[Intro/abstract?] The general interest in “is there life elsewhere than on Earth?” evolved to “Are there planets capable of supporting life outside our solar system?” In the last fifty years, astronomers have been working on the simpler question: “Are there planets outside of our solar system – so called, extrasolar planets?” A planet would perturb the motion of its parent star, that is, slightly changes the “color” of the star. From the magnitude of the color changed – called the Doppler shift – we now know there are over three hundred large Jupiter-sized extrasolar planets.
The fundamental concept behind this planet searching method is the Doppler effect, which is the change in frequency of a wave, such as a sound wave or light, that is proportional to velocity of the moving source. The best example to demonstrate the Doppler effect is in the situation where a car sounding a horn comes towards and then passes away from an observer. He or she will hear the horn in higher frequency when the car is approaching than when the car is driving away. This amplitude of the shifted frequency is proportional to the velocity of the moving source, that is, the faster the movement, the larger the shift.
The Doppler method for finding extrasolar planets detects the light from the star where its frequency would shift up and down by amount proportional to the planet's mass because the planet pulls the star back and forth. Consider an example where there are two people of different weights, spinning very fast with both of their hands joined. Although the heavier person won't move by much, he or she is still being pulled back and forth by the lighter person. In a star-planet system, a planet would perturb the motion of the star in the same way, and the speed of the moving star is proportional to the planet’s mass. Because of this perturbed motion of the star, the light from the star will be Doppler shifted, where the magnitude of the frequency shift hints the mass of the planet.
With the current observational equipment, the Doppler method only detects planets roughly a thousand times the size of Earth. The accuracy in detecting the motion of the star determines the size of the planets we can find since the star's motion is proportional to the planet's mass. With the current technology, astronomers can only detect the motion of the star down to roughly 1 m/s. For comparison, Jupiter causes our Sun to move 13 m/s, but Earth only causes our Sun to move 0.1 m/s (Radial Velocity Method, 2010). In other words, the detection limit falls in between the size of Earth and Jupiter. Therefore, the majority of the extrasolar planets found by the Doppler method are as big as Jupiter, and none are close to the size of Earth (Search for Extrasolar Planets, 2010).
Astronomers detect the Doppler effect on the light from the motion of the parent stars to search for Jupiter-sized extrasolar planets. The Doppler effect is the change in frequency that is proportional to how fast the source is moving. In a star-planet system, a planet perturbs the motion of the star and causes the observed Doppler shifted light. The amplitude of the frequency change is proportional to the mass of the planet. The Doppler method can only detect Jupiter sized extrasolar planets, with a resolution determined by current technology.