Why Pluto is No Longer a Planet

Article Updated: 24 Dec , 2015


This article was originally written in 2008, but we created a cool video to go along with it yesterday

Let’s find out why Pluto is no longer considered a planet.

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. Only 22 at the time, Tombaugh was given the laborious task of comparing photographic plates. These were two images of a region of the sky, taken two weeks apart. Any moving object, like an asteroid, comet or planet, would appear to jump from one photograph to the next.

After a year of observations, Tombaugh finally discovered an object in the right orbit, and declared that he had discovered Planet X. Because they had discovered it, the Lowell team were allowed to name it. They settled on Pluto, a name suggested by an 11-year old school girl in Oxford, England (no, it wasn’t named after the Disney character, but the Roman god of the underworld).

The Solar System now had 9 planets.

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.

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. This region extends from the orbit of Neptune out to 55 astronomical units (55 times the distance of the Earth to the Sun).

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.

Here’s the problem. Astronomers had been turning up larger and larger objects in the Kuiper Belt. 2005 FY9, discovered by Caltech astronomer Mike Brown and his team is only a little smaller than Pluto. And there are several other Kuiper Belt objects in that same classification.

Astronomers realized that it was only a matter of time before an object larger than Pluto was discovered in the Kuiper Belt.

And in 2005, Mike Brown and his team dropped the bombshell. They had discovered an object, further out than the orbit of Pluto that was probably the same size, or even larger. Officially named 2003 UB313, the object was later designated as Eris. Since its discovery, astronomers have determined that Eris’ size is approximately 2,600 km (1,600 miles) across. It also has approximately 25% more mass than Pluto.

With Eris being larger, made of the same ice/rock mixture, and more massive than Pluto, the concept that we have nine planets in the Solar System began to fall apart. What is Eris, planet or Kuiper Belt Object; what is Pluto, for that matter? Astronomers decided they would make a final decision about the definition of a planet at the XXVIth General Assembly of the International Astronomical Union, which was held from August 14 to August 25, 2006 in Prague, Czech Republic.

Astronomers from the association were given the opportunity to vote on the definition of planets. One version of the definition would have actually boosted the number of planets to 12; Pluto was still a planet, and so were Eris and even Ceres, which had been thought of as the largest asteroid. A different proposal kept the total at 9, defining the planets as just the familiar ones we know without any scientific rationale, and a third would drop the number of planets down to 8, and Pluto would be out of the planet club. But, then… what is Pluto?

In the end, astronomers voted for the controversial decision of demoting Pluto (and Eris) down to the newly created classification of “dwarf planet”.

Is Pluto a planet? Does it qualify? For an object to be a planet, it needs to meet these three requirements defined by the IAU:

  • It needs to be in orbit around the Sun – Yes, so maybe Pluto is a planet.
  • It needs to have enough gravity to pull itself into a spherical shape – Pluto…check
  • It needs to have “cleared the neighborhood” of its orbit – Uh oh. Here’s the rule breaker. According to this, Pluto is not a planet.

What does “cleared its neighborhood” mean? As planets form, they become the dominant gravitational body in their orbit in the Solar System. As they interact with other, smaller objects, they either consume them, or sling them away with their gravity. Pluto is only 0.07 times the mass of the other objects in its orbit. The Earth, in comparison, has 1.7 million times the mass of the other objects in its orbit.

Any object that doesn’t meet this 3rd criteria is considered a dwarf planet. And so, Pluto is a dwarf planet. There are still many objects with similar size and mass to Pluto jostling around in its orbit. And until Pluto crashes into many of them and gains mass, it will remain a dwarf planet. Eris suffers from the same problem.

It’s not impossible to imagine a future, though, where astronomers discover a large enough object in the distant Solar System that could qualify for planethood status. Then our Solar System would have 9 planets again.

Even though Pluto is a dwarf planet, and no longer officially a planet, it’ll still be a fascinating target for study. And that’s why NASA has sent their New Horizons spacecraft off to visit it. New Horizons will reach Pluto in July 2015, and capture the first close-up images of the (dwarf) planet’s surface.

Space enthusiasts will marvel at the beauty and remoteness of Pluto, and the painful deplaneting memories will fade. We’ll just be able to appreciate it as Pluto, and not worry how to categorize it. At least now you know why Pluto was demoted.

