Tuesday, April 21, 2009

How We Designate Planets Nowadays

(GEEK ALERT! Only interesting if you are a science or science-fiction geek. Mundanes may skip without penalty.)

How do we designate planets? The old way, the venerable science fiction way, was to give planets roman numerals in order of their distance from their sun. In this system, Earth was "Sol III," Khan was exiled on "Ceti Alpha V," and in the Dune universe, the planet Ix derived its name from the fact that it was the ninth planet from its sun.

That's not how it works in the real world. According to the all-knowing Wikipedia:
The most common way of naming extrasolar planets is almost the same as binary stars, except that a lowercase letter is used for the planet instead of the uppercase letter for stars. A lowercase letter is placed after the star name, starting with "b" for the first planet found in the system (51 Pegasi b). The next planet found in the system could be labeled the next letter in the alphabet. For instance, any more planets found around 51 Pegasi would be catalogued as "51 Pegasi c" and then "51 Pegasi d", and so on. If two planets are discovered around the same time, the closest one to the star gets the next letter, while the last planet would get the last letter. For example, in the Gliese 876 system, the most recently discovered planet is referred to as Gliese 876 d, despite the fact that it is closer to the star than Gliese 876 b and Gliese 876 c. The suffix "a" was intended to refer specifically to the primary, as opposed to the system as a whole, but this did not catch on. At present, the planet 55 Cancri f (being the fifth planet found in the 55 Cancri system) is the only planet to have "f" in its name, the highest letter currently in use.

In practical terms, this basically means that we're assigning letters in decreasing order of mass, since we detect extrasolar planets by their mass.

I assume that the old "roman numerals" system would still apply, once we get close enough to another planetary system to be sure that we've detected all the planets in the correct order.

Anyway, it occurred to me that I haven't seen the new way applied to the Solar System. So here it is:

    Sol b - Jupiter
    Sol c - Saturn
    Sol d - Neptune
    Sol e - Uranus
    Sol f - Earth
    Sol g - Venus
    Sol h - Mars
    Sol i - Mercury

I'm going to let the dwarf planets fight it out among themselves.

I suppose there's an analogous system for satellites, under which Ganymede = Jupiter b, Callisto = Jupiter c, Io = Jupiter d, Europa = Jupiter e; Titan = Saturn b; Triton = Neptune b; and even Charon = Pluto b. Saturn is going to cause problems, since it had more than 25 satellites.

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