Pluto: our favorite Dwarf Planet

Pluto-the dwarf planet

Pluto, the most loved planet! Wait, what! Did I say planet? A dwarf planet actually.

Used to be a planet, before the IAU decides to draft new rules and our poor little Pluto doesn’t obey one of it. 

let us take a look at the reason why Pluto was demoted from a planet to a dwarf planet.

According to IAU (International Astronomical Union) the rules for a celestial body to be considered as a planet, in other words, what makes a planet a planet:

1. It should be revolving around the Sun.  

Pluto – check

2. It should have enough gravity to form into a sphere (well approximately)

Pluto – check 

3. It should have a clear space around its self. 

Pluto – speechless 

pluto fails to obey the 3rd rule since its orbit is, umn.. not so clean with a considerable amount of space debris. These rules came into play in 2006 after which Pluto was demoted as a planet. 

So what is Pluto considered now?

well as the title suggests, Pluto is a dwarf planet. 


Do you remember the story behind the discovery of the planet Neptune

well if you don’t, you can check it out right here-

Planet Neptune, everything you want to know

Here’s a glimpse of what happened, 

Urbain Le Verrier, a French astronomer, and a mathematician. He notices perturbations in the orbit of Uranus.

He used nothing but mathematics to pinpoint the location of planet Neptune on 23rd September 1846, the reason for the disturbance in the orbit of Uranus.  

The quest for the 9th planet also began in a similar way. 

Long after the discovery of the 8th planet, Neptune, the orbit of Uranus was analyzed again and it was noticed that it still was disturbed by yet another body that was beyond the orbit of Neptune.

If it has the potential to disturb the path of a planet, then the object must be substantial in size. 

The problem triggered the dose of mystery and there it was, the chase to find the ninth planet. 

The search starts with this American businessman, author, mathematician, and astronomer – Percival Lawrence Lowell. Founder of the Lowell Observatory in Flagstaff Arizona (1894).

He was the one who first started the project to search for Pluto in the year 1906. In 1916, after Percival Lowell passed away, the research was discontinued.

Even after 10 years of prolonged research, there wasn’t much that was found.

Surprisingly, in the data they had extracted, there remained two pictures that contained a faint presence of Pluto which they happened to miss out for some reason. These pictures were taken on March 19th and April 7th, 1915. 

After his death, Percival Lowell had willed most of his estate to the observatory. But his wife Constance contested the will.

Because of this reason the search continued to be on hold until 1927 when the litigation was resolved in the favor of the Lowell observatory’s favor. 

When the search restarted, the observatory director – Vesto Melvin Slipher handed the job of finding he ninth planet to the 23 years old, newly hired Lowell Observatory assistant – Clyde Tombaugh.

He was hired by Slipher after he saw a sample of his astronomy drawings. 

After a year of searching, Tombaugh discovered a possible moving object in the photographic plates using a blink comparator. 


He took multiple images of a specific region of the sky where Lowell had predicted the ninth planet to be. Next, he compared the photos that are some days apart using the blink comparator

Blink operator
blink comparator

What the blink comparator did was, it rapidly blinked the images after superimposing them. When seen, the stars appeared to remain stationary whereas this planet X (the ninth planet/pluto) appeared to change position. 

Finally, on 18th February 1930, Pluto was discovered.  


The next thing that came after the discovery of this very awaited, mysterious body was what to name it?

The news of the discovery was opened to the world on 13th March 1930

Lowell Observatory at that time had received over a thousand suggestions from all over the world. 

Falconer Madan was the head of the Bodleian Library at the University of Oxford read the news with his 11 years old granddaughter Venetia Burney.

Venetia was very interested in mythology and suggested the name Pluto to her grandfather which meant – Roman God of the underworld.

Madan loved the name and immediately contacted his friend Herbert Hall Turner who was an astronomer. Herbert then contacted the Lowell university to suggest the name. 

Finally, 3 names were shortlisted – 




Pluto got all the votes. Tombaugh and his fellow astronomers also decided to choose the name ‘Pluto’ because its first two letters had the initials of Percival Lowell, the man who started it all.

The Awkward Nature Of Pluto

When mentioning about the awkwardness of our dear Pluto, one must take into account its unusual orbits! Oh yes, didn’t you know about it? 

It takes Pluto 248 Earth years to complete its one orbit around the Sun. That means, from the day it was discovered and until now, it hasn’t completed even one revolution around the Sun. 

Imagine we lived there! Our birthdays would never be celebrated! 

That would be such a sad thing. 

If you want to witness the first Plutonian year, you will have to wait until 23rd March 2178. I know, it’s a long wait!

Pluto’s orbit is slightly inclined at an angle of 17 degrees. Also, the orbit is moderately eccentric.

This eccentricity means that one half of the orbit is smaller than the other half. Due to this a part of Pluto’s orbit lies closer to the Sun than Neptune orbit. 

Pluto came to its Perihelion on September 5, 1989. And it was last recorded to be closer to the Sun than Neptune between February 7, 1979, and February 11, 1999. 


On Pluto, one day = 6.39 Earth days. 

Similar to Uranus, the axis tilt of Pluto is 120 degrees. Therefore it experiences extreme seasonal variations just like Uranus does. 

related article – Everything you want to know about planet Uranus

A very weird phenomenon

At solstices, Pluto’s 1/4th surface remains exposed to continued daylight and another 1/4th is in continues darkness. 

Why does this happen?

