Dr Michael E Brown & Julia Sweeney in conversation, June 2007

Michael Brown is a professor of planetary astronomy at the California Institute of Technology. He is best known for his discovery of Eris, the largest object found in the solar system in 150 years, which led to the debate and eventual demotion of Pluto from a "real" to a dwarf planet.

Comedian Julia Sweeney is best known for her roles on Saturday Night Live, especially as the androgynous character "Pat." Her 1996 one-woman monologue, God Said, Ha!, addressed her experience of surviving cancer. More recently she has written and performed In the Family Way and Letting Go of God, the Latter explaining her conversion to atheism.