12th Mar 2010
Why are exoplanets called, "HD and then some number after that"?
Why HD plus a number?
http://en.wikipedia.org/wiki/Henry_Drape...
(So this specifies the star around which the exoplanet orbits.)
This is a catalogue of stars published from 1918 to 1949. It includes (along with extensions to the catalogue) about 359,000 stars down to magnitude 9 (and somewhat fainter), and includes the spectroscopic classification for each star.
Most stars have several different designations, because there are lots of different star catalogues and naming systems. The Henry Draper catalogue is a widely used one, and gives astronomers a convenient way to refer to the "brighter" stars in the sky. (Stars of magnitude 9 are too faint to be seen with the naked eye, but can be seen with binoculars in a dark sky.)
-- edit
Each star catalogue has a somewhat different mission. The HD catalogue lists stars down to a certain magnitude and gives spectroscopic information. The Gliese catalogue (which is used for some other exoplanets) lists all known stars closer than a certain distance. For a fuller discussion of star catalogues, see the following web sites:
http://www.seds.org/~spider/spider/Misc/...
http://en.wikipedia.org/wiki/Star_catalo...
This is part of one of the more fundamental problems in science. There are vast numbers of molecules in chemistry, species in biology, stars in astronomy. How do you refer to these so that other people know what you're talking about? These catalogues are the way that astronomers deal with this problem. Star catalogues are not the end of the story; for instance, there are catalogues of galaxies and other "nebulae", catalogues of peculiar galaxies, catalogues of clusters of galaxies, etc. A good part of science is simply collecting and cataloguing a vast amount of data in order to make it manageable.
Currently, Gliese 581 d, the third planet of the red dwarf star Gliese 581
So the name of the planet should be HD-579 a or something like that, and HD-579 wouldbe the name of the star it orbits.
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