February 26 – Defining Minor Planets: A Major Headache

1842  –  Birth of Nicolas Camille Flammarion, astronomer and science fiction author, in  Montigny-le-Roi, France.

1880  –  Birth of Kenneth Edgeworth, the first person to propose the existence of what would eventually become known as the Kuiper Belt.

1965  –  Launch of the experimental meteorological satellite Cosmos 58 by the USSR.

Today, as we have no “asteroid” discoveries to discuss, we’re going to have a crash course in a couple of groups of large flying things that used to be called asteroids, but are now commonly known as minor planets.  The term refers to all sorts of things, and can be broadly defined as anything orbiting the Sun that isn’t a planet or a comet.   The terminology, however, is confused and confusing, and unlikely to become any clearer as more bodies are discovered beyond the traditional confines of the Solar System.  So nowadays you have to decide whether you’re talking about a dwarf planet, an asteroid, a trojan, a centaur, a comet, a small Solar System body,  a Kuiper Belt object or a trans-Neptunian object.

The IAU prefer the term small Solar System body for comets and anything too small to use gravity to maintain an ellipsoidal shape, and have done so since their General Assembly of 2006 (IAU 2006), the same one that decided the fate of Pluto.  According to this classification, anything that is not a planet, but which is able to become roughly planet-shaped is a dwarf planet.

So now we have to briefly ask “What is a planet?”  well, according to Resolution 5A of IAU 2006 a planet has to (a) be orbiting the Sun, (b) be able to use its own gravity to keep a nearly round shape, and (c) be sufficiently well-developed to have “cleared the neighbourhood“.  This might be a new phrase to you, but all it means is that the object under consideration has become the dominant one in its orbit, so that there is nothing left nearby of comparable size (except possibly its own satellites).  A dwarf planet meets conditions (a) and (b), but not (c).

So, let’s have a quick look at a couple of the various types of non-planets (I don’t want to drive you away by tackling them all at once).

There are presently known to be five DWARF PLANETS in the Solar System.  These are Pluto, Ceres, Eris, Haumea and Makemake.  Several further trans-Neptunian objects (including Sedna and Quaoar) may well swell their ranks shortly, but their size and distance makes pinning them down difficult.  However, as the outer reaches of the Solar System are explored it is thought that hundreds, possibly thousands, more will turn up.  It is probably only our inability to see beyond the Kuiper Belt that is keeping the numbers down.

Ceres from the DAWN spacecraft (image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

Ceres from the DAWN spacecraft (image credit: NASA/JPL-Caltech/UCLA/MPS/DLR/IDA)

I’m not fond of the term dwarf planet, as it suggests that these are small planets, whereas the idea was, I believe, that they aren’t planets at all.  For this reason I would maybe prefer the older term planetoid.  The Japanese have got the right idea: their name for dwarf planets, junwakusei, can be translated as “almost a planet”, which I like almost as much as the suggestion of Alan Stern and Harold Levison from their paper to the IAU in 2000.  They adopted the words überplanet for the big eight, and unterplanet for the rest.
TROJANS are interesting characters. I don’t want to get bogged down explaining Lagrangian points and barycentres (yet) so I’ll just say that a trojan shares an orbit with another (larger) body at the Lagrangian points, approximately 60° ahead of or behind it. We have more interchangeable terminology here: a trojan can also be called a Lagrangian object, and the Lagrangian points are sometimes called trojan points. Saturn has a great collection of trojan moons: they are (i) Telesto and Calypso (trojans of Tethys), and (ii) Helene and Polydeuces (trojans of Dione).

Jupiter also has a large collection of trojans, traditionally named after the two camps of the Trojan Wars. Greek trojans are located at one Lagrangian point, and Trojan trojans are at the other. There are two exceptions to this rule (because their discovery pre-dates it): they are 617 Patroclus ( a Greek, but with the Trojans), and 624 Hektor (a Trojan with the Greeks). I never liked Hektor much when I read the Iliad, so this doesn’t surprise me in the slightest.

Next time I get a quiet day I’ll move onto centaurs and Kuiper Belt objects (if I remember).



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