Far beyond the orbit of Neptune, five billion kilometers away, a small world made of ice and rock moves along its 248 year long orbit around the Sun.
It has icy mountains, nitrogen glaciers, and peaks which are thought to be cryovolcanos. Underneath its varied surface, the possibility of a liquid ocean.
Officially discovered on February 18th, 1930 by Clyde Tombaugh. He had been looking for a planet for almost a year. And then it happened. The movement of a point of light on a photographic plate, measured in millimeters, had revealed the existence of a whole new world.
The classical planets, visible to the naked eye, have been known since antiquity. Mercury, Venus, Mars, Jupiter, and Saturn. Though Uranus had been visible to astronomers, it wasn’t considered a planet. It made the official list only after William Herschel spotted it with a telescope on March 13th, 1781.
The case of Neptune was more interesting. Invisible to the naked eye, the presence of the eighth planet was first predicted mathematically. It was calculated based on the perceived disturbances in the orbit of Uranus. And then on the night of 23rd September 1846, Johann Galle found Neptune within one degree of the predicted position.
Celestial mathematics and telescopes were pulling away at the covers and extending the boundaries of our solar system like never before.
So when the orbit of Neptune didn’t make sense, yet again experts hypothesized the presence of an unknown planet causing the perturbations.
And so began, the search for planet X.
One that would go on for over a hundred years. To this day, that hypothetical planet believed to be at least 5 to 10 times the mass of the Earth has not been found.
Astronomer Percival Lowell dedicated his life to finding it. He founded the Lowell Observatory to aid this task. But it wasn’t to be. He died in 1916, unaware of the mark he would leave on the history of astronomy.
It was the search for planet X that led Clyde Tombaugh to his discovery. After nearly a year of painstaking effort which included photographing images of parts of the sky and then studying them to spot tiny differences.
Two images of the sky, taken over the course of six days. January 23rd and 29th, shows the movement of a point of light.
That point of light, was Pluto.
It was clear right away. Pluto wasn’t the planet they were actually looking for.
5 to 10 times the mass of the Earth, it was not. An object that size wouldn’t have been as faint as Pluto appeared to be. The estimate was brought down to one Earth mass.
After rechecking and validating their results, it was announced to the world on March 13th, 1930, the birth anniversary of Percival Lowell. Also the same day that Uranus was discovered 149 years earlier.
Newspapers announced the discovery of a new planet at the edge of the solar system. The first in 84 years. Falconer Madan read the news to his granddaughter at breakfast. He wondered what the new planet would be called. The eleven year old Venetia Burney suggested they should call it Pluto, after the Roman God of the underworld.
This suggestion made its way to the Lowell Observatory through intermediaries and beat out other names like Minerva and Cronus.
The name ‘Pluto’ fit nicely for the dark and mysterious planet at the edge of our solar system. At the observatory they also appreciated the fact that it had the initials of Percival Lowell.
The debate over Pluto is hardly a new one. Right from its discovery, experts were not convinced it was a planet. The earliest fact learned about the new planet was its unusual orbit. While all the other eight planets have orbits that are nearly circular and lie in the same plane, Pluto’s orbit was highly elliptical and was titled upwards.
Citing this as evidence, some claimed that Pluto was probably a large asteroid or a comet. But it didn’t matter. The excitement would win over the skepticism. At least for another seventy six years.
Astronomers now had a whole new world to study. But being so far away, it wasn’t easy and it took some astronomical detective work and improvements in technology to uncover its secrets.
One of the first things to be understood was Pluto’s day length. By studying the rhythm at which it brightened up and dimmed down, its light curve, astronomers estimated Pluto took about 6.39 Earth days to rotate on its axis once.
Estimates of its size would continue to shrink over the decades but in 1978, the discovery of Pluto’s moon Charon would finally give us clarity on the matter.
First the orbit of Charon was determined by observing its motion. Once this was done, using the laws of physics they were able to calculate Pluto’s mass. And it turned out to be quite small. About 1/400th the mass of Earth. Smaller than our moon.
The trend with Pluto seemed to be that the more we learned about it, the less it seemed like the traditional planets.
In 1992, the discovery of 1992QB1 would prove the existence of the Kuiper Belt. A region in the outer solar system consisting of small icy bodies. Pluto’s neighborhood.
In the following years, hundreds of these Kuiper belt objects were discovered giving further insight into what Pluto is, or isn’t.
The growing list of Kuiper belt objects fueled further doubt about Pluto but nothing changed. In 1999, the then head of the IAU even responded to speculation of demotion by saying Pluto would remain a planet.
