New planets

Today (55468) two more planets were announced orbiting the star Gliese 581, one of which is only three times as heavy as earth and orbiting in the "Goldilocks zone" where liquid water, and therefore life as we know it, can exist. In fact, one astronomer believes that "chances of life on this planet are 100 percent."

This seems like a ridiculous thing to say, since we only have one data point so far, which is not enough to determine probabilities. Just because life might be possible does not make it certain, and there are still too many variables which we do not know.

The planet is the sixth to be discovered around this red dwarf star, which is about 20 light-years (190 terameters) away from us, and called Gliese 581g. This is what bugs me. The six planets are lettered b-g, and the parent star is letter a.  This is how astronomers name extrasolar planets.  The first object in another system is named a, which is always a star, the second object, which may or may not also be a star, is lettered b, and so on. 

But it's not supposed to be that way!  Any sci-fi fan knows that planets are numbered in order from the parent star, which always has a pronounceable name.  For instance, earth, the third planet from the sun, is Sol IIIKhan was marooned on the fifth planet of a star, Ceti Alpha V, shortly before the sixth planet, Ceti Alpha VI, was destroyed.  It's not that bad that they use letters instead of numbers, but the letters do not distinguish between stars and planets, and go in order of discovery, which usually relates to size, rather than distance.  How stars, themselves, are named is much more complicated, and more often have just letters and numbers than proper names.

Of course, it has to be that way.  There are hundreds of billions of stars in the Milky Way, only a few thousand of which are visible to the naked eye from earth.  Only the brightest of these have short names.  Up to a couple of dozen per constellation have a name that is a Greek letter combined with the name of the constellation, like Alpha Centauri, which is actually a binary star, the components being lettered A and B.  (Proxima Centauri, the closest star to us, may or may not be part of the same system, so it is sometimes called Alpha Centauri C.)  There are various other naming schemes, some based upon star catalogues such as the Gliese Catalogue of Nearby Stars, which lists nearly a thousand stars that are within 20 parsecs (617 petameters).  Stars often have multiple names, e.g. Alpha Centauri is also called Rigil Kent or Toliman, as well as various other designations.

It is currently impossible to name extrasolar planets based upon orbital order, since we cannot see all of the ones that might be orbiting a given star.  So the best we can do is to name them as we find them, which necessarily means that they are named in order of discovery.  In our own solar system, we have proper names for all the planets.  The six innermost have been known since prehistory, long before it was known what they actually were, so there is no order of discovery, although we could order them by apparent brightness.  If we make the sun Sol a, then earth could be Sol b, Venus Sol c, Jupiter Sol d, Mars Sol e, Saturn Sol f, and Mercury, which is hard to see even at its brightest, Sol g.

One advantage of this system is that discovering a new planet would not require renaming known planets.  If a planet was discovered between Sol IV (Mars) and Sol V (Jupiter), then it would become Sol V and Jupiter would become Sol VI.  This actually happened a couple of hundred years ago, when Ceres was discovered, although now Ceres is classified as an asteroid or dwarf planet.  The first planet discovered in modern times, Uranus, would be Sol h, and Ceres would be Sol i.  But due to our vantage in the inner solar system, the last major planet, Neptune, was discovered after several minor planets, which were later reclassified as asteroids.  We all know that Pluto was likewise demoted from major planet to dwarf planet.  As we find more and more planets, such distinctions will become even more arbitrary, but they will tend to be discovered in order of size and named in that order, rendering it moot.

Of course, nothing precludes us from switching to the other system once we discover warp drive.  If we were to colonize extrasolar planets, we would find proper names for them and their parent stars, and at the same time we could refer to their orbital number, just as we call earth Sol III.  We use what is convenient for now, but in the future something else may be more convenient.

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  1. Not to be too pedantic, but until about 500 years ago, the Earth wasn't regarded as a planet- That is, it didn't transit the plane of the sky with the other "planets"-- The Sun, Moon, Mercury, Venus, Mars, Jupiter and Saturn. (This is why I get pissed at the "Pluto is not a planet, because we don't want any more planets" behaviour of the last few years in the astronomical community- They say we should stick to the "classical" planets, except "classical" meant the Earth didn't count, but the Sun and Moon did!)

    My, have times changed. :)

    That said, naming conventions for exosolar planets are such out of necessity, but do note that stars in a multistellar system are capitalised while planets are given lowercase designations-- Makes it interesting, should there be an Alpha Centauri B b (which likely does exist).


  2. Stardate.org covered the same subject on their radio show yesterday.