NASA plans to launch two multibillion-dollar spacecraft in the next decade to search for Earth-sized planets. The decision, which expands the agency's previous plans for a single mission, was prompted by President George W. Bush's new vision for space exploration.
Scientists so far have detected more than 120 extrasolar planets. But finding and studying large numbers of terrestrial-sized spheres requires a major leap in technology (Science, 2 January, p. 30). Astronomers and engineers in the United States and Europe have been competing fiercely in recent years to develop those technologies, and the European Space Agency (ESA) and NASA both hoped to launch their respective missions, called Darwin and Terrestrial Planet Finder (TPF), before 2020. The president's January speech added new impetus to those plans. In addition to proposing human visits to the moon and Mars, Bush called for greater emphasis on the search for Earth-like worlds around other stars.
NASA is likely to team up with ESA on at least one of the missions. ESA has focused on free-flying interferometers, in which light collected by several telescope mirrors flying in formation is combined to form a single infrared image. The distance between the mirrors allows astronomers to cancel out the glare from the central star and capture light coming from an angle, which could denote a small planet. NASA, meanwhile, has pursued a variety of options, including an optical-light coronagraph, in which an extraordinarily smooth mirror is designed to suppress a star's light by a factor of 10 billion and allow astronomers to concentrate on photons from the zone in which planets might exist.
In a 12 April memo to scientists, TPF project scientist Charles Beichman of the Jet Propulsion Laboratory says that NASA wants to build the smaller coronagraph first, for launch around 2014, followed several years later by a free-flying interferometer. The combination of monitoring both visible and infrared wavelengths, he adds, could lead to a "reliable and robust determination of habitability and the presence of life" on planets beyond Earth's system by providing independent confirmation in some cases. Zlatan Tsvetanov, TPF project scientist at NASA Headquarters, says that pursuing both technologies will enable scientists to develop instruments for subsequent missions that are capable of measuring specific spectral lines of gases, such as oxygen and methane, that could denote life.
Although funding for the projects remains uncertain, scientists are glad that NASA decided to strike while the political iron is hot. "I'm delighted," says Princeton University astrophysicist Marc Kuchner, a member of the TPF science working group.