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Microlensing by Exoplanets
- University of Arizona Press
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79 Microlensing by Exoplanets B. Scott Gaudi The Ohio State University Gravitational microlensing occurs when a foreground star happens to pass very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the background source star into two images, which are typically unresolved. However, these images of the source are also magnified, by an amount that depends on the angular separation between the lens and source. The relative motion between the lens and source therefore results in a time-variable magnification of the source: a microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived signature of the planet. This chapter provides an introduction to the discovery and characterization of exoplanets with gravitational microlensing. The theoretical foundation of the method is reviewed, focusing in particular on the phenomenology of planetary microlensing perturbations. The strengths and weaknesses of the microlensing technique are discussed, highlighting the fact that it is sensitive to low-mass planetary companions to stars throughout the galactic disk and foreground bulge, and that its sensitivity peaks for planet separations just beyond the snow line. An overview of the practice of microlensing planet searches is given, with a discussion of some of the challenges with detecting and analyzing planetary perturbations. The chapter concludes with a review of the results that have been obtained to date, and a discussion of the near- and long-term prospects for microlensing planet surveys. Ultimately, microlensing is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets except Mercury, as well as to free-floating planets, and will provide a crucial test of planet formation theories by determining the demographics of planets throughout the galaxy. 1. INTRODUCTION Gravitational lensing generally refers to the bending of light rays of a background light source by a foreground mass. Gravitational microlensing, on the other hand, traditionally refers to the special case when multiple images are created but have separations of less than a few milliarcseconds, and hence are unresolved with current capabilities. Although the idea of the gravitational deflection of light by massive bodies well predates the theory of general relativity, and can be traced as far back as Sir Isaac Newton, the concept of gravitational microlensing appears to be attributable to Einstein himself [see Schneider et al. (1992) for a thorough recounting of the history of gravitational lensing]. In 1936 Einstein published a paper in which he derived the equations of microlensing by a foreground star closely aligned to a background star (Einstein, 1936). Indeed, it seems that Einstein had been thinking about this idea as far back as 1912 (Renn et al., 1997), and perhaps had even hoped to use the phenomenon to explain the appearance of Nova Geminorum 1912 (Sauer, 2008). However, by 1936 he had dismissed the practical significance of the microlensing effect, concluding that “there is no great chance of observing this phenomenon” (Einstein, 1936). Indeed, there is no “great chance” of observing gravitational microlensing. The optical depth to gravitational microlensing, i.e., the probability that any given star is being appreciably lensed at any given time, is on the order of 10–6 toward the galactic bulge, and is generally similar or smaller for other lines of sight. (The phenomenon of gravitational lensing of multiply imaged quasars by stars in the foreground galaxy that is creating the multiple images of the quasar is also referred to as microlensing. In this instance, the optical depth to microlensing can be on the order of unity. However, in this chapter we are concerned only with gravitational microlensing of stars within our galaxy or in nearby galaxies, where the optical depths to microlensing are always small.) Thus at least partly due to this low probability, the idea of gravitational microlensing lay mostly dormant for five decades after Einstein’s 1936 paper [with some notable exceptions, e.g., Liebes (1964) and Refsdal (1964)]. It was not until the seminal paper by Paczynski (1986) that the idea of gravitational microlensing was resurrected, and then finally put into practice with the initiation of several observational searches for microlensing events toward the large and small Magellenic clouds and galactic bulge in the early 1990s (Alcock et al., 1993; Aubourg et al., 1993; Udalski et al., 1993). The roster of detected microlensing events now...