Naming New Extrasolar Planets
An artist's impression of the extrasolar planet HD 189733b, seen here with its parent star looming behind--astronomers said its sunset looks similar to a hazy red sunset on Earth. The planet is slightly larger than our own solar System's Jupiter, and its atmosphere is a scorching eight hundred degrees Celsius.
Credit: ESA/NASA/Frederic Pont, Geneva University Observatory

There has been more public furor over the demotion of Pluto to ?dwarf planet? than I have seen about other astronomical issue in a long time.

It?s good to have large participation and interest in astronomy, and many folks really care significantly about the naming of celestial objects. In a recent science meeting many colleagues discussed the naming of new extrasolar planets. I happened to disagree with the majority, and was surprised when a young scientist stated, with what appeared to be frustration (and not a little enthusiasm), the naming of extrasolar planets was already a tradition and could not be changed. Wow, a tradition in only 15 years! But the emotional involvement was a surprise, especially from a scientist. So here is the issue at hand.

Extrasolar planets are currently named in order of discovery using lower-case letters of the English alphabet. For example, we have the first extrasolar planet-mass bodies discovered around the pulsar PSR 1257+12 and they are named PSR 1257+12-b, PSR 1257+12-c, and PSR 1257+12-d in order of discovery. (?a? is reserved for the central star.) This has worked out well for this stellar system since the planets happen to also be in order of increasing orbital period (i.e., distance from their star).

Now, we also now have Gliese 876-d, Gliese 876-c, and Gliese 876-b which is the correct order from the star, but in this case the outer planet (?b?) was discovered first, followed by the second farthest (?c?), followed by the closest planet to the parent star, Gliese 876-d. The star HD160691?s planets (discovered so far) are, in order of distance from their star, HD160691-d, HD160691-e. HD160691-b, and HD160691-c. The planetary system 55 Cancri is really fun ? there the planets, listed in order from their parent star, are 55 Cancri-e, 55 Cancri-b, 55 Cancri-c, 55 Cancri-f, and 55 Cancri-d. You get the idea. Each planetary system preserves the historic order of discovery in their names but tells us nothing astronomical about the planets, or their proper relationship to each other. In addition, it seems like it could create quite a confusing situation, after many planets are discovered. Perhaps akin to what Greek or Indonesian children must have to go through when learning the geography of their country?s thousands of islands.

My suggestion, which was not adopted, was that each planet be named for its stellar parent as usual, but then be designated by its orbital period in days, to one decimal point . The orbital periods may certainly be expected to be constrained to within a tenth of a day or so. No two planets could be confused (unless there are Trojan planets which share orbits but may be rare, and at any rate might have the additional unambiguous designations of i and ii, as needed.) Thus we would have Gliese 876-1.9, Gliese 876-30.9, and Gliese 876-60.1. Those with a bent for history would have to look up the discovery order, instead of the astronomer having to sort through a list of letters to figure out which is the one to observe for a transit or radial velocity data.

Similarly we would have 55 Cancri-2.9, 55 Cancri-14.6, 55 Cancri-44.3, 55 Cancri-260, and 55 Cancri-5218. Now why make such a fuss about this now (if writing an article about it can be considered a fuss)? Over 340 extrasolar planets have already been discovered and things have seemed to work out OK so far, right? The reason is because current space missions may soon discover thousands of additional planets as well as many more smaller planets than in already known planetary systems. Some space missions will be able to detect Earth- or even Mars-sized bodies. If our Solar System is typical in terms of having eight or more planets to a system, then there could be designations consisting of combinations of the letters b, c, d, e, f, g, h, and i! And all sorts of permutations on these could occur. We could get: (star name)- b, g, f, d, e, h, i, c or (star name)-e, d, i, b, h, c, f, or any other of the 46,233 possible combinations of eight letters taken one, two, three, and on up to eight ways at a time. The possible combinations go up as the factorial of the number of planets so things do not really get ?bad? at one, two, three, four, or even five planets to a system (still only 120 possible combinations ? on the order of learning the states of the US). But as one gets up into six or more planets to a star system the possible combinations really begin to take off.

