On Oct. 6, 1995, scientists Michel Mayor and Didier Queloz announced they had observed a planet orbiting the star 51 Pegasi, which is just over 15 parsecs from Earth (that's about 50 light years or 1.28 Han Solo Kessel Runs) in the middle of the constellation Pegasus. The planet is now called 51 Pegasi b, with the lowercase letter indicating that it was the first object found in the 51 Pegasi system besides the star itself. Thus was born the formal exoplanet naming convention which, like so many scientific traditions, starts out logical but can get really confusing.
The trouble with the International Astronomical Union's exoplanet naming conventions is twofold: They weren't honored for the first exoplanets discovered, and they get pretty screwy when applied to multi-exoplanet star systems. As noted above, 51 Pegasi b was the first "traditional" planet found orbiting a major star. That is to say, it was the first planetary body found orbiting a star not unlike our own sun. 51 Pegasi b was not, however, the first planet found outside our own solar system.
Planets PSR B1257+12B and PSR B1257+12C were found orbiting the pulsar PSR B1257+12 in 1992, three years before 51 Pegasi b. Note the uppercase designations of the planets, rather than the lowercase tradition started with 51 Pegasi b. Since the pulsar planets were the first discovered and because they orbit a pulsar rather than a regular star, their naming convention was largely ignored when "real" planetary discovery started. The original designations of the first planets were grandfathered into official IAU catalogs rather than retroactively changing their names. (When a third, more closely orbiting planet was found around PSR B1257+12, they called it PSR B1257+12A, just to keep the confusion going.)
As noted, the accepted IAU convention is to label planets in order of discovery, rather than in order of orbital distance from the star. Thus, 55 Cancri e is the innermost known planet in the 55 Cancri system, but has the later letter designation because it was the fourth planet discovered around that star. As more massive planets are easier to find, and more massive planets tend to orbit farther from parent stars than do less massive planets, this erratic lettering system will likely become more common.
Planetary naming issues are also more complicated in multi-star systems, as stars are designated with uppercase letters, and those designations are combined with lowercase planet labels. Thus the second planet around the second star in the 16 Cygni system is 16 Cygni Bb.
No wonder 51 Pegasi b is referred by many scientists by its common name, Bellerophon, rather than by its formal IAU designation. We all grew up calling Spock's home planet Vulcan, rather than 40 Eridani Ac. It's a wonder more planets haven't been given common names.
In fact, how many extrasolar planets have been given official IAU-approved common names?
Officially, exactly zero planets have been given IAU-approved common names, because it is against International Astronomical Union policy to bestow common names on planets. (Yes, this was a trick question.)
The nicknaming of 51 Pegasi b as Bellerophon -- after the Greek hero who tamed the Pegasus -- was strictly informal. Yes, we were all trained by various science fiction universes that every planet had a common name, and scientists watch Star Trek, too. But naming every exoplanet after a mythical figure is actually pretty impractical, given how many planets we've found since the first pair were discovered in 1992.
According to the Extrasolar Planets Encyclopedia, we have to date observed 374 exoplanets in 316 star systems besides our own, with 39 of those being multiplanet systems, and some planets being rogue or orphan bodies that orbit no stars. That's an average rate of 22 planets discovered every year since 1992. And the rate of discovery is increasing. And there are between 100 and 400 billion stars just in our own galaxy, many of whom we haven't even begun to look for stars around. Thus, even if we scraped the name of every recognizable person, place, or thing from the Encyclopedia Mythica and slapped them on planets, it wouldn't be terribly long before we had more exoplanets than available names.
Being the first traditional exoplanet discovered, 51 Pegasi b received decidedly more publicity and scientific interest than many subsequent exoplanet discoveries. As such, its informal designation as Bellerophon was more a grassroots nicknaming than an official term, given that it had more of lay audience that liked the catchier name.
Only two other exoplanets have what you would call generally accepted nicknames: HD 209458 b is sometimes called Osiris, and PSR B1620-26 b is known informally as Methuselah. Osiris -- named for the Egyptian god of the afterlife and rebirth -- earned its nickname due to being the first exoplanet discovered with an atmosphere. Methuselah -- named for the oldest known person depicted in the Hebrew Bible -- is the oldest currently known exoplanet, with an estimated age of roughly 12.7 billion years.
Thus, while the IAU is not in the business of giving common names to exoplanets, scientists and the media are, and they'll throw a nickname on any extrasolar orb that has enough publicity star-power. That's not just a serendipitous slice of self-regulating self-promotion, it's a systematically stellar sample of the Truly Trivial.
SOURCES
51 Pegasi | Wikipedia
51 Pegasi b | Wikipedia
55 Cancri e | Wikipedia
Extrasolar planet - First discoveries | Wikipedia
Candidate Planet Catalog - All | Extrasolar Planets Encyclopedia
HD 209458 b | Wikipedia
PSR B1620-26 b | Wikipedia
Vulcan system | Memory Alpha
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After years of writing trivia columns I've come to embrace the fact that almost everyone reading these things is smarter than me, which is why you always seem to find omissions, mischaracterizations, and outright screw-ups in my work. And like all great businessmen, I intend to profit from my own ignorance and sloth. Thus, I invite you, the reader, to call me out on these entertaining little glitches in each Truly Trivial column. The responder with the smartest, best-sourced, and/or most amusing gotcha will earn a place of honor in the subsequent column by having his/her/its comment highlighted next week in this space, along with an excuse, rebuttal or (more likely) half-hearted mea culpa from me. Yes, that is your sense of self-importance rising to the bait. Don't fight it. I dare you.
