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Why Pluto is No Longer a Planet


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.

References:
NASA Solar System Exploration Guide
Caltech

About 

Fraser Cain is the publisher of Universe Today. He's also the co-host of Astronomy Cast with Dr. Pamela Gay.

Comments on this entry are closed.

  • ruvim udodik April 21, 2009, 10:22 AM

    so in conclusion my penis is a dwarf planet

  • justin tape April 21, 2009, 10:27 AM

    i think that pluto is no planet becuz it was a small planet and i think that is hard for me 2 remember all 9 planets at the same time so eight makes it easier

  • Generation of Wipers April 24, 2009, 7:42 AM

    Reading so many hostile and emotional comments here about a scientific classification is depressing.

    So many people so angry and frustrated by something they simply do not understand.

    None of you will ever go there.

    Most of you have never even seen Pluto in a telescope.

    Science is science, not a popularity contest. This isn’t American Idol, where everyone gets a vote.

    Someone said “what gives them the right?”
    I say what gives YOU the right to tell scientists how science should progress?

  • Clay Dante Tuvalu McDermott May 13, 2011, 6:17 PM

    In August of 2006, the International Astronomical Union came up with a definition of the word “Planet”. According to this new definition, Pluto is no longer a planet. It is a “Dwarf Planet”. However, this has created controversy around the world among Astronomers.

    The IAU members gathered at the 2006 General Assembly agreed that a “planet” is defined as 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, and (c) has cleared the neighbourhood around its orbit.

    A “dwarf planet” is 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, and (d) is not a satellite.

    The definition is flawed, relating to “cleared the neighborhood”. Every 228 years Pluto crosses inside of the orbit of Neptune, so technically speaking, it does not clear its neighborhood. But that also means that Neptune does not clear its own neighborhood. Mars and Jupiter don’t clear their neighborhoods as they “interfere” with the Asteroids, and the Earth actually orbits the Sun with thousands of Asteroids. So the Earth doesn’t clear its own neighborhood either.

    So if we use the definition set forth by the IAU, Pluto, Neptune, Jupiter, Mars, and the Earth, are NOT planets! Also, why aren’t “Dwarf Planets” known as planets? Dwarf Stars are still stars, and Dwarf Galaxies are still galaxies.

    These new definitions ONLY apply to objects in OUR Solar System, making the definition even more un-scientific.

    Along with the definition being both linguistically and scientifically flawed, so was the voting process. Although there are over 10,000 Astronomers in the IAU, only 237 of them voted and approved this definition. Therefore, there was NOT a majority consensus of what a planet is. Hundreds of Astronomers around the world (and this planetarium) have signed petitions to ignore the new definition and still refer to Pluto as the ninth planet in our Solar System.

    Discovered in 1930, Pluto orbits the Sun, has three moons, has an atmosphere, has weather, and even polar caps. It is not that much different than any of the other planets. It has been known as a planet for more than 75 years, and to change its status with a poor definition and process, is bad science.

    The Suits-Bueche Planetarium recognizes the fact that Astronomy changes as our knowledge grows, but we do not go along with the IAU’s flawed voting process and flawed definition.

    Therefore, our official policy is that Pluto is STILL the ninth planet in the Solar System!

  • Joe Zemalkowski May 17, 2011, 3:20 AM

    I guess NASA will have to scrub all flights to Pluto since it’s no longer a plant.

  • Joe Zemalkowski May 17, 2011, 3:22 AM

    Oh, wait….it’s a planet again. Sorry for the last post. Can’t wait for the 2015 pix and data.

  • Anonymous May 19, 2011, 3:28 AM

    Hello my name is Nikos Foundas. I am 6th grader at Le Jardin Academy.Clay Dante Tuvalu McDermott is in my class and copy/pasted this site http://www.look-to-the-skies.com/why_pluto_should_still_be_a_plan.htm
    I think that Pluto is not and never was a planet.I strongly agree that Pluto is a dwarf Planet. It is a dwarf because it has not cleared its orbit. Every 20 years Pluto is closer to the sun than Neptune. Pluto also has the most elliptical and the most eccentric orbit.

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