CCNet ESSAY, 25 September 2000


Michael Paine explains why the International Astronomical
Union should adopt a less confusing nomenclatura which
is in line with the new discoveries about our Solar System.

By Michael Paine <>

25 September 2000

Although I am not a scientist and have not been a party to
IAU discussions I would like to propose some name changes
to improve the PR image of research into asteroids and
comets. My experience is that reporters tend to lose
interest when they hear scientists talk about 'minor'
planets and especially 'objects' - Near Earth Objects,
Potentially Hazardous Objects (this could, of course,
include a kitchen knife but the play on the word 'foe' is
useful), Kupier Belt Objects and Trans-Neptune Objects. It
seems that the term 'object' gets used when researchers
can't distinguish between asteroids and comets. Something
that sounds more exciting is needed.

I suggest the IAU takes a lesson from a very successful
businessmen and promotes the idea that small is beautiful.
That certain businessman incorporated the prefix 'micro' in
the company name. My suggestion therefore, is that the
collective name for asteroids, comets and other small
bodies that orbit the sun should be 'microplanet'. Before
you start typing a hasty note to Benny, please consider
some of my reasoning.

The term 'planet' probably has some meaning to
non-scientists. Most school children are taught (and some
actually remember) that planets orbit the Sun. Remarkably,
my enlightened Geddes and Grosset dictionary defines
'planet' as a celestial body that orbits the sun or other

To scientists and engineers the term 'micro' has a strict
scientific meaning but this has become blurred in recent
years and my dictionary just defines it as a prefix meaning
'small'. So 'microplanet' means a small celestial body that
orbits a star - that seems promising. 'Celestial body' is
intended to exclude artificial objects but that might need
a specific mention.

When does a planet become a microplanet?

A review of the postings on CCNet concerning the planetary
status of Pluto (starting early 1999) gives a fascinating
insight into this highly sensitive issue.

It does not seems wise to define these objects by their
composition. Don Yeomans is reported as saying 'the
distinction between asteroids and comets is now hopelessly
blurring'. Others have pointed out that Pluto is icy
whereas the other planets are rocky or gaseous. Some
postings even suggested that the issue would have to wait
for the Pluto-Kupier Express space mission reached Pluto.
Sadly the proposed launch in 2004 has been cancelled and it
could be 20 years before Jupiter is once again available
for a gravity sling-shot.

On several occasions size has been considered as the
distinguishing feature. One CCNet report (8 Feb 1999)
indicates a suggestion by Michael A'Hearn that objects with
a diameter of at least 1000km be regarded as 'major'
planets was rejected by an IAU committee as being too
arbitrary. However, many conventions in society are
arbitrary. It turns out that 1000km might be quite a handy
threshold. Pluto is 2274km in diameter and its partner
Charon is 1172km. Ceres, the largest asteroid, is 933km

If a microplanet is defined as a celestial body with a
diameter (better still, semi-major axis) less than 1000km
that orbits a star then  most circumstances seem to be
covered. But what about Pluto and Charon? Is Charon a moon
of Pluto? My suggestion is that the pair be regarded as a
binary planet. One possible way to distinguish between a
binary planet and a planet/moon system is the location of
the barycentre, or common centre of mass. If the barycentre
is contained within the larger object then the smaller
object could be regarded as a moon. Thus our Moon remains a
moon (barycentre below the Earth's surface) but Charon and
Pluto form a binary planet (barycentre, I understand, about
1500km above Pluto's surface).

Incidentally, under this proposal Clyde Tombaugh becomes
the first person to discover a binary planet.

There are other reasons for fixing on a simple, objective
definition of 'major' planets. There may well be objects
larger than 1000km orbiting the Sun beyond Pluto and it
would be nice if astronomers knew straight away whether
their discovery would be regarded as a 'major' planet.

Is it appropriate for 'microplanet' to cover all sub-1000km
objects? I think so but if there are strong objections then
perhaps 'miniplanet' would be appropriate for objects
between, say, 200km and 1000km. 'Microplanet' would then
apply to those under 200km. I hestitate at 'nanoplanet'
instead of meteoroid for those under 10 metres.

Some advantages of the name microplanet:

* The intended meaning should be evident to non-scientists

* It sounds more exciting and is more concise than than
  minor planet, planetesimal, planetoid, interplanetary
  small body, small body of the solar system, minor
  demizens of the solar system...

* The Minor Planet Centre could become the Microplanet Centre and its
  initials could remain MPC.

* Near Earth Objects would become Near Earth Microplanets or NEMPs -
  which, appropriately, sounds a little like an impish
  character from a Tolkien novel.

Some possible problems are:

* Uncertainty about the actual size of small, distant objects.
  Apparently the size of Pluto and Charon was determined
  during a fortunate alignment with the Earth that produced

* Dealing with ususal objects such as comets captured by
  Jupiter ('captured microplanets'?) or interstellar

* A US software company already has the name Microplanet
  Inc but maybe they would welcome the term becoming
  (hopefully) a household name.

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