Remarkably, there was early debate among physicists as to whether Doppler’s principle could even be applied to light waves (Vogel 1900). An early and elegant demonstration of the Doppler effect applied in astronomy was made by James E. Keeler in his seminal paper “A Spectroscopic Proof of the Meteoritic Constitution of Saturn’s Ring” (Keeler 1895, p. 416). It is obvious now, but at that time it was not known whether the rings were solid or consisted of small particles in orbit around Saturn. He also could foresee the power of the Doppler method: “I have recently obtained a spectroscopic proof of the meteoritic constitution of the ring, which is of interest because it is the first direct proof of the correctness of the accepted hypothesis, and because it illustrates in a very beautiful manner (as I think) the fruitfulness of Doppler’s principle, and the value of the spectroscope as an instrument for the measurement of celestial motion.”
His results, published in the first issue of The Astrophysical Journal (Figure 1.1), clearly show the solid body motion of the planet and the Keplerian motion of the ring. Coincidentally, Keeler was eager to apply Doppler measurements for spectroscopic measurement of the velocity of galaxies (Osterbrock 2002). Unfortunately, he died tragically in 1900 at the young age of 42, and the discovery of the expanding universe had to await the work of Edwin Hubble.
Figure 1.1. A reproduction of the figure from Keeler (1895) showing the Doppler velocity along Saturn using long-slit spectroscopy of Saturn. The spectrum crossing the planet’s disk shows the Doppler motion of solid body rotation. The spectrum from the rings show Doppler motion consistent with Keplerian motion.
Doppler’s principle could indeed be applied to light with fruitful results, and it has produced some of the most fundamental discoveries in astronomy. Some examples include
Hubble’s relationship between the distance of a galaxy and its redshift (velocity). This established the fact that the universe was expanding as one of the fundamental principles of cosmology (Hubble 1929).
The flat rotation curves of galaxies, which was one of the first evidence of dark matter (Rubin et al. 1978).
The rotation of stars and the rotation break at spectral type mid-F stars. This was early evidence that magnetic activity is responsible for stellar angular momentum loss while a star is on the main sequence (Kraft 1967).
The discovery of exoplanets (Mayor & Queloz 1995).
This book is devoted to the last item—the use of stellar radial velocity (RV) measurements for the detection and study of exoplanets. Over the past two decades, the field of exoplanets has developed into one of the most vibrant fields of astrophysics. As of this writing, thousands of planets have been discovered orbiting other stars. This exciting field owes its existence to Doppler’s method, through which the first exoplanets were discovered.
1.2 Early Work on Stellar Radial Velocity Measurements
The first attempts to measure stellar RV measurements date to the 19th century, when Huggins (1868) visually observed the displacement of stellar hydrogen Balmer lines with respect to those from a hydrogen discharge tube. The “founder” of modern stellar RVmeasurement arguably falls on the German astronomer Herman Carl Vogel, who systematically applied photography in stellar RV measurements. He studied astronomy at the German universities of Leipzig and Jena, and his accomplishments were pioneering. Vogel was the first to measure the rotation of the Sun using Doppler shifts of the approaching and receding limbs. His RV measurements first detected an unseen stellar companion to an eclipsing binary (Vogel 1890, p. 27) using the Doppler method, noting that “…before a minimum Algol was moving away from the Sun, and after a minimum it was moving toward it.”
Because the RV precision was of the order of several km s−1, early work focused largely on the study of binary stars. Figure 1.2 shows the velocity curve of the spectroscopic binary star HD 36954 taken with the 36 inch refractor at Lick Observatory in the mid-1930s (Neubauer 1936). The RV measurements have a scatter of 6.9 km s−1, typical of the RV precision of that era.
Figure 1.2. Orbital motion of HD 36954 (curve) calculated from radial velocity measurements taken from 1932–1935 using the 36 inch refractor at Lick Observatory (Neubauer 1936). The scatter about the orbital solution is 6.9 km s−1.
The first astronomer to recognize that stellar RV measurements could be used to detect exoplanets was Otto Struve. He was a Russian-born astronomer who did most of his astronomical work in the United States. Struve served as director of the Yerkes, McDonald, and National Radio Astronomy Observatories. As a director, he could recognize talent, having hired Subrahmanyan Chandrasekhar and Gerhard Herzberg, two future Nobel Prize winners.
Struve was also a visionary. His remarkable paper “Proposal for a Project of High-precision Stellar Radial Velocity Work” (Struve 1952) was the first to propose using Doppler measurements to search for exoplanets. The discovery of 51 Peg b in 1995—a giant planet in a 4.2 day orbit—was foreseen by Struve. In his paper, p. 200, he argued that “we know that stellar companions can exist at very small distances. It is not unreasonable that a planet might exist at a distance of 1/50 of an astronomical unit. Such short-period planets could be detected by precise radial velocity measurements.” His predictive powers did not stop there. He goes on to say that “there would, of course, also be eclipses … and the loss of light in stellar magnitudes is about 0.02.” Struve not only foresaw the possibility of short-period Jupiter-mass planets, but the use of the transit method to characterize the density. His proposal did not result in the “powerful” spectrograph he advocated, which only shows that science has its own “prophets” who are often ignored. The discovery of exoplanets still had to wait another half century.
1.3 Toward Precise Stellar Radial Velocity Measurements
With 150 years of stellar RV measurements and even proposals from the mid-20th century to build spectrographs capable of such precise measurements, why did it take until the end of the 20th century to discover the first exoplanets? The short answer: a lack of precision.
The Doppler shift of a star due to the presence of planetary companions is small. We can use Kepler’s third law to get an estimate of the RV precision needed to detect the reflex motion of star due to the presence of a planetary companion:
P2=4π2a3G(Ms+Mp),(1.1)
where Ms is the mass of the star, Mp is the mass of the planet, P the orbital period, and a the semimajor axis.
