Pluto’s Out of the Planet Club

Article written: 24 Aug , 2006
Updated: 24 Mar , 2012
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Last year we had 9 planets. Recently we were informed it would grow to 12. Now we’ve only got 8. The International Astronomical Union, currently meeting in Prague, voted on August 24, 2006 to demote Pluto down from planethood status. Now Pluto, Charon, Ceres and the newly discovered 2003 UB313 (aka Xena) will merely be known as “dwarf planets”. Under the new definition, planets must orbit a star, be spherical in shape, and clear out their neighbourhood of orbital debris. Pluto has failed to fulfill the third requirement, so it’s official – Pluto is not a planet.

It is official: The 26th General Assembly for the International Astronomical Union was an astounding success! More than 2500 astronomers participated in six Symposia, 17 Joint Discussions, seven Special Sessions and four Special Sessions. New science results were vigorously discussed, new international collaborations were initiated, plans for future facilities put forward and much more.

In addition to all the exciting astronomy discussed at the General Assembly, six IAU Resolutions were also passed at the Closing Ceremony of the General Assembly:

1. Resolution 1 for GA-XXVI : “Precession Theory and Definition of the Ecliptic”
2. Resolution 2 for GA-XXVI: “Supplement to the IAU 2000 Resolutions on reference systems”
3. Resolution 3 for GA-XXVI: “Re-definition of Barycentric Dynamical Time, TDB”
4. Resolution 4 for GA-XXVI: “Endorsement of the Washington Charter for Communicating Astronomy with the Public”
5. Resolution 5A: “Definition of ‘planet’ ”
6. Resolution 6A: “Definition of Pluto-class objects”

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.

This means that the Solar System consists of eight “planets” Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. A new distinct class of objects called “dwarf planets” was also decided. It was agreed that “planets” and “dwarf planets” are two distinct classes of objects. The first members of the “dwarf planet” category are Ceres, Pluto and 2003 UB313 (temporary name). More “dwarf planets” are expected to be announced by the IAU in the coming months and years. Currently a dozen candidate “dwarf planets” are listed on IAU’s “dwarf planet” watchlist, which keeps changing as new objects are found and the physics of the existing candidates becomes better known.

The “dwarf planet” Pluto is recognised as an important proto-type of a new class of trans-Neptunian objects. The IAU will set up a process to name these objects.

Below are the planet definition Resolutions that were passed.

RESOLUTIONS
Resolution 5A is the principal definition for the IAU usage of “planet” and related terms.

Resolution 6A creates for IAU usage a new class of objects, for which Pluto is the prototype. The IAU will set up a process to name these objects.

IAU Resolution: Definition of a Planet in the Solar System
Contemporary observations are changing our understanding of planetary systems, and it is important that our nomenclature for objects reflect our current understanding. This applies, in particular, to the designation ‘planets’. The word ‘planet’ originally described ‘wanderers’ that were known only as moving lights in the sky. Recent discoveries lead us to create a new definition, which we can make using currently available scientific information.

RESOLUTION 5A
The IAU therefore resolves that “planets” and other bodies in our Solar System be defined into three distinct categories in the following way:

(1) A “planet”1 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, and (c) has cleared the neighbourhood around its orbit.

(2) 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) shape2 , (c) has not cleared the neighbourhood around its orbit, and (d) is not a satellite.

(3) All other objects3 except satellites orbiting the Sun shall be referred to collectively as “Small Solar-System Bodies”.

Original Source: IAU News Release


2 Responses

  1. Jennifer Williams says

    How does a planet clear its neighborhood around its orbit?

    What does it have to clear?

    Will Pluto ever be re-instated as a planet?

  2. Cameron says

    I totally disagree with you, just because Pluto has a bad orbit doesn’t mean it’s not a planet. Here’s what I think what happened:

    I agree that Pluto is a member of the Kuiper Belt, which means that it was once a comet. But then, out of nowhere, a meteorite or another comet smashes into Pluto, knocking it plain out of orbit. Pieces from the impact form back, giving Pluto a spherical look. Other pieces form Pluto’s neighbor moon, Charon.

    Also, I heard that during it’s 20 earth years in front of Nuptune, it had an atmosphere. That atmosphere development was likley pernament, so it probably still has it. I’ve seen blurry, close up pictures of Pluto, and it looks like it has an atmosphere. Also during those years, if they weren’t formed in the impact, two objects must have got caught in the orbit of the tiniest planet of the Solar System. Those were Pluto’s secondary moons, Hydra and Nix.

    The other thing is, New Horizons was already on the way, and it was unfair to kick Pluto out of the planet club when the first probe to get there was on the way. So what those IAU geeks say are fat-out lies. WAIT ‘TILL NEW HORIZONS HAS EVIDENCE, THEN YOU CAN TELL WHETHER PLUTO IS A PLANET OR NOT. So what those people say isn’t official. Those 2,500 scientists came there for nothing. Pluto IS a planet, and it always will be. Oh, sure, it has a bad orbit, and passed in front of Neptune for 20 years, but like I said in the beginning, just because the ninth planet has a bad orbit, doesn’t mean it’s not a planet. So it’s official. PLUTO IS A PLANET.

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