Jelaskan Pencarian planet kedelapan dari James Challis

Clerke, A. M. (2006) "Challis, James (1803–1882)", rev. David B. Wilson, Oxford Dictionary of National Biography, Oxford University Press, online edn, Oct 2006, accessed 17 September 2007

Eggen, O. J. (1970–1981) "Challis, James" in