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IAU definition of planet

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The IAU's draft proposal would immediately add 3 planets, shown here in a size comparison to Earth. Leftmost is 2003UB313, then Charon, Ceres, and the Earth

In 2006, a proposal was brought before the International Astronomical Union to redefine the term "planet" so as to include other objects beyond the traditional nine planets which have been historically considered a part of the solar system.[1] The proposal is denoted as Resolutions 5, 6 and 7 for GA-XXVI; members of the IAU will vote on it on August 24, 2006 in Prague, Czech Republic.

The redefinition would recognize three new planets: Ceres, Charon, and 2003 UB313. It is presumed that, after more observation and discussion, astronomers will accept more objects in the solar system as meeting this new definition.

On August 22 the original redefinition (which recognized 12 solar system planets, including Pluto) was dealt serious, probably fatal blows in two open IAU meetings. Jay Pasachoff of Williams College, who attended both meetings, is quoted as saying, “I think that today can go down as the ‘day we lost Pluto’ ”. [2]

Draft proposal

August 16

The IAU published the definition proposal on August 16, 2006. Its form follows loosely the second of three options considered by a 19-member IAU-appointed panel in 2005. In its exact wording, it is:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet.

The new definition would require that three celestial bodies immediately become planets:

A further dozen are possible candidates to join the list, pending refinements of knowledge regarding their physical properties. Some objects in the second list are more likely to eventually be adopted as "planets" than others. Despite what has been claimed in the media, the proposal does not necessarily leave our solar system with only 12 planets. Mike Brown, the discoverer of Sedna and 2003 UB313, has said that at least 53 known bodies in the solar system probably fit the definition, and that a complete survey would probably reveal more than 200.[3]

A pair of objects would be considered a double planet system if they independently satisfy the definition of a planet, and the common center of gravity of the system (known as the barycenter) lies outside both bodies.[4] Pluto and Charon would be the only known double planet in the solar system. Other planetary satellites (for example, in the Earth and Moon system) might be in hydrostatic equilibrium, but would still not be defined as a double planet, since the barycenter lies within the more massive celestial body (that is, Earth).

The dozen "candidate planets" that might be included under this new definition.

The term "minor planet" would be abandoned, replaced by the categories "small Solar System body" and a new classification of pluton. The latter would signify those planets with highly inclined orbits with large eccentricities and an orbital period of more than 200 years (that is, those orbiting beyond Neptune). Pluto would be the prototype for this class. The term "dwarf planet", which can be used to describe all planets smaller than the eight "classical planets" in orbit around the Sun, is not an official IAU classification.[5] Those objects underneath the "spherical" threshold are termed "Small Solar System Bodies" (SSSB). The IAU has not made recommendations in this draft resolution on what separates a planet from a brown dwarf.[6] The final decision on whether this draft resolution will be passed is expected by August 24, 2006.[7]

This redefinition of the term 'planet' could also lead to changes in classification for the trans-Neptunian objects 2003 EL61, 2005 FY9, Sedna, Orcus, Quaoar, Varuna, 2002 TX300, Ixion, 2002 AW197, and the asteroids Vesta, Pallas, and Hygiea.

On 18 August, the Division of Planetary Sciences of the American Astronomical Society, the world's largest international professional society of planetary scientists, endorsed the draft proposal[8].

The roundness factor generally results in the need for a mass atleast 5 x 1020 kg, or diameter atleast 800 km in size. [9]

August 22-24 developments

On 22 August the proposal was rewritten with two changes from the previous draft.

The first was a generalisation of the name of the new class of planets (previously the draft resolution had explicitly opted for the term 'pluton'), with a decision on the name to be used postponed.

Many geologists had been critical of the choice of name for Pluto-like planets [10], being concerned about the term pluton which has been used for years within the geological community to represent a form of magmatic intrusion. [11] [12] Pluton formations are fairly common hunks of rock, and confusion was thought undesirable due to the status of planetology as a closely allied field. [13]

Further concerns surround Pluton; in many European languages Pluto itself is called "Pluton", potentially adding to confusion.

The second change was a redrawing of the planetary definition in the case of a double planet system. There had been a concern that, in extreme cases where a double body had its secondary component in a very eccentric orbit, there could be a drift of the barycenter in and out of the primary body, leading to a shift in the classification of the secondary body as a satellite or planet depending on where in its orbit it was. Thus the definition was reformulated so as to consider a double planet system in existence if its barycenter lies outside both bodies for a majority of the system's orbital period.

