Subsequently, analogues of HD114762b have been found. Part of the skepticism of the planetary nature of the companion was due to the fact that its minimum mass was sufficiently high that it had a nonnegligible (although very small) chance of being a nearly pole-on binary, part was due to the high eccentricity and close period, and part was due to the fact that the definition of a planet at the time was anything with a mass of less than 10 M J. In the title of the article, the companion was referred to as a “probable brown dwarf,” although they also speculated that it might be a giant planet. (1989) announced a companion to the Sun-like (F9V) star HD 114762 with minimum mass of 11 M J and a period of roughly 84 days-that is, similar to the orbital period of Mercury. (2003) confirmed that the signal was, indeed, due to a planetary companion to gamma Cep Ab. While they were hesitant to definitively ascribe their observed Doppler signal with a period of roughly 3 years and an amplitude of roughly 25 m/s to a 2 M J companion with orbital separation of a few AU, subsequent observations by Hatzes et al. (1988) of a planetary companion to gamma Cephei Ab using the radial velocity technique. Perhaps the first suggested detection of a planetary companion that ultimately turned out to be confirmed was the claim by Campbell et al. Considering Jupiter analogues to Sun-like stars, the size ratio is roughly 1 to 10, the mass ratio is roughly 1 to 1000,Īlthough the concept that other stars might host planetary systems like Earth’s is ancient, up to roughly 30 years ago, scientists did not know whether other stars hosted planetary systems. It is useful to recall the relevant orders of magnitude that are involved. This is the primary reason that it was not until nearly the end of the 20th century that the first definitive detections of exoplanets were made. METHODS OF DETECTING AND CHARACTERIZING EXOPLANETS: APPLICATIONS, BIASES, AND LIMITATIONSīy essentially every physical measure, planets are exceptionally diminutive, in particular in comparison to their host stars. The field took another great leap forward with the launch of the National Aeronautics and Space Administration (NASA) mission Kepler ( Borucki et al., 2010), which brought the study of exoplanets into the statistical age. A few notable milestones include the discovery of the first transiting planet, HD 209456b ( Charbonneau et al., 2000 Henry et al., 2000) the discovery of the first exoplanet via transits, OGLE-TR-56b (whose photometric signal was first identified by Udalski et al., 2002, and whose planetary nature was confirmed via radial velocities by Konacki et al., 2003) the discovery of the first exoplanet via microlensing, OGLE 2003-BLG-235/MOA 2003-BLG-53Lb ( Bond et al., 2003) and the discovery of the first directly imaged planetary system around HR 8799 ( Marois et al., 2008). Since the discovery of 51 Peg b, many thousands of exoplanets have been discovered via many different techniques (see Figure 2.1). The discovery of 51 Peg b heralded a general principle that has since held in the exoplanet field-namely, that planetary systems are remarkably diverse, and to “expect the unexpected.” Indeed, this is still the prevailing wisdom, and thus the discovery of 51 Peg b led to the realization that at least some fraction of exoplanets undergo large-scale migration from their birthplaces. This is because the then-popular planet formation model predicted that such planets could not form in situ (e.g., Lin et al., 1996). The discovery of 51 Peg b, which has a minimum mass of roughly 0.5 times the mass of Jupiter (M J) but an orbital period of only about 4 days, surprised many. Although it was not the first detected exoplanet (see Box 2.1), the discovery of a planetary companion to the near solar analogue 51 Pegasi by Mayor and Queloz in 1995 launched the field of exoplanets.
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