Interferometer Lights Path in Search for Distant Planets

Richard Gaughan

Scientists at the University of Arizona's Steward Observatory have demonstrated principles that could allow the direct detection of Earth-like planets around distant stars.

The 10-µm emission of a gas cloud around Betelgeuse becomes visible when light from the central star is nulled (left). If the light from Betelgeuse were not interferometrically canceled, a bright point source would swamp the image (right).

The work, initially reported in the September 1998 issue of Nature, was done with two 1.83-m telescopes of the Multiple Mirror Telescope (MMT). By adjusting the relative optical path of light gathered by the two telescopes, the astronomers were able to destructively interfere the light from a distant star while retaining the light from the surrounding space. With the images being collected at 10 µm, visible point sources in the circumstellar region could indicate planets with a temperature range not too far outside our own. The Steward Observatory work was not designed to detect extrasolar planets but to demonstrate the feasibility of a detection technique first proposed by Ronald Bracewell 20 years ago.

"The MMT was the only telescope with multiple apertures on the same structure, a configuration that allows us to honestly image planets around other stars," said Roger Angel, professor of astronomy at the University of Arizona and a collaborator in this work.

The mirrors and paths in the telescope were all fixed; there was no wavefront distortion compensation. Light from each telescope was routed along the fixed separating structure to a zinc selenide beamsplitter that combined the beams and delivered them to a 128 × 128 arsenic-doped silicon blocked impurity band (BIB) array. The astronomers adjusted the position of the beamsplitter until the stellar image flickered, indicating they were near the null. They then continuously acquired images at 20 to 30 Hz. Later examination of the images showed that when atmospheric conditions were just right, the image of the central star was blanked out, leaving a glow from warm dust in the circumstellar environment.

The Multiple Mirror Telescope is now dismantled, but the Large Binocular Telescope (LBT), scheduled for operation in 2003, will continue this work. With two 8.4-m telescopes on 14-m centers, and adaptive optical control of both wavefront and relative path length, the Large Binocular Telescope should push beyond the Multiple Mirror Telescope sensitivity of one part in 20 to one in 10,000.

"That sensitivity will enable us to characterize the zodiacal dust around a couple dozen good candidate stars within a 30-light-year range," Angel said. "The ground-based measurements at LBT will be critical because the photon noise of that dust may fundamentally limit the acceptable targets of eventual space-based nulling telescopes."