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No 51 - December 2006

Pluto and the planets

The planet Pluto has been in the headlines for the last time. The 26th general assembly of the International Astronomical Union (IAU) in Prague was due to decide what is and what is not a planet. A committee had been formed three years earlier to come up with a proposal for the assembled astronomers to vote on. The initial proposal was rejected. Pluto would have remained a planet. But there would immediately have been three further planets: the asteroid Ceres, Pluto's moon Charon, and the trans-neptunian object 2003 UB313. There would have been about a dozen known trans-neptunian candidates for planetship, and more to discover.

In the end the astronomers decided otherwise. They removed much of the complexity of the initial proposal and added the orbit-clearance criterion for a planet. We now have only eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune. Pluto is demoted to being a "dwarf planet", Ceres will most likely be promoted to dwarf planet, and 2003 UB313 is another known dwarf planet. Two or three large asteroids may turn out to be dwarf planets, if they are large and massive enough to pull themselves into a round shape. The same holds for the dozen or so trans-neptunian objects that may fulfil that criterion. According to the IAU press releases the shape of objects with mass above 5 1020 kg and diameter greater than 800 km would normally be round due to self-gravity. Be that as it may, Pluto's moon Charon will be just that, a satellite and not a dwarf planet.

planets and dwarf planets
This image illustrates the relative sizes of the Sun and the planets. I have magnified the dwarf planets threefold to make them more apparent. (Image courtesy of the International Astronomical Union and Martin Kornmesser.)

The full resolutions passed by the IAU general assembly are:

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 5

The IAU therefore resolves that "planets" and other bodies in our Solar System, except satellites, be defined into three distinct categories in the following way:

  1. A planet1 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".

1The eight "planets" are: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.
2An IAU process will be established to assign borderline objects into either dwarf planet and other categories.
3These currently include most of the Solar System asteroids, most Trans-Neptunian Objects (TNOs), comets, and other small bodies.

IAU Resolution: Pluto

Resolution 6

The IAU further resolves:

Pluto is a "dwarf planet" by the above definition and is recognized as the prototype of a new category of Trans-Neptunian Objects1.

1An IAU process will be established to select a name for this category.

From opinions that I have heard directly, astronomers seem reasonably happy with or indifferent to this decision. There is some doubt that Pluto's demotion will go down well with others, and attempts to re-instate it as a planet can be expected.

Meanwhile, the Minor Planet Center stake their claim and assign numbers to the new dwarf planets: (134340) Pluto and (136199) 2003 UB313. The other new dwarf planet already has a minor planet number, because it is (1) Ceres. A little later 2003 UB313 is given the official name Eris.

Mike Brown - whose team discovered the "tenth planet" 2003 UB313 - changes his web page to call the object the largest known dwarf planet. The whole planet question had come to a head when it had become clear that Eris is larger than Pluto. Both will now not be planets. People involved with NASA's New Horizons mission to Pluto and the Kuiper Belt were initially not so happy. In September their web site says:

Poor New Horizons. When it launched in January 2006 it was with all the prestige of the first spacecraft to study Pluto, the last unvisited planet in the solar system. That changed seven months later, when astronomers decided that Pluto was not a planet. For the time being, New Horizons is at least the first mission to a dwarf planet - the new class of objects into which scientists dumped Pluto. But that doesn't mean it will be the first spacecraft to visit a dwarf planet. Under the new definition (it's still unclear), Ceres may be upgraded from asteroid to dwarf planet, and if NASA's Dawn mission launches as planned next summer, it will arrive at Ceres in February 2015, five months before New Horizons gets to Pluto.

For many of us amateur astronomers the new IAU ruling means that we now have seen all planets. To me Pluto is not a regular planet because it is so faint. Uranus is about magnitude 5.8, Neptune 7.9. They are of similar size and the brightness difference reflects their different distances from Earth. Pluto is currently about the same distance as Neptune, but is only magnitude 13.7. This is because Pluto is so small, not because it is so far.


Horst Meyerdierks


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Pluto and the planets

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