Letter to the Editor

Strong starlight suppression sufficient to enable direct detection of exoplanets in the habitable zone^*

¹ Jet Propulsion Laboratory, California Institute of Technology, 4800 Oak Grove Drive, Pasadena, California 91109, USA e-mail: stefan.r.martin@jpl.nasa.gov
² Sigma Space Corp., 4801 Forbes Boulevard, Lanham, MD 20706, USA

Received: 26 January 2010
Accepted: 4 February 2010

Abstract

Direct detection of photons from exoplanets in the habitable zone around nearby stars is challenging because of the much higher photon flux and close angular proximity of the star. At mid-infrared wavelengths, around 10 μm, the flux contrast ratio between a sun-like star and an earth-size planet is several million to one, favorable compared to the visible range, while the angular separation of the bodies is less than 1 μradian.
The wavelength range between 7 and 20 μm is worthy of study because it can yield information on the planetary atmospheric composition, particularly as it contains absorption lines for CO₂, ozone and water, which together can be considered a biomarker under some conditions. To achieve observations of the spectrum, strong and stable suppression of the excess starlight is required along with suppression of the local and exo-Zodiacal light, which also have much higher fluxes than the exoplanet signal.
Here we show in the laboratory with nulling interferometry, the suppression of artificial starlight by almost eight orders of magnitude, which is sufficient to detect a planet some three million times fainter than the star. The results show that a combination of starlight suppression techniques enables the detection of medium-sized planets in the habitable zone around nearby stars. Large space telescopes planned for future exoplanet studies will employ these techniques and one additional method, which brings earth-size planets within reach to obtain compelling data on the atmospheres of nearby exoplanets.