If you’d like more information about Pluto, we did two podcasts on this topic at Astronomy Cast. The first discusses the IAU’s decision, and the second is about Pluto and the Icy Outer Solar System. Check them out.

Here is much more info about Pluto, including pictures of Pluto.

NASA Solar System Exploration Guide

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April 10, 2008 2:10 PM

The most accurate measurement currently pegs Pluto at 1,195 km (743 miles) across. Although this is small, Mercury is only 2,440 km (1,516 miles) across.

I think those are radii, not diameters. Same for Eris.

John Molina
John Molina
April 10, 2008 3:30 PM

I thought that Pluto is not even considered its own planet, even in the dwarf category.

I am under the impression that Pluto and its counterpart, Chiron, rotate around a shared center of gravity, thus making it a binary system. Why am I thinking that Pluto-Chiron is classified by the IAU as a “Double Dwarf Planet?”

April 10, 2008 7:38 PM
Pluto is made up of the same materials as Uranus, Neptune, Titian, and Triton. This is only one factor that determines the type of planet it is. Mass is a second factor. Clearly, Pluto lacks the ice giants mass, but has enough to be round. Location is another. Pluto was exiled early the solar system’s history to the Kuiper Belt. The Neptune Effect made the current configuration of the solar system. It moved the ice giants, Uranus and Neptune to the outer solar system with Pluto and probably Eris. Dwarf stars are still considered stars. Dwarf galaxies are also considered galaxies. Pluto, Ceres, and Eris should be considered smaller planets. We are going to find a lot bigger… Read more »
Todd Sieling
April 10, 2008 2:04 PM

Sad? Really? I find it a great opportunity to demonstrate with a timely and easy example that scientific knowledge is provisional and changes as our understanding changes. It’s one of the best ways to steer learning minds clear of absolutism and dogma, which is often confused with actual science.

I was always excited by the idea of Pluto as a planet had gone un-noticed for a long time, but while the sentimentality of its status is lost, the change creates a great opportunity for education and understanding of science as a changing body of knowledge, not static.

April 10, 2008 2:17 PM

At least Bill Arnett had a sense of humor about it when his domain name proved to be incorrect (look at the title): http://nineplanets.org/

Plus he also had the foresight to add eightplanets.org to his stable to domain names.

April 10, 2008 2:30 PM

For one side, you can be happy as you are not Pluto, who has the right to demoting it?
There are animal protection groups, Pluto would have to have its own. (That lazy astronomers…).

But yes, it could be more acurate to say that pluto and its neighbour are a planet in formation and the New Horizonts is of prime importance to study this medium.

April 10, 2008 2:31 PM
Actually, I still think this definition makes very little sense. Not because of Pluto: Pluto’s status is, or should be, pretty much irrelevant in the debate, but because of other things: – The “clearing the neighborhood” thing is totally arbitrary in what constitutes a “clean neighborhood”, or even a “neighborhood”. – The fact that in our solar system planetary neighborhoods are clean, whatever that might mean, does not imply that the same is true in other planetary systems: theoretical studies have shown that 1:1 resonances between planets as large as gas giants are completely possible, and, given the vastness of space, what’s possible will almost certainly be found, somewhere. In my opinion, any definition of planet that isn’t… Read more »
April 10, 2008 2:47 PM
I have to agree with Gorge on this one. As far as I know as well, the definitions for the planet and dwarf planets are applicable to our solar system only. I mean if we find a young system with a body larger than earth that is still in the middle of a plantery disc surrounded with dust, rocks and what other junk you may find, it isn’t classified as a planet until a much later date. Seems to sound like a planet will go through a cycle much like a star being from main sequence to red giant, it will just take much less time for inner, terrestrial planets to gain the status, despite having massive jovian… Read more »
April 10, 2008 3:23 PM

I think that, with a nod to a colorful history (Percival Lowell), we should just keep Pluto as a planet.

April 10, 2008 4:25 PM

¿Por qué Plutón no es un planeta? Esta tiene que ser una de las preguntas más desgarradoras que me han hecho. Y me la preguntan seguido. Esperaba que algunos años después de que la Unión Astronómica Internacional (IAU) tomara esta decisión polémica, el debate decayera y la gente finalmente lo aceptara. Pero no, todavía es un asunto delicado para mucha gente. […] Fraser Cain para Universe Today

April 10, 2008 5:51 PM

I’ve never understood why this gets people so worked up.

