Today is a major anniversary in planet-hunting circles, as 14 years ago on this date scientists announced discovery of the first traditional planet orbiting a major star other than our own sun. That is to say, we found the first real alien planet.
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?
Image via WikipediaFour years ago today, the International Astronomical Union voted to revise the current IAU definition of a planet -- adopting the one that didn't include Pluto. This, of course, led to some blowback in the astronomy community (and the sci-fi/internet community, too). Team Pluto, however, didn't have much in the way of empirical evidence to back the position Pluto deserved to stay -- especially since it wasn't even the largest member of the newly minted dwarf planet group to which it now belongs.
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