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Radial Velocity

The First Method that Worked

The radial velocity method, also known as Doppler spectroscopy, is the most effective method for locating extrasolar planets with existing technology. Though other approaches hold great promise for the future, the vast majority of Exoplanets discovered so far were detected by this method.

The radial velocity method 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.

The 3.6 meter telescope dome in La Silla, Chile

S. Brunier/ESO

The 3.6 meter telescope dome in La Silla, Chile
This telescope was used to measure the radial velocity of numerous exoplanets discovered to date.

Using highly sensitive spectrographs, planet hunters on Earth can track a star's spectrum, searching for periodic shifts towards the red, blue, and back again. The spectrum appears first slightly blue-shifted, and then slightly red-shifted. If the shifts are regular, repeating themselves at fixed intervals of days, months, or even years, it 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 body orbiting the star, and if it is of a low enough mass it is called a planet.

The success of this method was made possible by the development in recent years of extremely sensitive spectrographs, which can detect even very slight movements of a star. The spectrograph used by Geoff Marcy's team of planet hunters can detect a star moving as slow as 3 meters per second. It is no coincidence that this U.C. Berkeley-based team is responsible for the discovery of over half of the extrasolar planets known to date.

Advantages

It's hard to argue with success. Radial velocity was the first successful method for the detection of exoplanets, and is responsible for identifying hundreds of faraway worlds. It is still the most effective method for detecting exoplanets from Earth.

Drawbacks

It is a fundamental feature of the radial velocity method that it cannot accurately determine the mass of a distant planet, but only provide an estimate of its minimum mass. This is a serious problem for planet-hunters, because mass is the leading criterion for distinguishing between planets and small stars. Some astronomers believe that at least some of the "planets" detected by spectroscopy are not planets at all but very low-mass stars.

The source of this trouble with radial velocity is that the method can only detect the movement of a star towards or away from the Earth. 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.

The radial velocity graph of 51 Pegasi

exoplanets.org

The radial velocity graph of 51 Pegasi
51 Pegasi was the first exoplanet detected and confirmed. The points on the graph indicate actual measurements taken. The sinusoid is the characteristic shape of the radial velocity graph of a star rocking to the tug of an orbiting planet.

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. Now the mass of the suspected planet is directly proportionate to the star's actual wobble. If -- as is usually the case -- only a portion of this wobble is detected, then the measured mass will be lower than the true one and provide only a minimum figure for the planet's mass.

The portion of a distant planet's mass that is detectable is determined by its 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). The mass of the planet as detected from Earth is therefore given by 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 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 too massive to be true planets.

Another drawback of the radial velocity method is that it is most likely to find the types of planets that are the least likely to be hosts to life. Early on, most of the planets detected by spectroscopy were 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 quick and relatively large 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 less likely 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 with this method are very unlikely abodes for life.

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