Development of Nulling Interferometry Devices for the Detection and Characterization of Extrasolar Planets

Abstract

Whithin less than 20 years, exoplanet science dramatically changed its status. From a hypothetic subject in between science fiction and astrophysics, it became one of the most popular fields of research of modern astronomy. The reason of this success is twofold. First, finding planets and especially inhabited ones has always been a subject of fascination for researchers but also for the public. Second, answering one of mankind’s biggest questions: “Are we alone in the Universe”would definitely put an end to the last bastion of helio-centrism left standing.

The quest for exoplanets

The beginning of the 90’s has seen the kick-off of exoplanet science with the detection of the first extrasolar planets, first around a pulsar with the radio-telescope of Arecibo (Wolszczan & Frail 1992) and then shortly later around a solar-type main-sequence star (Mayor & Queloz 1995). Since these pioneering discoveries, the number of detections has increased every year to reach today a total of 529 planets detected in 440 different systems (see Figure 1, left). A large majority of these planets have been unveiled by two techniques. First, by radial velocity (RV) measurements showing small Doppler shifts in the stellar lines as the star moves back and forth due to the gravitational pull of its planet. Second, by the photometric measurement of the apparent flux variations due to the transit of the planet in front of the stellar photosphere. These two techniques have the common disadvantage of measuring the effect of the presence of the exoplanet on its environment, and especially its host star, rather than directly collecting its photons. Consequently, they will be hereafter referred as indirect methods in opposition to the direct ones including for example directimaging.

Up to now, the detected sample of exoplanets is dominated by massive giant planets orbiting close to their stars, because the signature of their presence is easy to measure by indirect techniques (see Figure 1, right). These planets are referred to as hot Extra-solar Giant Planets (or hot-EGP) because they can be heated up to about 1000 K due to the proximity of their host star. The existence of EGPs at such small orbital distances was rather unexpected according to the classical planet formation theory, which predicted the formation of giant planets to occur further out in proto-planetary disks through core accretion mechanisms of solid particles (Pollack 1984). Indeed, only in these regions, behind the “snow line” (Sasselov & Lecar 2000), can proto-planetary cores find enough gas to accrete and become a gaseous giant planet. This accretion process, while proceeding slowly in the early phases, can eventually run away when the so-called critical mass is reached (at ∼ 10M⊕). The formation timescale of a gas giant through this mechanism was then estimated around 10 Myr (Pollack et al. 1996). This is dangerously close to the typical lifetime of protoplanetary diks which is believed to be in the 1-10 Myr range (Haisch et al. 2001). Another possible scenario that could explain the formation of EGP within shorter timescales is based on the local gravitational collapse of the protoplanetary disk (Boss 1998).