This is a guest article by Radhika Bangar, a computer science engineering student from North Maharashtra University, Jalgaon.
Pluto should be a planet again? Scientists argue anew! Pluto’s proponents push to reclassify the demoted Dwarf Planet.
For the first time in its history, IAU (International Astronomical Union) in year 2006 defined the characteristics of a ‘Planet’ in our Solar System. This consequently demoted Pluto from ‘Planet’ to ‘Dwarf Planet’. Formalizing the definition brought these cosmologies into direct collision, necessarily privileging some cosmologies over other. The story of Pluto, from discovery to demotion, illustrates the discursive disruption that stemmed from forced consensus.
History of Pluto
Clyde Tombaugh discovered Pluto in 1930. Astronomers who were studying the orbits of Uranus & Neptune observed that Pluto was at least as big as Earth and immediately classified as ninth planet. Pluto quickly rose to the status of a public icon, embraced by Americans as the first planet discovered by one of their own countrymen.
Pluto is largest known trans-Neptunian object. It is primarily made of ice and rock and is relatively small- about one-sixth the mass of the Moon and one-third its volume. It has moderately eccentric and inclined orbit during which it ranges from 30 to 49 AU from the Sun.
But after 1992, its status as planet was questioned following the discovery of several objects of similar size in Kuiper belt. In 2005, Eris, a dwarf planet in the scattered disc which was 27% more massive than Pluto, was discovered. This led the IAU to define the term “planet” formally in 2006, during their 26th General Assembly. The definition excluded Pluto and reclassified it as a dwarf planet.
But what is Dwarf Planet??
The dwarf planet was a new class added by IAU in 2006. These are small, almost round bodies that orbit the Sun in a belt of other objects. The Solar System is also full of other small bodies, including moons that orbit planets, asteroids, Kuiper belt objects that form a flattened belt beyond Neptune, and comets.
What is Planet then?
IAU tightened the definition of a planet. As per IAU a planet is a celestial body that
- Is in orbit around the Sun.
- Have sufficient mass for its self gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round shape) and
- Has cleared the neighbourhood around its orbit.
The latest volley comes from the study led by University of Central Florida planetary scientist Philip Metzger who says IAU got it wrong.
As per the definition standardised by IAU Pluto ticks first two boxes but fails to satisfy the third because it lives in Kuiper Belt (an area of Solar system packed with the icy bodies). But Metzger argues that the third requirement isn’t valid.
Metzger claimed that there is no support in the research literature for requiring a planet to clear its orbit. As he looked over 200 years back publications he found only one from year 1802 that used this requirement to classify a planet, and that publication was based on now disproven reasoning.
He refers Pluto as “second most complex and interesting planet in our Solar system” and calls IAU’s definition as “sloppy.” He suggests that the planets should be classified based on being large enough that their gravity allows them to be spherical. He points to the Pluto’s moons and its complex geology and atmosphere, saying “It’s more dynamic and alive than Mars.”
The Metzger-led study, which focuses on how asteroids came to be classified differently than the planets in scientific literature, was published online in the journal Icarus.
The same question was raised on IAU’s definition of planet by NASA’s New Horizons team, which guided a spacecraft in a close-up study of Pluto. In 2017, they proposed radical new definition for planets. Pluto was unofficially upgraded by NASA when they completed their mission. The head of American Space Agency Charles Bolden said he still viewed Pluto as a full planet.
Other space scientists also questioned whether Pluto should regain its position alongside the other eight planets after few measurements showed it was larger than previously thought.
“We’re calling Pluto a planet; technically it’s a dwarf planet. I call it a planet, but I’m not the rule maker.” says Bolden. Measurements sent back by New Horizon as it came close to Pluto showed that the dwarf planet was 20-30 kilometres larger than previously thought, with radius of 736 miles. Significantly, it makes it larger than Eris, a body discovered in 2005. The fact that the Pluto was smaller than Eris was one of the strongest arguments for reclassifying Pluto as a minor planet in 2006.
“Maybe we need to reconsider its status again,” said Dr. Brown, an astronomy expert in Nottingham Trent University.
However New Horizon’s principal investigator Dr. Alan Stern claims those terms would also exclude Earth, Mars, Jupiter, and Neptune, all of which also share their orbits with asteroids. Even without full planet status, Pluto still has many secrets to give up. Intriguingly it appears the dwarf planet is red, not the icy grey that was expected. Experts believe the colour arises from the chemical action of sunlight generating red components in the atmosphere that then fall on the surface.
Mr Bolden added: “I expected to see some cold grey icy planet. It has reddish tint, not unlike Mars. That’s fascinating. We continue to be mesmerized by this incredible planet and its moons.”
But the IAU press officer Lars Lindberg Christensen tells CNET there have been no resolutions proposed to revisit Pluto’s classification. “It is nevertheless good and healthy to debate such topics,” he says.
What do young astrophysicists think?
“Pluto should be considered as planet. Although it’s orbital plane is tilted by 17 degrees above. Its orbit is not clear as per IAU but no planet has its orbit clear. It’s just our language and our way of thinking that is changing our perspective towards it.”
-Akshat Pathrikar (PhD student from Indian Institute of Astrophysics, Bangalore.)
What is next....?
Tomorrow or day after tomorrow IAU will accept Metzger’s claims and NASA’s experimental observations as “Experiment is the truth, rest is poetry!!”