Various things to look at

The Science of Aliens (Basic Books, 1999).

http://io9.com/325109/four-theories-about-creatures-from-high+gravity-worlds

http://www.umassmed.edu/shriver/Research/Biomedical/Projects/gravity.aspx

http://www.astro.rug.nl/~onderwys/sterIIproject97/kleefman/index.html

http://astrobiology.nasa.gov/ask-an-astrobiologist/question/?id=53

http://www.molvray.com/sf/exobio/astron.htm <---- This