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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