What does the "2000" mean at the end of the coordinate system names in the Solar System folder?
You can also find reference to the number in the details of the *.prj file itself.
GEOGCS["GCS_Jupiter_2000",DATUM["D_Jupiter_2000",
SPHEROID["Jupiter_2000_IAU_IAG",
71492000.0,15.41440275981026]],
PRIMEM["Reference_Meridian",0.0],
UNIT["Degree",0.0174532925199433]]
The number suffix is, in fact, a date. It refers to the currently used standard equinox (and epoch) which is J2000.0.
The prefix "J" indicates that it is a Julian epoch and the number refers to January 1, 2000, 12:00 Terrestrial Time. There have been other standard equinoxes (and epoch) where the previous version was B1950.0, with the prefix "B" indicating it was a Besselian epoch. Julian equinoxes and epochs have been used for every equinox since 1984.
