https://pds.jpl.nasa.gov/planets/special/pluto.htm

http://nssdc.gsfc.nasa.gov/planetary/factsheet/plutofact.html

http://starchild.gsfc.nasa.gov/docs/StarChild/whos_who_level2/tombaugh.html

http://www.nasa.gov/audience/forstudents/k-4/stories/nasa-knows/what-is-pluto-k4.html