Sites

Asteroid Discovery Circumstances (Popularity: )
http://ssd.jpl.nasa.gov/dc_home.html
For each minor planet: Where and when found, and by whom.

Neptune and Pluto (Popularity: )
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Neptune_and_Pluto.html
Mathematical discovery of planets

Orbits and Gravitation (Popularity: )
http://www-groups.dcs.st-and.ac.uk/~history/HistTopics/Orbits.html
The development of the mathematics of planetary motion.

The Struggles to Find the Ninth Planet (Popularity: )
http://perso.wanadoo.fr/olivier.granier/meca/vulga/pluton/pluton.htm
A paper by Clyde Tombaugh relating his discovery of Pluto.

Exploring The Planets-Discovery (Popularity: )
http://www.nasm.si.edu/ceps/etp/discovery/etpdiscovery.html
The five known in antiquity, three more since the telescope, and Ceres.

Is Mars Habitable? (Popularity: )
http://www.wku.edu/~smithch/wallace/S730.htm
A. R. Wallace's 1907 critique of Lowell's Book "Mars and Its Canals"