1. Department of Physics and Astronomy, University of Western Ontario, London, ON N6A 3K7, Canada
2. Institute for Earth and Space Exploration, University of Western Ontario, London, ON N6A 3K7, Canada
3. Carl Sagan Center, SETI Institute, 189 N. Bernardo Avenue, Suite 100, Mountain View, CA 94043, USA
4. Department of Astronomy, University of Maryland, College Park, MD 20742, USA