Astronomer Dimitar Sasselov, director of Harvard's Origins of Life Initiative, joins us to discuss his new book "The Life of Super-Earths" and to explain why he thinks planets larger than Earth offer the best prospects for finding life as we know it.

A White House spokesman says President Obama is considering sending humanitarian aid to Syrians, while at the same time ramping up pressure against President Bashar Assad. Growing sectors of the Syrian population have mounted demonstrations against the Assad regime, sparking violent government crackdowns resulting in thousands of deaths. The U.S. and several Gulf countries have closed their embassies and recalled their diplomats. We discuss the next steps in the region.

The federal 9th Circuit Court of Appeals has ruled that Proposition 8, California's same-sex marriage ban, is unconstitutional. The court found that Prop. 8 violates the equal protection clause under the 14th Amendment of the U.S. Constitution. We discuss the ruling, which will likely be appealed to the U.S. Supreme Court.

The San Francisco Board of Supervisors will vote on whether to expand the city's ban on plastic bags Tuesday. The new plan would ban the bags from all businesses -- not just large grocery stores and pharmacies -- and it would impose a surcharge on other bags.

Internet activist and Google marketing executive Wael Ghonim was thrust into the international spotlight last year as his Facebook page helped spark the Egyptian uprising. He joins us to talk about his new book "Revolution 2.0," and about recent political developments in Egypt.

Painter Georgia O'Keeffe and photographer Alfred Stieglitz are among art history's foremost couples. And they left an amazingly detailed record of their relationship in the form of more than 5,000 letters. Sarah Greenough of the National Gallery of Art has edited a new volume of these letters. She joins us to discuss the couple's relationship and their work.

U.S. Food and Drug Administration Commissioner Margaret Hamburg joins us to discuss the role of the FDA in protecting public health, how the agency approves new drugs and her views on what she calls "smart regulation."

The Athena Breast Health Network is launching a series of face-to-face forums where patients, breast cancer experts and community members can exchange direct experience and research in breast health care. The first forum will examine breast cancer risk assessment and prevention - and the results will be fed back into the UC-based Athena Network of 150,000 women to improve survival and reduce suffering from breast cancer.