Astronomers usually try to gather precious starlight, not block it out. But researchers in Arizona are thrilled with new observations that wiped out most of the light from three stars. The study is the first demonstration of a technique proposed 20 years ago to erase starlight in order to spot planets normally hidden by a star's glare, according to a report in today's Nature. Further progress may help scientists unveil new worlds using ground-based telescopes.
Planets shine by reflecting starlight and by emitting their own warm glows in infrared light. However, stars in planetary systems swamp these feeble rays with light up to a billion times more intense. In 1978, astronomer Ronald Bracewell of Stanford University in California suggested a solution: "nulling interferometry." Light impinging on two adjacent telescopes will cancel out like water waves if special optics focus the light onto a detector after shifting one beam by half a wavelength, Bracewell reasoned, so that the peaks of one wave fill the troughs of the other. That nulls the central star, but leaves the dim light from nearby objects, which travels along slightly different paths.
To test the idea, a team at the University of Arizona, Tucson, used two 1.8-meter mirrors mounted 5 meters apart at the Multiple Mirror Telescope atop nearby Mount Hopkins. Optical components combined the beams with enough precision to erase 95% of the light from three stars. That let the group take the first direct image of a gauzy nebula of dust around Betelgeuse, a giant star in the constellation Orion. Exposing planets from a ground-based telescope would require a system that eliminates 99.99% of the starlight, says graduate student and lead author Philip Hinz. To reach that level, he says, the researchers plan to design adaptive optics using rapidly flexible mirrors, which will correct for the distortion of light in Earth's atmosphere.
"This is a marvelous paper," says Bracewell. "It's an ingenious use of clever optics." Future nulling interferometers in space, such as the proposed Terrestrial Planet Finder that NASA hopes to launch in 15 years, could hunt for planets as small as Earth around other stars. However, Bracewell notes, further proofs of his concept on the ground are essential before such a mission wins funding.