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Space Topics: Extrasolar PlanetsDoppler Spectroscopy: The Method that WorksDoppler Spectroscopy, also known as the Radial Velocity method, has proven to be by far the most effective method for locating extrasolar planets. Though other approaches hold great promise for the future, Doppler spectroscopy has proven to be the only effective means of detecting extrasolar planets with currently available technology. With the exception of two planet-bearing pulsars, all of the Extrasolar planets discovered so far were detected by this method. Doppler Spectroscopy relies on the fact that a star does not remain completely stationary when it is orbited by a planet. It moves, ever so slightly, in a small circle or ellipse, responding to the gravitational tug of its smaller companion. When viewed from a distance, these slight movements affect the star's normal light spectrum, or color signature. If the star is moving towards the observer, then its spectrum would appear slightly shifted towards the blue. if it is moving away, it will be shifted towards the red. Suppose now that a highly sensitive spectrograph on Earth detects spectrum
shifts in a star: the spectrum appears first slightly blue-shifted, and then
slightly red-shifted. And suppose, furthermore, that the shifts are periodical,
and repeat themselves at fixed intervals of days, months, or even years. This
means that the star is moving ever so slightly back and forth - towards the
Earth and then away from it in a regular cycle. This, in turn, is almost certainly
caused by a large body orbiting the star, or in other words - a planet. AdvantagesIt's hard to argue with success. Doppler spectroscopy is the only method for detecting planets outside our solar system that has proven feasible and reliable with existing technology. DrawbacksIt is a fundamental feature of the Doppler spectroscopy method that it cannot accurately determine the mass of a distant planet. It can only give an estimate of the planet's minimum mass, while its actual mass may be much larger. This is a serious problem for planet-hunters, because mass is the leading criterion for distinguishing between planets and small stars. Many astronomers believe that at least some of the "planets" detected by spectroscopy are not planets at all but small stars known as "brown dwarfs." The source of this trouble with spectroscopy is that the method can only detect the movement of a star towards or away from the Earth. The mass of the suspected planet is directly proportionate the star's wobble, and is calculated based on its detected wobble. This is not a problem if the orbital plane of the distant planetary system appears "edge-on" when observed from the Earth. In that case, the entire movement of the star will be towards or away from the Earth, and can be detected with a sensitive spectrograph. The mass of the planet, derived from this movement, will in this case be fully accurate. If, however, the orbital plane of the planet is "face on" when observed from the Earth, the entire wobble of the star will be perpendicular to an observer's line of vision. While the star may move significantly within the orbital plane, no part of its movement will be towards or away from the Earth. No spectrum shift will be detected, and the Earth-bound observer will remain ignorant of the presence of a planet orbiting the star. In most cases a distant planet's orbital plane is neither "edge-on" nor "face-on" when observed from the Earth. Most likely it is tilted at some angle to the line of sight, which is usually unknown. This means that a spectrograph would not detect the full movement of the star, but only that component of its wobble that moves it towards the Earth or away from it. The size of this component depends on the distant planet's orbital plane, when observed from Earth. If the angle of inclination from the "face-on" position is "i", then the component which is in line with the Earth is given by Sin(i). Since the orbiting planet's mass is proportionate to the stellar movement, this directly affects astronomers' mass estimate. Instead of calculating the full planetary mass M, they can only derive a figure for M*Sin(i). If "i" is large, i.e. the system is close to an "edge-on" position, then the derived figure is close to the true one. But if "i" is small, and the system is, in fact, close to a "face-on" position, then the true mass of the "planet" is much larger than the estimate. Only very rarely do astronomers know a planetary system's true angle of inclination. This leaves open the possibility that at least some of the objects detected are not planets but small stars. Another unfortunate feature of the spectroscopic method is that it is most likely to find the types of planets that are the least likely to be hosts to life. Most of the planets detected by spectroscopy are of a type known among scientists as "hot Jupiters." These are giant planets composed mostly of gas, similar to our neighbor, Jupiter, but orbiting at dizzying speeds at a very short distance from their star. Their size, short periods, and close proximity to their star ensures that they produce the sharp stellar wobbles that are most easily detected by spectroscopy. Cooler planets orbiting further away produce more moderate wobbles in their home star, and take years to complete each orbit, factors which make them much harder to detect with spectroscopy. But while "hot Jupiters" are relatively easy to find, they are unlikely homes to any form of life as we know it. Even worse, their presence at the center of a planetary system makes it unlikely that more Earth-like planets had survived in their neighborhood. In other words, while the discoveries made with spectroscopy established the presence and prevalence of planets outside our Solar System, most of the systems detected are of only limited use in our search for life in the universe. --Amir Alexander |
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