Read Frosty Drew Observatory and Science Center's Update on SARS-CoV-2 / Coronavirus Disease 2019 and our Reopening Plan. Updated: December 1, 2020

Log, Aug 22, 2006

Miss Victoria's Ears Make Jumping Shockingly Uncomfortable Near ===?===

August 22, 2006 is now being called the "The Day We Lost Pluto". Essentially, the definition of a planet by an IAU committee chaired by Owen Gingerich has been decisively overturned. It was radical revision and the IAU members were given less than a week's notice which did not go over well with many of them. I already explained my objection [the barycenter rule] to the distinction of planets versus moons but the IAU Convention had two additional major objections. They are:

  1. True planets are large enough to sweep their orbits zones clear of small objects (asteroids, comets and what have you). [This objection excludes Pluto/Charon, Sedna, Xena (still officially UB313) and Ceres for a variety of reasons].
  2. True planets congregate in the plane of the ecliptic (essentially a plane that roughly coincides with the Sun's equator) while the other possible planets have orbits at steep angles to the ecliptic. [Large inclination angles exclude Pluto/Charon (17o), Sedna (12o), Xena (43o) and marginally Ceres(10o)]

There is by no means consensus with the either the committee's or the revised definition of a planet. For example, the revision which creates four groupings (1) planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune), (2) dwarf planets (Ceres, Pluto, Sedna, Xena and possibly additional objects beyond Neptune), (3) minor objects (asteroids and comets) and (4) satellites (attendants to the major planets). It is interesting that the person with the most to lose, Dr. Mike Brown the discoverer of Xena, is willing to accept the revised definition with a good deal of grace.

The proposed name for objects beyond Neptune "plutons" has been dropped because of objections by geologists who already use the term. This really makes sense because many planetary astronomers are really better called astronomical geologists. In fact, geologists who study the geology of Mars are already being called by the subdivision "areologists".

The status of satellites will remain the same as they have defined historically. In spite of oddities like the Sun's gravity being three times as strong as the Earth's gravity on the Moon, the Moon will remain a satellite. So the Moon is a moon which makes a lot of sense. The attendants of the other bodies of the Solar System remain satellites (a.k.a. moons) no matter if they are huge full fledged worlds like Ganymede of chunks of rock no bigger than Block Island.

The main objection to this revised definition comes from the fact that it simply cannot be extended directly to other stellar systems. Even the more complex committee's definition really did not address all possible combination of worlds around other stars or world ejected from other stellar systems. Some really wild combinations are conceivable and may be discovered by large telescopes within the next decade or two (perhaps by the NextGen Telescope due to replace the Hubble).

Ultimately, I am comfortable with the revised definition of planets, satellites, dwarf planets and minor objects. In fact, all it changes is the formal definition of Pluto and creates a category name ["dwarf planet"] for things which properly are bigger than minor objects and smaller than real planets. Finally the anomalous status of "co-planet" for Charon is deleted. Charon is a satellite.

We can decide what to call worlds around other stars what we chose as they are discovered. The revised definition sure makes my job easier. I could explain the committee's "roundness" requirement easily to kids but the "barycentric" requirement would be hard to explain to Astronomy 101 folks from the local universities!

Well our old jingle is no longer viable. I might as well get into the swing of things. Hows this for a way to remember the order of the planets?

My Vitamin Enriched Mint Julip Seems Unusually Nutritious

Not likely!

-Les Coleman

Leslie Coleman
Author:
Leslie Coleman
Entry Date:
Aug 22, 2006
Published Under:
Leslie Coleman's Log
Subscribe to Leslie Coleman's Log RSS Feed