Pluto was first discovered in 1930 by Clyde W. Tombaugh at the Lowell Observatory in Flagstaff Arizona. Astronomers had long predicted that there would be a ninth planet in the Solar System, which they called Planet X.
The story behind Pluto’s name is also famous. It was suggested by an 11-year-old girl in England, who was interested in Roman legends and thought naming the icy planet after the god of the underworld was intriguing.
Planet X was named Pluto after the classical god of the underworld, but almost immediately there were doubts as to whether Pluto could really be Planet X. It was very faint, and its disc remained unresolved even in the world’s largest telescopes.
Astronomers weren’t sure about Pluto’s mass until the discovery of its largest Moon, Charon, in 1978. And by knowing its mass (0.0021 Earths), they could more accurately gauge its size. The most accurate measurement currently gives the size of Pluto at 2,400 km (1,500 miles) across. Although this is small, Mercury is only 4,880 km (3,032 miles) across. Pluto is tiny, but it was considered larger than anything else past the orbit of Neptune.
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Pluto has an icy shell, dunes made of solid methane ice, and mountain peaks covered in methane snow (but the snow is red instead of a fluffy white). It’s also home to the largest known glacier in the solar system.
In fact, Pluto is so cool that its temperature is around 400 degrees Fahrenheit below zero, and it gets even colder as it orbits farther away from the sun. Typically, Pluto is so far from the sun that sunlight is only as bright as a full moon on Earth.
Astronomers estimate that there are at least 70,000 icy objects, with the same composition as Pluto, that measure 100 km across or more in the Kuiper Belt. And according to the new rules, Pluto is not a planet. It’s just another Kuiper Belt object.
When Pluto was demoted, it prompted a wave of science textbook reprints to ensure that students of the new millennium would be taught Pluto is a dwarf planet. Pluto received a new classification: it was a dwarf planet, but this decision is not without its critics. Pluto may have been demoted, but it plays a vital role in understanding the conditions of the early Solar System.
Pluto planetary days are remembered fondly — for decades it was notable for being our solar system’s smallest and farthest planet. It’s only about half the width of the United States and lies in a far out region of the solar system called the Kuiper Belt, which requires a telescope to see.
Over the last few decades, powerful new ground and space-based observatories have completely changed previous understanding of the Outer Solar System. Instead of being the only planet in its region, like the rest of the Solar System, Pluto and its moons are now known to be just a large example of a collection of objects called the Kuiper Belt.
Pluto was obviously small, perhaps only the size of Earth or even Mars. Observing the planet was extremely difficult, but a handful of dedicated astronomers continued the work. The small size, combined with its unusual orbit, began to cast serious doubt as to whether Pluto could really be a planet at all.
In 2015, NASA’s New Horizons Program flew past Pluto to take close-up photos and measurements of the dwarf planet, ultimately revealing that Pluto is bigger than scientists originally thought.
Even the principal investigator for the New Horizons spacecraft, planetary scientist Alan Stern, didn’t agree with the IAU and claimed Pluto was demoted simply because of its distance from the sun.
Some astronomers point out that there are inconsistencies in the wording of Resolution 5A – while it is true that Pluto hasn’t cleared its orbit, neither has Earth or Jupiter. Earth orbits with 10,000 near-Earth asteroids, while 100,000 Trojan asteroids lie within Jupiter’s orbit.
According to NASA, the data gathered by the New Horizons flyby “clearly indicated that Pluto and its satellites were far more complex than imagined,” prompting space enthusiasts to wonder if it would regain planet status.
Planets are celestial objects large enough to be made rounded by their gravitational orbit around the Sun and to have shooed away neighboring planetary objects and debris.
The main event of the 2006 General Assembly of the IAU, the proposal that would come to demote Pluto, from its position as the ninth planet from the Sun to one of five “dwarf planets”. It was a defining moment for the rest of the solar system as well.
Pluto is now classified as a dwarf planet because, while it is large enough to have become spherical, it is not big enough to exert its orbital dominance and clear the neighborhood surrounding its orbit.
Dwarf planets are celestial bodies that only meet the first two criteria in the new definition of a planet. Dwarf planets, like Pluto, have not yet cleared the neighborhoods of their orbits — and still have some cleaning to do if they will ever become “true” planets.
Criteria of a Planet
A celestial body that (a) is in orbit around the Sun, (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, (c) has not cleared the neighbourhood around its orbit.
Pluto met the first two criteria, but the last one proved pivotal. “Clearing the neighbourhood” means that the planet has either “vacuumed up” or ejected other large objects in its vicinity of space. In other words, it has achieved gravitational dominance.
Pluto is relatively round and orbits the sun, but it does not meet the criteria because its orbit crosses Neptune’s orbit. Critics of the resolution argue that other planets in the solar system, including Earth, have not cleared the neighborhood around their orbits. Earth, for example, regularly encounters asteroids in and near its orbit.
Because Pluto shares its orbital neighbourhood with other icy Kuiper Belt Objects, the resolution effectively stripped the distant world of a planetary designation it had held for some 76 years.
It was immediately relegated it to the distinct category of “dwarf planet”, alongside the biggest body in the asteroid belt, Ceres, and other large Kuiper Belt Objects such as Eris, Quaoar and Sedna.
Pluto’s orbit is erratic. The planets in our solar system all orbit the sun in a relatively flat plane. Pluto, however, orbits the sun at a 17-degree angle to this plane. In addition, its orbit is exceptionally elliptical and crosses Neptune’s orbit.