Colloquium: Stars, Planets, and Metals
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REVIEWS OF MODERN PHYSICS, VOLUME 75, JANUARY 2003 Colloquium: Stars, planets, and metals Guillermo Gonzalez* Iowa State University, Department of Physics and Astronomy, Ames, Iowa 50011 (Published 22 January 2003) The discovery in 1995 of the first planet orbiting another Sun-like star stimulated renewed interest in planet formation and evolution processes. A number of trends among the properties of the planets have become evident in the years since. An interesting pattern began to emerge in 1997—stars hosting planets tend to be more metal rich (i.e., have more abundant elements with ZϾ2) than the average nearby star. Other, more subtle, trends are beginning to appear as the sample size continues to grow; for example, the masses of stars hosting planets are found to correlate with their metallicities. The author reviews the state of our knowledge concerning the observed trends, their possible causes, and their possible implications for astrophysics and astrobiology. CONTENTS B. Recommendations for future observational research 118 Acknowledgments 118 I. Introduction 101 References 118 II. Properties of Stars with Planets 102 A. The sample 102 B. Spectroscopic observations 103 C. Spectroscopic analysis methods 103 D. Photometric analysis methods 104 I. INTRODUCTION E. Results 104 1. Spectroscopic 104 The announcement of the first candidate planet orbit- 2. Photometric 104 ing another Sun-like star by Mayor and Queloz (1995) III. Comparison with Field Stars 106 opened up a new field of empirical study for astrono- A. Control samples 106 mers. The low-mass object orbiting 51 Pegasi (51 Peg), B. Comparison of metallicity distributions 107 with a minimum mass a little less than the mass of Jupi- 1. Spectroscopic 107 ter and an orbital radius of 0.05 astronomical units 2. Photometric 107 (AU’s), turned out to contradict their expectations.1 As- C. Predictions: Confirmed and not 107 tronomers had adopted the Solar System as the model D. Biases 108 for all planetary systems, having assumed that gas giant E. Cautions 109 planets would be found at least 5 AU’s from their Sun- F. Incidence of giant planets 109 like host stars. They had also expected that the planets IV. Chemical Abundance Anomalies Among Stars with Planets 110 would be found in nearly circular orbits. Both assump- A. Light elements 110 tions turned out to be wrong. 1. Lithium 110 These discoveries were made possible by great ad- 2. Beryllium 110 vances in stellar spectroscopic Doppler techniques be- B. Other elements 110 ginning in the 1980s. Instrumental in that revolution C. Trends with condensation temperature 110 were the increased use of echelle spectrographs with D. Common proper-motion pairs 111 large telescopes, which allow large-wavelength coverage V. Galactic Kinematics 111 and high spectral resolution, the development of large VI. Causes of Trends Among Stars with Planets 112 charge-couple devices (CCD’s), and the adoption of an A. Introduction 112 iodine absorption cell as the velocity reference. A giant 1. Primordial 112 planet induces typical velocity amplitude in its host star 2. Self-enrichment 112 3. Migration 114 of tens of meters per second relative to the center of B. Evaluation 114 mass of the star-planet system. Doppler velocity preci- VII. The Solar System 115 sion of 1 to 2 meters per second has been reported, but VIII. Implications and Future Research 116 3 to 5 meters per second is more typical. Achieving a A. Present research 116 Doppler precision around 1000 times better than the 1. Learning about planet formation and evolution 116 2. Distinguishing planets from brown dwarfs 117 1 3. Implications for astrophysics 117 For the sake of historical completeness, it is important to 4. Implications for astrobiology 117 note that the first substellar-mass object was found around the star HD 114762. Latham et al. (1989) reported its minimum mass as 11 times that of Jupiter and suggested it was probably a brown dwarf but possibly a giant planet. The true nature of *Electronic address: [email protected] this object is still an unsettled issue. 