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.

 Australian Planetary Science

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  businessman 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 [CCNet Moderator], 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 star.

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 across.

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 sizes of Pluto and Charon were determined during a fortunate alignment with the Earth that produced eclipses.

* Dealing with unusual 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.


Jan 2013. I have just been re-reading the 1985 book Comet by Carl Sagan and Ann Druyan. They use the term "microplanets" several times in the book but are referring to the grains that cause meteors in our skies. They also use the term "planetoid" in Broca's Brain (includes a great history of space exploration).

Nov 2015 Discovery News: Pluto Flunked the Planet Test, Again.

Dec 2017: The  December issue of Scientific American ($) has a brilliant feature by Alan Stern: Pluto's Secrets Revealed - this lends support to my suggestion that the Pluto-Charon system be regarded as a binary planet.

May 2018: Science Alert: These NASA Scientists Think Pluto IS a Planet, And Here's Why + Forbes: You Won't Like The Consequences Of Making Pluto A Planet Again  - Also Alan Stern advises me that the binary planet nomenclature has been used for Pluto-Charon since the late 1980s

Aug 2018: Planet Nine: 'Insensitive' Term Riles Scientists - "...we the undersigned believe the use of the term 'Planet 9' for objects beyond Pluto is insensitive to Professor Tombaugh's it Planet X"

October 2019 New Scientist: The solar system has too many moons – it's time for a cull