Is There A System For Naming TNOs?

Question posed by Mark in a conversation on G+. If you don't know what a TNO is, you might like to check this post out first!

Astronomical objects sometimes go through two naming processes. The first one is the rather formal-sounding 'provisional designation', which is little more than a cataloguing procedure and gives names like 2003 VB12*. The second one is the one which gives it the more friendly, familiar names that we remember more easily because they're distinctly more name-like.

Each type of astronomical object has its own system of provisional designation, and a lot of these are complex and confused further by older conventions having been used for older discoveries. These older conventions have been superseded largely due to an upsurge in the discovery rate of many types of object - the old systems just couldn't cope with the frequency of discovery.

In the case of TNOs, the naming convention for minor planets (which also includes asteroids and centaurs) is used. The current system has been in place since 1925, and goes like this:

  • We start with the 4-digit year of discovery: this bit's easy. In the designation quoted above, 2003 VB12, we can see that this object was discovered in 2003.
  • Next up is a letter that describes the half-month of discovery. 'A', for example, stands for the first half of January. 'B' denotes the second half of January, 'C' the first half of February, and so on up to 'Y', which is a label for the second half of December**. In our example, 2003 VB12, the 'V' tells us that this object was discovered in early November (of 2003).
  • The second letter in combination with the subsequent numbers denotes the object's place in the order of discovery that month: 'A' means it was first. 'B' means it was in second place, and so on up to 'Z' being 25th in line that half-month***. The 26th discovery (and this is where it gets tricky) is denoted by 'A1', the 27th by 'B1' and the 28th by 'C1', etc - as the 25 letters are used up, they are recycled, and the number shows how many times we've gone through the cycle. With 2003 VB12 we can see that 'B12' involves 12 cycles through the letters (totalling 12 x 25 = 300) plus an extra 2 to get to B, making 2003 VB12 the 302nd discovery in the first half of November 2003.
The second naming procedure is a bit simpler: Minor planets are given a name by their discoverer****. This can be any name (as long as it is unique), but TNOs are (by international agreement, I think) generally given the names of humanity's many and varied gods. I can't back this up, but I'd like to think that this followed on from 11 year-old schoolgirl Venetia Burney's naming of the first (to be discovered) TNO in 1930, Pluto.

* The 12 should really subscripted, but it isn't always. This particular object has the more familiar name 'Sedna'.
** Shrewd readers will have scratched their heads at this point: There are 12 months in a year, and therefore 24 half-months, so how is it that we can get as far as Y, the 25th letter of the alphabet? The answer is that the letter 'I' is not used, being so easily confused with the number '1'.
*** Head-scratchers have probably figured out that the 'I' is skipped again.
**** This isn't strictly true: the discoverer has the privilege of suggesting a name which is considered by a committee at the International Astronomical Union.


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