The call goes against a controversial resolution from 2006 by the International Astronomical Union that decided Pluto is only a “dwarf planet” — but the researchers say a rethink will put science back on the right path.
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 must be spherical, orbit the sun and have gravitationally “cleared” its orbit of other objects.
Pluto meets two of those requirements — it’s round and it orbits the sun. But because it shares its orbit with objects called “plutinos” it didn’t qualify under the new definition.
As a result, the IAU resolved the solar system only had eight major planets — Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune — and Pluto was relegated from the list.
But a study announced in December from a team of researchers in the journal Icarus now claims the IAU’s definition was based on astrology — a type of folklore, not science — and that it’s harming both scientific research and the popular understanding of the solar system.
The researchers say Pluto should instead be classified as a planet under a definition used by scientists since the 16th century: that “planets” are any geologically active bodies in space.
As well as Pluto, that definition includes many other objects — the asteroid Ceres, for example, and the moons Europa, Enceladus and Titan. But the researchers say the more the merrier.
“We think there’s probably over 150 planets in our solar system,” said Philip Metzger, the study’s lead author and a 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 debate about Pluto’s status, planetary geologist Paul Byrne of North Carolina State University said.
“There was such interest from the New Horizons flyby,” said Byrne, who was not involved in the study. “But every time I gave a talk and I put up a picture of Pluto, the first question was not about the planet’s geology, but why was it demoted? That’s what stuck with people, and that’s a real shame.”
The researchers argue 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 by 2006, and so the IAU engineered its definition to exclude them, Metzger said.
That led to the IAU — and therefore the public — adopting the “astrological” concept that Earth and the other planets were few and special, instead of a better classification that would have greatly increased the number of planets, he said.
The result is that most planetary scientists now disregard the IAU’s definition, he said.
“We are continuing to call Pluto a planet in our papers, we are continuing to call Titan and Triton and some other moons by the term ‘planet’,” he said. “Basically, we are ignoring the IAU.”
The definition has gained new importance as better techniques and telescopes — such as the James Webb space telescope — will discover more “exoplanets” around distant stars.
Metzger said most star systems are not like ours. Instead of a handful of planets orbiting at large distances, they often have a few very large planets, possibly orbited by large moons, circling very close to their star.
That means any definition based on our solar system won’t be relevant to most of the others.
“Because of the diversity of planetary architectures that we’re discovering, we think it’s important to get it right at this time,” Metzger said.
But it seems there is no impetus in the IAU to change its definition, and the campaign to make Pluto a planet again is not welcomed by champions of the 2006 resolution.
Caltech astronomer Michael Brown, the 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 fixed an embarrassing mistake that had been perpetuated for generations,” he said in an email. “The solar system is now sensible.”
Jean-Luc Margot, a professor and astronomer at the University of California, Los Angeles, added in an email that the IAU definition aids the study of exoplanets by correctly classifying them, because it would usually be impossible to determine if an exoplanet was geologically active or not.
Another recent study looks at a curious feature seen in the New Horizons photographs — the polygonal patches visible on Pluto’s surface.
Lead author Adrien Morison, a physicist at the University of Exeter in the United Kingdom, said the polygons are caused by the sublimation — the process of melting directly from a solid to a gas — of nitrogen ice. The ice left cools and becomes denser than before, and so it sinks and is replaced by ice from below. The result is a landscape that’s been likened to a “lava lamp.”
“The boundaries of the polygons are where the cold ice goes down, while the center of the polygons are where the hotter ice from below goes up,” he said in an email.
The polygons show Pluto is changing from low-temperature geological processes. But explanations are needed for other features, such as its mountains and surface faults, he said. “We still know very little about all the processes that could go on there.”
Both Morison and Byrne agree the IAU classification has had a scientific impact, and think Pluto and similar bodies should be classified as planets.
But “it’s not particularly crucial whether the IAU agrees,” Morison said. “It doesn’t prevent us, as scientists, from using a more convenient definition for our purposes.”