Later on the 22nd, two open meetings were held that ended in an apparently abrupt aboutface on the basic planetary definition. The position of astronomer Julio Ángel Fernández (see below) gained the upper hand in the committee and was described as unlikely to lose its hold by the 24th. This position would result in eight major planets, with Pluto ranking as a "dwarf planet" or "planetoid".[14] The discussion at the first meeting was heated and lively, with IAU members in vocal disagreement with one another over such issues as the relative merits of static and dynamic physics. In an indicative vote members heavily defeated the proposals on Pluto-like objects and double planet systems, and were evenly divided on the question of hydrostatic equilibrium. The debate was said to be "still open", with private meetings being held ahead of tomorrow's vote. [15]

At the second meeting in the evening, following 'secret' negotiations, a compromise began to emerge after the Executive Committee moved explicitly to exclude consideration of extra-solar planets and to bring into the definition a criterion concerning the dominance of a body in its neighbourhood. [16] The newly proposed, third draft definition is:

A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid-body forces so that it assumes a hydrostatic-equilibrium (nearly round) shape, (b) is the dominant object in its local population zone, and (c) is in orbit around the Sun.

Advantages

The proposed definition has found support among many astronomers as it uses the presence of a physical qualitative factor (the object being round) as the defining feature. Most other potential definitions depend either on a limiting quantity (e.g., a minimum size or maximum orbital inclination) tailored for our solar system. According to members of the IAU committee, this definition would not use man-made limits but instead "nature" would decide whether or not an object is a planet. [17]

It also has the advantage of measuring an observable quality. Suggested criteria involving the nature of formation would be more likely to see accepted planets later declassified as scientific understanding improves.

Additionally, the definition keeps Pluto as a planet. Pluto's planetary status is fondly thought of by many, and the general public could be alienated from professional astronomers after the uproar that occurred last time the media raised the possibility of demotion.[18].

Criticism

There has been some criticism regarding the proposed redefinition with respect to ambiguity. Astronomer Phil Plait [19] and NCSE writer Nick Matzke [20] have both written about why they do think the redefinition is not, in general, a good one. The redefinition defines a planet as orbiting a star, which would mean that any planet ejected from its star system or formed outside of a solar system (a rogue planet or interstellar planet) cannot be called a planet, even if it fits all other definitions. Of course, a similar situation already applies to "moon", which cease to be moons once they are ejected from planetary orbit, a term with widespread acceptance.

Similarly the redefinition does not differentiate between planets and brown dwarf stars. It is thought that the IAU will attempt to clarify this differentiation at a later date.

There has also been criticism of the definition of double planet: while the Moon is defined as a satellite of the Earth, over time the Earth-Moon barycenter will drift outwards (see Tidal acceleration) and be situated outside of either body. This would then upgrade the Moon to full planet according to the redefinition. The time taken for this to occur is expected, however, to be billions of years. [21]

Another criticism is that the proposal will lead to too many new planets: while only twelve bodies in the Solar System are currently known to match the new definition, this number is almost certain to increase, and may eventually reach 200 or more.[22] It is felt by some that having this many planets effectively renders the term meaningless.

In an August 18 Science Friday interview, Mike Brown expressed doubt that a scientific definition is even necessary:

"The analogy that I always like to use is the word "continent". You know, the word "continent" has no scientific definition ... they're just cultural definitions, and I think the geologists are wise to leave that one alone and not try to redefine things so that the word "continent" has a big, strict definition."[23]

Alternative proposal by Julio Ángel Fernández

According to Alan Boss of the Carnegie Institution of Washington, a subgroup of the IAU met on August 18, 2006, and held a straw poll on the draft proposal: only 18 were in favour of the draft proposal, and over 50 against. The 50 in opposition preferred an alternative proposal drawn up by Uruguayan astronomer Julio Ángel Fernández.[24]

(1) A planet is a celestial body that (a) is by far the largest object in its local population[1], (b) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape [2], (c) does not produce energy by any nuclear fusion mechanism [3].

(2) According to point (1) the eight classical planets discovered before 1900, which move in nearly circular orbits close to the ecliptic plane are the only planets of our Solar System. All the other objects in orbit around the Sun are smaller than Mercury. We recognize that there are objects that fulfill the criteria (b) and (c) but not criterion (a). Those objects are defined as "dwarf" planets. Ceres as well as Pluto and several other large Trans-Neptunian objects belong to this category. In contrast to the planets, these objects typically have highly inclined orbits and/or large eccentricities.

(3) All the other natural objects orbiting the Sun that do not fulfill any of the previous criteria shall be referred to collectively as "Small Solar System Bodies".[4]

Under this proposal, our current solar system would remain unchanged, but Pluto would be demoted to a dwarf planet. One unanswered question is: what is the objective or mathematical definition of "by far the largest"?

