Judging by the flood of resulting press coverage, the plan, formally known as Breakthrough Starshot, has captured the public's imagination. The effort has an advisory committee stacked with reputable scientists, and a board of directors that includes cosmologist Stephen Hawking and Facebook founder Mark Zuckerberg.

Breakthrough Starshot is connected to The Planetary Society through its founders and efforts to advance solar sailing technology. Ann Druyan, a Starshot management and advisory committee member who co-wrote the original Cosmos documentary series with her late husband, Society co-founder Carl Sagan, described this connection just 20 minutes into the press conference.

"I remember in the 1970s, when [Sagan] first described to me this means of moving through the Cosmos," she said. "I couldn't get over what a mythic achievement it seemed to me that it would be to be able to ride the light. What an immaculate way of traveling through the Cosmos."

Sagan championed solar sailing with fellow Society co-founders Louis Friedman and Bruce Murray, who were working on the concept at NASA's Jet Propulsion laboratory in Pasadena, California. There, scientists and engineers hoped to build a kilometers-wide solar sail spacecraft to rendezvous with Halley's Comet when it flew past Earth in 1986. The plan never came to fruition, but the idea lived on in the form of Cosmos 1, the Society’s privately funded solar sail that was ultimately doomed by a rocket failure in 2005.

"A few years after Carl died, I became involved with The Planetary Society, working with Louis Friedman, who really wrote the textbook on solar sailing," Druyan said. "We were working on a proof-of-concept mission. But one thing we didn't think of was making the sail tiny—making the actual craft tiny. And I think that's the inspiring audacity of [Breakthrough Starshot]."

Cosmos 1 had a sail area of 600 square meters, whereas the Alpha Centauri-bound "nanocrafts," as Breakthrough calls them, would have sails with diameters of about four meters. Other solar sail spacecraft that have flown to date include Japan's IKAROS (196 square meters), NASA's Nanosail-D (9.3 square meters), and The Planetary Society's LightSail 1 (32 square meters). If successful, LightSail 2 (also 32 square meters) will be just the second solar sail spacecraft to demonstrate purposeful flight by light—IKAROS, thus far, is the first and only craft to do so.

In terms of sail size, then, Starshot falls somewhere between Nanosail-D and LightSail. But whereas LightSail weighs about 5 kilograms, the Starshot nanocrafts might weigh just a gram—a mass reduction by a factor of 5,000. And the sail would also need to be lighter and more reflective, possibly built from an exotic material like carbon nanotubes rather than Mylar.