Nulling interferometry: impact of exozodiacal clouds on the performance of future life-finding space missions

Abstract

Context. Earth-sized planets around nearby stars are being detected for the first time by ground-based radial velocity and space-based transit surveys. This milestone is opening the path toward the definition of instruments able to directly detect the light from these planets, with the identification of bio-signatures as one of the main objectives. In that respect, both the European Space Agency (ESA) and the National Aeronautics and Space Administration (NASA) have identified nulling interferometry as one of the most promising techniques. The ability to study distant planets will however depend on the amount of exozodiacal dust in the habitable zone of the target stars. Aims. We assess the impact of exozodiacal clouds on the performance of an infrared nulling interferometer in the Emma X-array configuration. The first part of the study is dedicated to the effect of the disc brightness on the number of targets that can be surveyed and studied by spectroscopy during the mission lifetime. In the second part, we address the impact of asymmetric structures in the discs such as clumps and offset which can potentially mimic the planetary signal. Methods. We use the DarwinSIM software which was designed and validated to study the performance of space-based nulling interferometers. The software has been adapted to handle images of exozodiacal discs and to compute the corresponding demodulated signal. Results. For the nominal mission architecture with 2-m aperture telescopes, centrally symmetric exozodiacal dust discs about 100 times denser than the solar zodiacal cloud can be tolerated in order to survey at least 150 targets during the mission lifetime. Considering modeled resonant structures created by an Earth-like planet orbiting at 1 AU around a Sun-like star, we show that this tolerable dust density goes down to about 15 times the solar zodiacal density for face-on systems and decreases with the disc inclination. Conclusions. Whereas the disc brightness only affects the integration time, the presence of clumps or offset is more problematic and can hamper the planet detection. Based on the worst-case scenario for debris disc structures, the upper limit on the tolerable exozodiacal dust density is approximately 15 times the density of the solar zodiacal cloud. This gives the typical sensitivity that we will need to reach on exozodiacal discs in order to prepare the scientific programme of future Earth-like planet characterisation missions.

An interferometric study of the Fomalhaut inner debris disk

“…Under favorable 100% cloudcover conditions, it would extend from 1.4 to 8.1 AU. At these distances, the level of warm dust emission around Fomalhaut is high and represent therefore a threat for future spectroscopic and direct imaging missions (e.g., Defrère et al 2010;Roberge et al 2012). In turn, the existence of a massive asteroid belt may be an indication that there is no planet in these region as it would have cleared its neighborhood around its orbit.…”

Debris disks as signposts of terrestrial planet formation

“…A major obstacle to the direct imaging of terrestrial exoplanets is the amount of bright dust close to those planets, i.e., their exo-zodiacal dust clouds (Cash 2006;Defrère et al 2010;Noecker & Kuchner 2010). The Solar System's zodiacal dust has been shown to derive almost entirely from Kuiper Belt comets that were scattered by the giant planets into the inner Solar System, where they partially sublimated to produce warm dust before eventually being ejected (Nesvorný et al 2010).…”

The Transit Light Curve of an Exozodiacal Dust Cloud

“…Asymmetric structure in exozodiacal clouds may be a critical source of astrophysical noise for future exo-Earth imaging missions (Defrère et al 2010;Lawson et al 2009). This new method of resonant ring transit detection provides the only current probe by which we can place upper limits on the degree of exozodi structure.…”