Definitions and clarifications

  1. The local population is the collection of objects that cross or close approach the orbit of the body in consideration.
  2. This generally applies to objects with sizes above several hundred kilometers, depending on the material strength.
  3. This criterion allows the distinction between gas giant planets and brown dwarfs or stars.
  4. This class currently includes most of the Solar System asteroids, near-Earth objects (NEOs), Mars-, Jupiter- and Neptune-Trojan asteroids, most Centaurs, most Trans-Neptunian Objects (TNOs), and comets.[25]

It is not yet clear what the IAU intends to do with this new proposal, or whether it will be subject to the vote on August 24.[26]

Procedure

Any IAU vote on the subject of redefinition will take place at the XXVIth IAU General Assembly in Prague on 24 August 2006. Following a reversion to the previous rules on 15 August, as this is a primarily scientific matter, every individual member of the Union attending the Assembly will be eligible to vote. As of 22 August, the number having registered their attendance at the Assembly stood at 2398. [27]

The IAU Executive Committee may decide to put either the original proposal, an amended version or the alternative before the Assembly for decision. On 18 August, Owen Gingerich, a historian and astronomer emeritus at Harvard who led the committee which generated the original definition, predicted the Executive Committee, "will undoubtedly come before the membership with a single resolution. They may make some adjustments." He added that correspondence he had received had been evenly divided for and against the proposal. [28]

See also

References

  1. ^ Connor, Steve (2006-08-16). "Solar system to welcome three new planets". New Zealand Herald. {{cite news}}: Check date values in: |date= (help)
  2. ^ Overbye, Dennis (2006-08-22). "Pluto Seems Poised to Lose Its Planet Status". New York Times. {{cite news}}: Check date values in: |date= (help)
  3. ^ Mike Brown (2006). "How Many Planets Are There?". CalTech. Retrieved 2006-08-16.
  4. ^ Robert Roy Britt (2006). "Nine Planets Become 12 with Controversial New Definition". Space.com. Retrieved 2006-08-16.
  5. ^ "Draft Resolution 5 for GA-XXVI: Definition of a Planet". International Astronomical Union. 2006. Retrieved 2006-08-16.
  6. ^ "Planet Definition" Questions & Answers Sheet". International Astronomical Union. 2006. Retrieved 2006-08-16.
  7. ^ Gareth Cook (2006). "Nine no longer: Panel declares 12 planets". Boston Globe. Retrieved 2006-08-16.
  8. ^ "Planetary Scientists Support Proposed Redefinition Of A Planet". SpaceDaily. 2006. Retrieved 2006-08-19.
  9. ^ Jenny Hogan (2006). "Planets are round. Will that do?". Nature.
  10. ^ "Star-gazers puzzled by Pluto". Independent Online. 2006. Retrieved 2006-08-18.
  11. ^ "Geologists Force Astronomers To Rethink Pluto Plan". 2006. Retrieved 2006-08-18.
  12. ^ Elise Kleeman (2006). "Planet, pluton or rock?". Pasadena Star News. Retrieved 2006-08-20.
  13. ^ Geoff Brumfiel (21 August 2006). "Plutons, planets and dwarves : Geologists and astronomers wrangle over words". news@nature.com.
  14. ^ Overbye, Dennis (2006-08-22). "Pluto Seems Poised to Lose Its Planet Status". New York Times. {{cite news}}: Check date values in: |date= (help)
  15. ^ "Astronomers divided over 'planet' definition". Deutsche Presse-Agentur. 2006-08-22. {{cite news}}: Check date values in: |date= (help)
  16. ^ Tresch Fienberg, Richard (2006-08-22). ""The Day We Lost Pluto"". Sky & Telescope. {{cite news}}: Check date values in: |date= (help)
  17. ^ Robert Roy Britt (2006). "Nine Planets Become 12 with Controversial New Definition". Space.com. Retrieved 2006-08-19.
  18. ^ Pearson education (2006). "The Flap over Pluto". infoplease.com. Retrieved 2006-08-19.
  19. ^ Phil Plait (2006). "Congratulations! It's a planet!". Bad Astronomy. Retrieved 2006-08-18.
  20. ^ Nick Matzke (2006). "Wherein I argue emotionally about the definition of "planet"". The Panda's Thumb. Retrieved 2006-08-18.
  21. ^ Robert Roy Britt (2006). "Earth's moon could become a planet".
  22. ^ Mike Brown (2006). "How Many Planets Are There?". CalTech. Retrieved 2006-08-16.
  23. ^ Ira Flatow and Mike Brown (2006-08-18). "Pluto's Planet Status / String Theory". Talk of the Nation - Science Friday. National Public Radio. Retrieved 2006-08-22.
  24. ^ Robert Roy Britt, 'Pluto May Get Demoted After All'. Space.com. 18 August, 2006.
  25. ^ "Details Emerge on Plan to Demote Pluto". 2006. Retrieved 2006-08-18.
  26. ^ Robert Roy Britt, 'A Wild Week: Planet Definition Left Up in the Air'. Space.com. 19 August, 2006.
  27. ^ IAU General Assembly Welcome page 19 August, 2006.
  28. ^ Robert Roy Britt, 'Pluto May Get Demoted After All'. Space.com. 18 August, 2006.
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