And this is just the start of the fun. If we consider multiple star systems?where stars are usually designated by the capitols, A, B, C, etc.?we get situations where, for example, alpha Centauri A-b is a planet, while alpha Centauri B-a is a star. (The "a" designation would, of course, not be used unless a planet is discovered and then the "a" for the star is implied, using the present nomenclature.) Let's look at the 6-star system Castor (alpha Geminorum) where we actually already have the stellar components named Aa, Ab, Ba, Bb, Ca, and Cb. (The second components of close binaries are referred to with lower case letters in these cases.) These are each star names, but if a planet was discovered about star Ab it would, by the current nomenclature, be designated alpha Gem Ab-b, while a second planet would be alpha Gem Ab-c. Similarly, planets discovered around star Bb in order would be alpha Gem Bb-b, alpha Gem Bb-c, and so on, and planets discovered around alpha Gem Cb would be alpha Gem Cb-b, alpha Gem Cb-c, etc. So in this case alpha Gem Bb is a star, even though the first planet around the double star 16 Cygni B is 16 Cyg B-b. I guess the hyphen would be the only way to tell. Wouldn't it be easier, if three new planets are discovered around alpha Gem Bb, to have the designation be something more like alpha Gem Bb-12.3, alpha Gem Bb-20.2, and alpha Gem-43.6? Incidentally, double star systems are not rare?more than half of all stars are in binary systems.

Again, I don't want to ruffle any fur, but with thousands of new extrasolar planets down to terrestrial-body sizes being discoverable within the next decade or so, shouldn't we be taking stock of our existing nomenclature? I know the IAU commission on naming celestial objects is very diligent (and very busy) and so hope they have considered (along with other extrasolar planets astronomers) some of these possible permutations. You can see further interesting suggestions for nomenclature in Wikipedia. I shall leave others to do the possible combinatorics and nomenclature for future multiple star system planet discoveries for up to eight planets around each of the stars in, say, a triple star system. The number of triple star systems is non-trivial.

Now I must add that this is not the first time astronomers have run into trouble with confusing nomenclature (aside from the usual beginner?s confusion over larger magnitude numbers referring to fainter stars, and things like that). The historic names of variable stars is particularly amusing. A gentleman named Fredrich Argelander decided that the first variable star in a given constellation would have the designation R put in front of it. He chose R because he knew the earlier letters in the alphabet were already being used (for multiple star systems, for example, as noted above). But he also felt that there could certainly be no more than 9 variable stars per constellation. Unfortunately, he was off by thousands and thousands. After R of course came S, continuing down the (English) alphabet to Z. Rather than going back to A the designation that follows Z was then dubbed RR, then RS, then RT on down to RZ. For example, I've done work on the eclipsing variable RT Andromeda. After RZ comes SS (not SR, by the way), followed by ST, on down to SZ. Then comes TR to TZ, and so on down to ZZ. (ZZ Ceti is a famous star, for example.) OK, now what? RRR? Well, no.

It turns out that after ZZ comes AA (which apparently would not be confused with double stars using Aa since in this case the letters are both capitals). Then comes AB, on down to QZ finally, skipping the letter J for some unknown reason ? maybe to keep us on our toes? Since there are thousands of variable stars in each constellation (the number of which, of course, grows all the time) the 334 variable names possible with this system (up to QZ) was clearly not going to keep up. OK, so now would it be time for RRR? Alas, no. Astronomers gave up after this and began to use V335, then V 336 and so on, with the "V" for variable star. Might have been good to think of this ahead of time but after all is it all part of the rich tapestry of the history of astronomy, right??

Now, back to extrasolar planets. Why would current astronomers prefer letter designations (not dissimilar to what early variable star folks used) rather than numbers that would mean something more astronomical? Would planetary orbital periods, for example, change? I would assert that most planets would not be changing their orbital periods by a tenth of a day or more over thousands of years but special nomenclature might be noted (similar to the i or ii above) for such rare cases. Thus with planetary orbital period designations we might hope to avoid impending nomenclature confusion by using numbers that also mean something astronomically, (with apologies to science historians, of course).

Finally, an example of smart nomenclature, I have always felt, was the inclusion of the position of the star system in the name, like BD 16 + 516 and the pulsar system given above, PSR 1257+12. In this case the first number is the rough Right Ascension (i.e. essentially the star?s longitude on the map of the sky) and the second number the rough Declination (essentially the star?s latitude on the map of the sky). So, by only seeing the name it can be determined if these stars are "in season" and observable at night at this time of year, as well as if they are high enough above the horizon at a given observatory. It is true that stars change position according to the processional motion of the Earth?s rotation axis, but this has not been a problem because the designation is so coarse?precession is a rather tiny effect over a century.

Within this decade we may expect to discover thousands of new extrasolar planets which, using the current designation, could produce tens of thousands of possible (and frankly essentially meaningless) letter combinations designating them. Therefore I humbly make a suggestion to my fellow astronomers: it is not too late to bail on this system (as "traditional" as it is). We may not want to go through the variable star experience of the 19th Century again in spite of its rich historical tapestry. But if we do, I guess I can always practice on Greek or Indonesian island names. And when I think of this as an emotional issue, what will happen if astronomers start to discover Pluto-like bodies around other stars? Are they going to be planets with letter designations or dwarf planets with other designations? I think, as they say in England, I'll give that one a miss.

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