Direct imaging of extrasolar planets

(Sometimes called exoplanets) might be easier than astronomers thought, according to a new study. Evidence for the existence of planets around nearby stars is mostly indirect—tiny Doppler shifts in a star’s spectra or a minute dimming of a star’s emission. Direct imaging of an exoplanet is problematic because of the overwhelming brightness of the nearby star. One proposed way of getting around the glare is to use nulling interferometry, which combines the light waves from two or more telescopes so as to minimize the total signal. With this technique, a dim object, like a planet, might suddenly emerge from what had been irrepressible glare. Now, William Danchi (NASA’s Goddard Space Flight Center in Greenbelt, Maryland) and his colleagues have extensively studied the capability of nulling infrared interferometry. They found that, for two reasons, the instrument’s angular resolution can be an order of magnitude better than conventionally assumed. First, the interferometer’s response decreases quadratically inside the null while the number of signal photons increases exponentially as the planet gets closer to the star. An exponential always wins, so the planet’s signal remains strong. Second, the team used the ratio of two IR wavelengths in a way that rendered the observation insensitive to fluctuations in the optical pathlength of the system. The astronomers simulated observations of all known exoplanets and found that several could be directly imaged with even a modest instrument—two 0.5-m telescopes set 12.5 m apart—and spectra could be obtained of their atmospheres. (W. C. Danchi et al., Astrophys. J. Lett. 597 , L57, 2003 http://dx.doi.org/10.1086/379640 .)