Over 300 extrasolar planets have been found since 1992, showing that planetary systems are common and exhibit an outstanding variety of characteristics. As the number of detections grows and as models of planet formation progress to account for the existence of these new worlds, statistical studies and confrontations of observation with theory allow to progressively unravel the key processes underlying planet formation. In this chapter we review the dominant contribution of Doppler spectroscopy to the present discoveries and to our general understanding of planetary systems. We also emphasize the synergy of Doppler spectroscopy and transit photometry in characterizing the physical properties of transiting extrasolar planets. As we will see, Doppler spectroscopy has not reached its limits yet and it will undoubtly play a leading role in the detection and characterization of the first Earth-mass planets.
Deep Dive into Detection and Characterization of Extrasolar Planets through Doppler Spectroscopy.
Over 300 extrasolar planets have been found since 1992, showing that planetary systems are common and exhibit an outstanding variety of characteristics. As the number of detections grows and as models of planet formation progress to account for the existence of these new worlds, statistical studies and confrontations of observation with theory allow to progressively unravel the key processes underlying planet formation. In this chapter we review the dominant contribution of Doppler spectroscopy to the present discoveries and to our general understanding of planetary systems. We also emphasize the synergy of Doppler spectroscopy and transit photometry in characterizing the physical properties of transiting extrasolar planets. As we will see, Doppler spectroscopy has not reached its limits yet and it will undoubtly play a leading role in the detection and characterization of the first Earth-mass planets.
The question of the existence of other worlds has been present in human history for millennia but it is only recently that scientific evidence has confirmed what many had anticipated: planets do exist and are common outside the Solar System. The first robust detection of another planetary system came in 1992 with the discovery of two terrestrial-mass planets orbiting the pulsar PSR 1257+12 (Wolszczan & Frail 1992). Interestingly, this discovery did not receive all the attention that could have been expected, probably because these two planets orbit a "dead star" very different from the Sun and much less likely to host life in its vicinity. From this anthropocentric perspective, the major milestone in the search for extrasolar planets was the discovery in 1995 of 51 Peg b, the first extrasolar planet found to orbit a Sun-like star (Mayor & Queloz 1995). Although of Jovian nature, 51 Peg b orbits at 0.052 AU from its parent star, a striking characteristic when compared to the giant planets in the Solar System, which all orbit beyond 5 AU. This proximity has been a major surprise and a serious challenge to planet formation theories.
Thirteen years after the discovery of 51 Peg b, over 300 extrasolar planets have been detected, including many fascinating systems1 . Five different techniques have contributed to these discoveries: pulsar timing (4 planets detected; e.g. Wolszczan 1997), Doppler spectroscopy (292 planets; e.g. Udry et al. 2007), photometric transits (52 planets; e.g. Charbonneau et al. 2007), microlensing (8 planets; e.g. Gaudi 2007), and direct imaging (4 objects with a mass possibly below 20 M Jup ; e.g. Beuzit et al. 2007). These observational techniques have considerably improved in recent years and Doppler spectroscopy, which has contributed the bulk of the discoveries so far, now allows the detection of planets with half the mass of Uranus.
Likewise, since the discovery of 51 Peg b, planet formation theory has made rapid progress. To overcome the challenge posed by the unexpected properties of the newly found planets, two different formation models have been proposed: core accretion and disk instability. According to the core accretion model, dust grains coagulate to form planetesimals, which then accumulate to build up planetary cores. Planetary cores reaching the critical mass of 5-15 M ⊕ before the dissipation of the gaseous protoplanetary disk subsequently accrete significant amounts of nebular gas and become giant planets (e.g. Lissauer & Stevenson 2007). The remaining solid cores merge through giant impacts to form terrestrial planets (e.g. Nagasawa et al. 2007). In the alternative disk instability model, giant planets form by direct fragmentation of the protoplanetary disk (e.g. Durisen et al. 2007). Quantitative predictions based on the disk instability scenario are still sparse because the simulations are computationally challenging and involve complex physics. In contrast, core accretion is now mature enough to allow for detailed calculations, and explicit comparisons with the observed population of extrasolar planets are possible.
In this chapter we review the dominant contribution of Doppler spectroscopy to planet discoveries and to our general understanding of planetary systems. In Sect. 2 we describe the Doppler technique itself. Then, we present the observational results and we discuss their interpretation within the current theoretical framework. In Sect. 3 we consider the results on giant planets for which an extended statistics is available. In Sect. 4 we present recent results on low-mass planets of Neptune-and Earth-type, and we discuss the present limitations on Doppler precision. We end this review by describing in Sect. 5 the role played by Doppler spectroscopy in the characterization of transiting planets. All these achievements and results are summarized in Sect. 6, where we also outline future perspectives.
2 Doppler spectroscopy
Doppler spectroscopy is an indirect detection method which uses the starlight to measure the gravitational influence of a planet on its host star. Specifically, Doppler spectroscopy is based on the following key observations:
In a planetary system, the star and the planet orbit their common barycenter according to Newton’s law of gravitation and to the laws of motion. The two barycentric and the relative orbits have the same periods and eccentricities, but semimajor axes in the proportions a ⋆ : a p : a = m p : m ⋆ : (m ⋆ + m p ), where m ⋆ is the mass of the parent star and m p the mass of the planet. The three orbits are coplanar and the orientations of the two barycentric orbits differ by 180 • within that plane.
According to the Doppler-Fizeau effect (hereafter simply the Doppler effect), the light emitted by a source approaching (receeding from) the observer is shifted towards shorter (longer) wavelengths. In its simplest relativist form, the Doppler formula writes
where z is the so-called redshift, λ and λ 0 are the obse
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