0034-6861/2003/75(1)/101(20)/$35.00 101 ©2003 The American Physical Society 102 Guillermo Gonzalez: Stars, planets, and metals typical stellar linewidths is possible because (1) the metallicity3 of the host star and the presence of giant wavelength coverage is large (ϳ1000 Å), and includes planets. thousands of absorption lines, (2) the signal-to-noise The first evidence purporting to link the metallicities (S/N) ratios are high (ϳ300), and variations in the point of the SWP’s with the presence of planets was published spread function of the spectrograph across a given spec- by Gonzalez (1997). Only six SWP’s with accurate me- tallicities were known at the time, but all were found to tral order are corrected by the software. have solar metallicity levels or greater; one of them, 1 Of course, a number of phenomena can induce appar- 55 Cnc, was known to be one of the most metal-rich ent Doppler variations in the atmosphere of a star. stars in the solar neighborhood. About the same time, These include short- and long-term variations from star Fuhrmann et al. (1997) confirmed the high metallicity of spots, pulsations, and planets. Indeed, for about two 51 Peg. The following year Gonzalez (1998a) and Fuhr- years following its announcement, a debate raged mann et al. (1998) published the results of spectroscopic around the source of the Doppler variations in 51 analyses of several more SWP’s and continued to con- Peg. Gray and Hatzes (1997) cited spectral line profile firm the high mean metallicity of the group. The close variations in their spectra of 51 Peg as evidence for non- association between the super-metal-rich class of stars radial pulsations in its atmosphere. These claims were (discussed in the literature since the 1960s) and the later disproven by Hatzes et al. (1998). This was a worth- SWP’s was also noted at the time. These types of studies while lesson. The interpretation of periodic low- have continued until the present (e.g., Gonzalez, Laws, amplitude Doppler variations around a given star as due et al., 2001; Santos et al., 2001). Although spectroscopic to the presence of a planet should be considered as pre- analyses of SWP’s have not kept pace with the rapid rate liminary until other data are obtained to eliminate alter- of planet discoveries, the sample size is now sufficient for us to begin to look for trends linking the properties nate explanations; in particlar, high-precision photomet- of the giant planets and those of their host stars. In this ric and ultraviolet Ca II emission data can be used to Colloquium, I shall review the current state of our test for pulsations and star spots, respectively. Recently, knowledge on this topic. the Doppler variations of HD 192263, previously as- cribed to the presence of a planet, were shown from II. PROPERTIES OF STARS WITH PLANETS photometry to be due to spots (Henry, Donahue, and Baliunas, 2002). A. The sample More direct methods of planet detection could even- tually move planets from the ‘‘candidate’’ column to the At the time of this writing (spring, 2002), 77 planets 4 ‘‘secure’’ column on our scorecards. One such method have been reported orbiting 69 Sun-like stars. All the would involve resolving the reflex motion of a star on discoveries to date have been accomplished with the the sky with high-precision astrometry (something that Doppler technique, whereby the reflex motion of the has yet to be achieved with any planet candidates). An- host star about the center of mass is measured over at other method would involve the detection of transits of least one orbital period of the most prominent planet. The survey stars are selected from the brightest mid-F to a planet in front of its host star via high-precision pho- M spectral type main-sequence stars5 in the solar neigh- tometry. Charbonneau et al. (2000) employed this method to confirm the planetary nature of the object in orbit around HD 209458, and the OGLE III photomet- 3 ric survey towards the Galactic bulge has recently pro- Throughout this paper the term ‘‘metallicity’’ refers to the abundances (by number) of elements heavier than He in a star duced many planet transit candidates.2 (sometimes Li is not grouped with the metals). Today, astrono- Much theoretical work has also been spurred by the mers term these elements metals because they contribute a discoveries of planets around Sun-like stars. Some of large fraction of the electrons in a stellar interior, and the met- this work has focused on the formation and dynamical als were produced only after star formation began in the uni- evolution of the planets to their present configurations verse. Historically, though, the word was chosen because the [see the review by Nelson (2001)]. Other work has fo- strong lines in most stellar spectra were known to be due to cused on trying to model the interiors and atmospheres metals, such as Ca, Na, and Fe, and astronomers of the early of substellar objects [see the review by Burrows et al. twentieth century were confident that the mix elements in the stars would be the same as in the Earth, where Fe, Ni, and Mg (2001)]. are very abundant. Unless otherwise indicated, when referring Early on, far less attention was given to another to the metallicity of a star, the relative abundances of the met- anomalous property of the 51 Peg system: the relatively als are assumed to scale with respect to the Solar System abun- high metallicity of the host star. Intrigued by this dance pattern, as determined from meteorites and the solar anomaly, the present author started a long-term project spectrum.