Scientists say that Pluto should instead be classified as a planet under a definition used by scientists since the 16th century: that “planets” are all geologically active bodies in space.

In addition to Pluto, this definition includes many other objects – the asteroid Ceres, for example, and the moons Europe, Enceladus and Titan. But researchers say the more the better.

“We think there are probably over 150 planets in our solar system,” said Philip Metzger, the study’s lead author and planetary physicist at the University of Central Florida.

The study comes amid research based on data from NASA’s New Horizons probe, which flew by Pluto in 2015.

The probe’s revelations have revived the debate over Pluto’s status, said planetary geologist Paul Byrne of North Carolina State University.

“There was such an interest from New Horizons flyby,” said Byrne, who was not involved in the investigation. “But every time I gave a lecture and I posted a picture of Pluto, the first question was not about the geology of the planet, but why was it degraded? That was what got stuck in people, and that’s really annoying. ”

The scientists claim that the IAU definition contradicted a definition of a planet that had stood for centuries.

Objects similar to Pluto, such as Eris and Makemake, had been found in 2006, and so the IAU constructed its definition to exclude them, Metzger said.

This led to the IAU – and therefore the public – adopting the “astrological” concept that the Earth and the other planets were few and special, instead of a better classification that would have significantly increased the number of planets, he said.

The result is that most planetary scientists now ignore the IAU’s definition, he said.

“We continue to call Pluto a planet in our newspapers, we continue to call Titan and Triton and some other moons with the term ‘planet’,” he said. “Basically, we ignore the IAU.”

The definition has taken on new meaning as better techniques and telescopes – such as the James Webb Space Telescope – will detect more “exoplanets” around distant stars.

Metzger said most star systems are not like ours. Instead of a handful of planets orbiting at great distances, they often have a few very large planets, possibly orbiting large moons orbiting very close to their star.

This means that any definition based on our solar system will not be relevant to most of the others.

“Because of the diversity of planetary architectures that we are discovering, we think it’s important to get it right at this point,” Metzger said.

But it seems that there is no driving force in the IAU to change its definition, and the campaign to make Pluto a planet again has not been welcomed by proponents of the 2006 resolution.

Caltech astronomer Michael Brown, author of the memoir “How I Killed Pluto and Why It Had It Coming,” says the IAU made the right call by correctly classifying it as a dwarf planet.

“I think the IAU has corrected an embarrassing mistake that had been perpetuated for generations,” he said in an email. “The solar system now makes sense.”

Jean-Luc Margot, 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 correctly because it would normally be impossible to determine if an exoplanet was geologically active or not.

Another recent study looks at a strange feature seen in the New Horizons photographs – the polygonal spots visible on Pluto’s surface.

Lead author Adrien Morison, a physicist at the University of Exeter in the United Kingdom, said that polygons are caused by sublimation – the process of melting directly from a solid to a gas – by nitrogen ice. The ice left behind cools and becomes denser than before, and then it sinks and is replaced by ice from below. The result is a landscape that has been compared to a “lava lamp”.

“The boundaries of the polygons are where the cold ice goes down, while the center of the polygons is where the warmer ice from below goes up,” he said in an email.

The polygons show that Pluto changes from geological processes at low temperature. But explanations are needed for other features, such as its mountains and surface defects, he said. “We still know very little about all the processes that could take place there.”

Both Morison and Byrne agree that the IAU classification has had a scientific impact, and believe that Pluto and similar bodies should be classified as planets.

But “it’s not very crucial whether the IAU agrees,” Morison said. “It does not prevent us, as scientists, from using a more convenient definition for our purposes.”

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