In 2005, Eris was discovered at the outer edges of the Kuiper belt. At the time it was believed to be bigger than Pluto. And if Pluto was a planet, then so was Eris was the claim.
Until then no official definition for the word ‘planet’ existed. The discovery of Eris however, would force the experts to reconsider and come up with one.
That definition would end Pluto’s planethood.
In January 2006, the New Horizons mission was launched. The first ever spacecraft to visit Pluto. Seven months later in August, the IAU met in Prague and voted on the definition of a planet. By this definition, to be a planet in the solar system, a celestial body must -
- Be in orbit around the sun.
- Have sufficient mass to achieve hydrostatic equilibrium (Nearly round shape)
- Cleared the neighborhood in its orbit.
Pluto fails to meet the third criteria as it shares its neighborhood with other Kuiper belt objects.
A new category, dwarf planet, was created. One that Pluto would be the prototype for. A dwarf planet is the same thing as a planet except that it doesn’t clear its neighborhood and is not a moon. There are now five of these bodies in our solar system. Ceres, Pluto, Eris, Haumea, and Makemake.
This new definition would give rise to one of the most popular astronomical debates in recent times. Though officially concluded, 14 years later the debate rages on.
It essentially boils down to this. Should a planet be defined by its intrinsic qualities like composition and geological activity or its dynamical relationship with other bodies in the solar system?
While arguments are raised on both sides, there is one thing the two sides agree on. It is more than a matter of semantics. What is defined as a planet or a dwarf planet has importance. Classification is a significant part of doing science. How they decide to classify the planets is where the division stems from.
The matter is far from settled and is likely that it will go on until a new definition is created.
But what we call Pluto and which category we put it in has no impact on how it behaves out in space. Being reclassified as a dwarf planet didn’t make it any less interesting.
As the New Horizons fly by would show us, Pluto wasn’t a dead ball of ice floating in the outer edges of the solar system, it was a complex and active world.
And on July 14th 2015, humanity would get its first real look it.
After a nine year journey, the New Horizons spacecraft had arrived. Apart from all the instruments on board, it carried nine mementos. One of them, probably the most symbolic, a container holding Clyde Tombaugh’s ashes.
The spacecraft was greeted by the large brightly colored heart shaped surface, now called Tombaugh Regio. The western lobe of this region, Sputnik Planitia, is the wide smooth region that doesn’t have any crater impacts and is estimated to be as young as 10 million years old, indicating active geology on Pluto.
Another point in favour of this idea is that Sputnik Planitia’s surface is covered with polygonal shaped cells believed to be caused by sublimation due to internal heating.
One of the most interesting features in this region is the mountain named Wright Mons which shows a depression. This is believed to be a cryovolcano. A volcano that erupts water ice instead of lava.
In contrast to this relatively young surface that is devoid of craters. The darkly colored region on Pluto’s surface is covered with a large number of craters and is thought to be billions of years old.
The dark red substance on the surface is speculated to be a hydrocarbon called 'tholins' that form from the methane and nitrogen in Pluto’s atmosphere interacting ultraviolet light and cosmic rays from the Sun.
A geologically active world with mountains and glacial flows certainly excited scientists. But the most interesting speculation to come out has been that Pluto could have a subsurface liquid ocean.
While there are different lines of reasoning as to why this may be the case, scientists cannot be absolutely certain, though they are confident. A future orbiter mission would make things clearer and the New Horizons mission has definitely made a case for such an undertaking.
Though it lost its planetary status by the time the spacecraft arrived, there was no doubt when the flyby was completed that Pluto was bigger than Eris, making it officially the largest object in the Kuiper belt.
Pluto was an accidental discovery. It happened to be in the right place at the right time.
It wasn’t the first object in space to be reclassified but it was the first to be at the center of a large scale debate in regards to definitions. One that even the general public seemed to be interested in.
Definitions and categories are created to ease research and study. Conventions for convenience. Changes in these areas are usually due to updated understanding. Disagreements and debates on other hand show us the field is active and healthy.
Some scientists say that Pluto has not been ‘demoted’. Its status in fact has elevated due to the change in classification. It has gone from being the runt of the family of planets to becoming the king of the Kuiper belt.
And as the prototype for a new class of planets, it has become a representative of a new system.
Objects like Pluto beyond Neptune are now called ‘Plutoids’. And objects that have an orbit similar to Pluto are called ‘Plutinos’.
Whatever you decide to call Pluto, it is a fascinating object with an interesting history worth looking into.