Everything is got to do with the huge distance of Pluto from the Sun. Why the distance you ask? Well here’s why, 

Due to the huge distance, nitrogen on the far side of Pluto freezes and solidifies. This causes Pluto to reorient itself due to the imbalance of mass. This gives Pluto an axis tilt of 120 degrees. 

Pluto-Neptune relationship

Having said that Pluto for some time enters the orbit of Neptune, isn’t it possible that they might both end up colliding with each other?

Nope! Not possible and here’s why, 

Pluto’s orbit indeed crosses Neptune’s, but this is only as viewed directly from above the solar system model. While in reality they both never happen to collide or even approach close to each other. 

As viewed from above, when Pluto is closest to the Sun and hence close to the orbit of Neptune’s orbit, it appears as if both the orbits intersect, but in reality, the orbit of Pluto is far above the orbit of Neptune.

In fact, it is 8 Au above Neptune’s orbit. Therefore a collision is impossible. 

Another reason is that Pluto and Neptune are locked by a 2:3 orbital resonance. This means that for every 2 orbits Pluto makes around the Sun, Neptune takes 3.

This resonance between the two bodies is highly stable and has been followed for millions of years. 


Pluto’s surface is made up of 98% Nitrogen and traces of methane and carbon monoxide that are pretty abundant on the anti-Charon face.

Pluto has some very interesting geographical features we can’t miss. 

1. Tombaugh region – the heart 

2. Cthulhu Macula – the whale 

3. Brass knuckles –series of equatorial dark areas that are semi-regularly spaced with irregular boundaries. 

4. Sputnik Planitia – this region lies in the west lobe of the heart (Tombaugh Reigo). It is a 1000 km wide basin of frozen nitrogen and carbon monoxide ices divided into polygonal cells.

sputnik Planitia
Sputnik Planitia
NASA/JHUAPL/SwRI [Public domain]

The surface has no signs of craters which tells that the surface is not more than 10 million years old. 

The latest studies have revealed that the surface of Pluto is approx. 180,000 years old.


The core comprises of rocky materials and a mantle of water ice surmounts this core. 

Pluto interior
Pluto image: NASA / Johns Hopkins University Applied Physics Laboratory / Southwest Research Institute, Internal Structure of PlutoCC BY-SA 4.0

Scientists expect the core to be approximately 1700 km in diameter. That is 70% of Pluto’s diameter.

Scientists further say that the decay of radioactive elements generates a substantial amount of heat from the core that melts the ices and separates the rock from it. 

Many believe that this heating takes place till today, therefore creating a subsurface ocean of liquid water at the core-mantle boundary that is 100 to 180 km thick. 

In September 2016, at Brown University, scientists simulated the impact that might have formed the liquid water upwelling after hitting Pluto’s surface and revealing the subsurface ocean. 


Pluto has been visited by only one spacecraft – New Horizon

New Horizon
New Horizon

Launched on 19 January 2006, New Horizon became one of the fastest spacecraft with speeds of up to 57,936 km/h (36,000 mph). 

It took 9 long years to finally make a flyby across Pluto. On 14th July 2015, it made the flyby. 

It made its closest approach to Pluto when it came to about 12,500 km (7,750 miles) from Pluto’s surface. 

It took close up pictures of Pluto in both visible and ultraviolet wavelengths. 

true image of Pluto
Source: NASA/Johns Hopkins University Applied Physics Laboratory/Southwest Research Institute/Alex Parker

Above is the most accurate natural-color image of Pluto taken by NASA’s New Horizons spacecraft in 2015.

Magnetic field 

Sorry, Pluto has got no magnetic field. 



Does Pluto have rings?

Nope! new horizon triple checked. 

Pluto temperature

when closest to the Sun, Pluto reaches temperatures of -223 degrees Celcius (-369 degrees Fahrenheit) and on the furthest part of the orbit, the temperature drops down to -233 degrees Celcius (-387 degrees Fahrenheit)


The first question in mind, how many moons does Pluto have? 

Pluto has got 5 moons – Charon, Styx, Nix, Kerberos, and Hydra. 


Discovered by James Christy on 22nd June 1978.

Pluto and Charon
Pluto and Charon

It is the largest moon of Pluto, about half its size. Has 1/8th the mass of Pluto. 

Surprisingly, the barycentre of Charon and Pluto lies 960 km above Pluto’s surface. 

The other moons revolve approx. 4 times the distance of Charon 

Styx – 42,000 km and Hydra – 64,000 km from the barycentre of the system. They rotate in the same orbital plane as Charon and have nearly circular prograde orbits. 

All the 4 moons are very small as compared to Charon. They are also irregular. 

On their longest axis, Hydra is 55 km long, Nix – 42 km, Styx – 7 km and Kerberos – 12km.

Do you love reading about dwarf planets? Well if you do then you should definitely check this out

Haumea: The egg-shaped dwarf planet

The dwarf planet that nobody told you about


The reason why Pluto is not a planet but a dwarf planet is that there are many bodies out there that are just like Pluto far beyond the orbit of Neptune.

If all of them were to be considered as planets then we would probably end up having thousands of planets on the list.

Percival Lawrence Lowell was the person responsible for starting the search for Planet X.

Important dates –

18th February 1930 – Pluto was discovered.

13th March 1930 – discovery was opened to the world.

24th August 2006 – Pluto demoted from full planet status to a dwarf planet.

23rd March 2178 – completion of one Plutonian year


Sahil Asolkar

writer and co-founder

Sahil Asolkar is a writer, poet, and shows a good interest in astronomy. His work can be seen in the articles he writes for Astronomiac.

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