Nulling Interferometry to Study Extra-Solar Planets

Roger Angel and Nick Woolf
Center for Astronomical Adaptive Optics,
Steward Observatory, Univ. of Arizona
Tucson AZ

Interferometric techniques offer two advantages for the detection and analysis of thermal radiation from planets: destructive interference to strongly suppress the stellar emission, and the possibility of high resolution imaging to resolve planets and distinguish them from dust emission. We describe new interferometric configurations of co-linear elements in which the conflicting requirements for these goals are reconciled. Very strong, broad interference nulls are realized, so high resolution fringes can be used while maintaining good suppression of the stellar disc. Cross correlation techniques analogous to aperture synthesis can recover true images.

When operated 5 AU from the Sun to escape background emission from local zodiacal dust, the interferometer's sensitivity will be fundamentally limited by noise in the photon flux from warm zodiacal dust in the planetary system under observation. In order to design for adequate sensitivity, the 10 micron emission from such dust could be determined early on by the Large Binocular Telescope, configured as a Bracewell interferometer, and using adaptive optics correction at the secondaries for low thermal background.

If stars at 10 pc distance have zodiacal clouds like our own, a 50 m long space interferometer with four 1 m elements should see individual planets like the Earth in images taken over 10 hours. Simultaneous infrared spectra of planets like Earth, Venus, Jupiter and Saturn could be obtained during a 3 month integration, with the sensitivity to detect carbon dioxide, water and ozone at the levels seen in Earth's spectrum.