1. Astronomers can detect extra-solar planets by radial velocity, astrometry, transit method, and optical detection.
2. Astronomers detect extra solar planets by radial velocity which is also known as the Doppler spectroscopy. This is when they detect the movement of a star which is caused by the gravitational pull of another planet. They use the Doppler effect to detect the frequency of waves that are coming to or away from us. When it is moving away, the light of the spectrum is red, and vise-versa it is blue. They then can determine the size and orbit of the planet.
3. About 75 planets have been discovered by radial velocity.
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