Wasn’t the thinking behind the “clears its orbit” criteria basically that there probably are a large number of probably spherical objects orbiting the sun, and it would become confusing if one day there were, say, 35 planets.

In that case we’d probably have to create a categorization of “major planets”, which would exclude pluto anyway.

April 10, 2008 8:35 PM
I saw a good t-shirt the other day: it had a picture of Pluto with a sad looking face on it, and said: “Don’t worry Pluto – I’m not a planet either…” But seriously… Look, the term planet is just an arbitrary human classification anyway. In reality, nature is more continuous than discreet, and has more variety in its processes than we could ever hope to have a label for. It reminds me of an interview with Richard Feynmann, where he was talking about how his father had taught him to look beyond superficial labels – it went something like this: His dad took him for a walk and showed him a bird. He said “people will tell… Read more »
Emission Nebula
April 10, 2008 11:04 PM
Personally, I could care less if its a “dwarf planet” or other wise. I do think that todays elementary school books should include ALL of these known massive solar bodies. When I was in school Pluto was a planet. But they also taught us in school that South America and Africa werent ever connected!! This is obviously ludicris now. We now know that at one time all the continents were one giant continent. Anyway, my point being that schools should teach about all the massive solar objects beyond Neptune. Infact, Im a little dissapointed in the fact that I wasnt taught about Varuna or Orcus or any of the objects beyond Neptune except Pluto Even in a book… Read more »
April 10, 2008 11:39 PM

And isn’t there also one called Ixion…?

April 11, 2008 2:49 AM
Let’s not forget that the asteroid Ceres in the main asteriod belt between Mars and Jupiter is almost 600 miles in diameter and is spherical like a planet. It, like Pluto, meets only two of the three IAU criteria for defining a body as a planet. If Pluto is a planet than Ceres and all the outer dwarf planets must be full-fledged planets as well. Considering that there may be HUNDREDS of large spherical bodies not yet discoverd in the outer solar system, we better be happy with just Eight planets…I, for one, do not want to have to remember the names of the dozens and dozens of planets in our solar system the next time I play… Read more »
Brian Sheen
April 11, 2008 1:31 AM

Hi There,

Cleared its own orbit? Well even Jupiter has Trojans, small asteroids 60 degees ahead and behind the planet but they are there because of the gravity of Jupiter.

Also Pluto comes inside the orbit of Neptune for some 25 years and its inclination to the plane of the ecliptic is high. Two features that are not like conventional planets.

I like the idea of eight planets – for young kids it is easy to grasp, then when older they can be taught dwarf planets, then again asteroids and all the families of bits of rock that orbit our Sun.

April 11, 2008 3:10 AM
What a mess! A lot of people seem concerned with the “how many are there?, 8?, 9?, 12?” issue that has no scientific meaning. Although I understand the criteria, the motivation behind it seems taylored to the “How many” issue, in result the current definition of planet is not very good beacause, with it, for you to classify an object, you must know where it is, how does it moves and what’s arround it. This is not very practical; Check these examples: Ex 1: Think of all those exoplanets that are being discovered… According to the current definition some of them might turn up to be “exo-dwarf-planets”… Ridiculous! Ex 2: Imagine a planet arround a sun (A planet… Read more »
April 11, 2008 3:17 AM

…And let’s face it…

A lot of them fires up our imagination (at least mine).

Wouldn’t it be more poetic to go on a turistic travel arround, say, Planet Ceres than the same turistic travel arround asteroid Ceres?


George G. Kountouris
George G. Kountouris
April 11, 2008 6:22 AM

I agree with the above responders, by my own objections – questions:
which other body is in Pluto’s orbit?
Charon is a Pluto`s satellite or Pluto-Charon is a “binary” planet (now an ex planet), as MANY binary stars, orbiting each other by them gravitational center.
A point with i will agree to this demotion, is that Pluto`s orbit is crossing the Neptune`s respective, as Ceres is crossing Earth`s, and it is an asteroid.
Finally as a Greek citizen i have emotional reasons, due to ancient Greeks history, when Pluto was the underworld`s God, ALL THE OTHER PLANETS are named with ancient Greek Gods.