Astronomy

The Wobble Method for Discovering other Worlds



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The wobble method, so called, is a means of detecting the existence of extra solar planets, i.e. any planet that accompanies a star other than Sol (hence solar), our own local star, from an earthbound position. Given the extreme distances between the stars (our nearest stellar neighbour is Alpha Centauri, a triple star system which lies some 1.35 parsecs or 4.4 light years i.e. some 37 or more trillion kilometres away from our earthbound position), even stellar sized bodies appear to us here on earth as just pinpoints of light. When we consider that, in the Solar System, the sun itself constitutes more than 99.9% of the total mass of the system, then the difficulty of locating an extra solar planetary body from here on earth is placed in clear perspective. If stellar sized masses, i.e. suns, are so small from our viewpoint, then, in order to detect less than stellar sized masses some more dynamic methods must be employed.

In the last decade or so, direct observations of extra solar planets have been made, which is not surprising given the improved facilities that are available to investigators. Notwithstanding, most of the extra solar planets that have been discovered to date have been found by means other than direct observation, and it seems likely that for the foreseeable future, such means will be the primary method of discovering such planets.

The wobble method, or Doppler spectroscopy, has proven to be the most successful method for the discovery of planetary bodies around alien suns. This procedure was first suggested by the astronomer Otto Struve (1897-1963) in 1952, and, in hindsight, seems wonderfully simple and straightforward. Since every body is in motion and at the same time exerts some gravitational pull on every other body, there is always some wobble as one body goes along its own path; even the smaller body will exert some pull on the larger one and cause it to deviate from its path (see, for instance, the tidal effects of the moon on the land and water bodies of Earth). Given the proper tools, it would be possible to detect the effect of a single grain of sand from any of earth's beaches on the movement of Jupiter as it makes its journey around the sun. Starting from this premise, any star with a planetary following will exhibit this kind of wobble. Yet, even with less than ideal tools, the effects of giant planets like Jupiter upon their primary ought to be discernable. Simply stated, this is what is meant by the wobble effect when we speak of planetary and extra-planetary masses.

But, how to detect such gravitational interference. The Doppler effect. Almost a century before Otto Sturve proposed the idea, the means had been determined. Christian Doppler (1803-1853), the Austrian physicist, had set out, in 1842, the law of change of wave frequency when a source of vibrations is moving towards or away from the observer of the frequency. Because all bodies are in motion relative to one another, the frequency of any wave, light waves for instance, increases or decreases depending upon whether the observer and the observed are moving closer to or farther away from each other. This simple observation, simple only in retrospect, has been used ever since to measure the radial velocity of the stars. Depending on whether a star is moving away from or towards us on earth, the light which reaches an earthbound observer is shifted toward the red or blue end of the visible spectrum. This particular phenomenon was first observed and recorded by the astronomer, Edwin Hubble (1889-1953), for whom the Hubble Space telescope is named, in 1923.

Using the mathematical formula known as Hubble's constant, the Doppler shift of any particular star can be determined and it is then possible to determine whether or not such a star has a planetary following or not by observing its light as it shifts between the red end and the blue end of the visible spectrum; shifts brought about by the slight deviations from its natural path as it wobbles, be it ever so slightly, as a result of the gravitational impact of its accompanying planet/planets.

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