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Squares May Resolve Exosolar Planets

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Daniel S. Burgess

With one exception, astronomers have observed all of the known exosolar planets indirectly by monitoring the changes they elicit in the radial velocities of their parent stars. NASA has an ambitious plan, the Terrestrial Planet Finder mission, that would use nulling interferometry to collect the light from Earth-like planets 50 light-years away. However, it depends on technologies that have not yet been developed. A new approach based on apodized square apertures promises a simpler alternative.

Computer simulations suggest that apodized square apertures could enable astronomers to observe exosolar planets. Here, the brightness ratio of the star to the planet is 109. The left image exhibits no wavefront aberration. The right features λ/50 error rms, demonstrating that optical quality is key to isolating the light from the target from noise in the system.

As proposed by Peter Nisenson of the Harvard-Smithsonian Center for Astrophysics and Costas Papaliolios of the center and Harvard University, this new technique still would require extremely high quality telescopes equipped with adaptive optics. Nevertheless, the physicists' simulations suggest that it should enable a single 8- to 10-m instrument to observe the planets targeted by Terrestrial Planet Finder, which is envisioned as a fleet of instruments flying in precise formation.

"One nice thing about this approach is that, even with only a 2-m telescope, one could survey a few dozen nearby stars for Earth-like planets and many hundred stars for larger planets," Nisenson said.

The technique, which they reported in the Feb. 20 issue of Astrophysical Journal, relies on two principles: diffraction by a square aperture and apodization. Light diffracts as it passes through an aperture, uniformly in the case of round apertures. Because the visible light coming from a star is nine orders of magnitude more intense than that reflected from its planets, its diffracted light effectively obliterates the planets' signals in a telescope. Imaging in the IR reduces the star-to-planet brightness ratio to 106, the researchers note, but it requires interferometric baselines or telescope diameters 10 times as large.

Hip to be square

A square aperture, however, diffracts most of the light perpendicularly to its sides. Adding an apodizing mask with a peak transmission in the center reduces the intensity of the diffracted light even more. Therefore, by rotating the aperture so that the planets fall in the diagonals to the star, an astronomer should be able to isolate the planetary light and look for indicators of life in their spectra, such as O3.

The researchers have had the apertures made for laboratory testing, but many questions remain to be addressed before apodized square apertures find their way to the Terrestrial Planet Finder. Even if this mission ultimately does not discover life on other worlds, there is important scientific knowledge to be learned.

"Direct detection of the reflected light, even from giant planets, would allow spectral analysis of their atmospheres, determining their chemical composition, which would be very exciting," Nisenson said.

Photonics Spectra
Apr 2001
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