Direct Imaging of Extrasolar Planets

Direct imaging of extrasolar planets might be easier than astronomers thought, a new study shows. Evidence for the existence of planets around nearby stars comes mostly in the form of tiny Doppler shifts in the star's spectra as one or more orbiting planets tug on the star. In a few cases the transit of a planet across the face of a star can be detected from a minute dimming of the star's emission. These approaches are indirect. The problem of imaging extrasolar planets directly is that the planet is far outshone by the nearby star. One proposed way of getting around this glare problem is to use nulling interferometry. In ordinary interferometry the light waves from two or more telescopes are added together in such a way that the resulting observation is equivalent to one made with a single telescope with a much wider diameter than any of the component scopes. But instead of maximizing the composite signal from the distant object, it can be minimized (see past Update item). By doing this, a weaker nearby object, like a planet, might suddenly emerge from what had been irrepressible glare.

In a new paper, William Danchi (Goddard Space Flight Center) and his colleagues have performed extensive studies of the interferometry nulling technique, especially the way in which increasing the precision of component detectors increases the degree to which the star's image is truly nulled, the better to see either smaller planets or planets that are closer in toward their parent star. Both the smaller and closer criteria are pertinent when searching for earth-like extrasolar planets. Danchi (301-286-4586) says that the new study shows that with the right configuration of detectors, the spatial resolution of the overall interferometer (which is related to its size) can be less than have been thought, an important consideration for what would be an orbiting space-based observatory. Danchi envisions that a first-round nulling interferometer using two half-meter-sized telescopes separated by a 12-meter boom could observe already discovered extrasolar planets (including spectroscopic studies of atmospheres). With a later, larger version of the nulling interferometer one could hope to search for earthlike planets harboring characteristic molecules such as ozone, and/or oxygen, plus carbon dioxide, water, and methane. Detecting these molecules could help determine the age of the planet and what life processes might be occurring there. (Danchi, Deming, Kuchner, and Seager, Astrophysical Journal Letters, 1 November 2003; preprint astro-ph/0309361)