We knew they had to be there. Once people realized that the Sun is a star and that the planets travel around it, it was only natural to suspect that other stars had planets of their own. Confirmation, however, didn't come until the early 1990s, when astronomers found compelling signs of smaller bodies orbiting first pulsars and then normal main-sequence stars.
Today, it is estimated that our galaxy contains at least as many planets as stars. Almost 900 have been detected, and a few thousand others are under investigation. . . .
Exoplanets, one of the works cited in an article ("Exoplanet Habitability") included in the special section, has the LCSH "Extrasolar planets." LCSH lists "Exoplanets" as a variant of "Extrasolar planets." Exoplanets is classed in 523.24 Extrasolar systems, which has a class-here note: “Class here extrasolar planets.” The Relative Index entries "Extrasolar planets" and "Extrasolar systems" both point to 523.24, and the LCSH "Extrasolar planets" and "Habitable planets" have both been mapped to 523.24. Other examples of works classed in 523.24 Extrasolar systems are:
Are There Other Earths?
A Decade of Extrasolar Planets around Normal Stars: Proceedings of the Space Telescope Science Institute Symposium, Held in Baltimore, Maryland, May 2-5, 2005.