How do astronomers find planets outside our solar system?
Astronomers have discovered far more planets outside our solar system, known as extrasolar planets, than within it. Astronomers can not only detect a planet's presence, they can deduce a lower limit to its mass and distance from its parent star, and the length of its year.
The most successful search method used to date is called the radial velocity method. As a star is tugged to and fro by a planet's gravitational pull, astronomers measure a slight shift in the frequency of the star’s light.
Astrometry is another detection method. It is sometimes called positional astronomy. Astronomers measure a tiny shift in a star's position on the sky caused by the gravitational pull of a planet. They can use this information to calculate the planet's mass and orbit.
If a planet passes directly between a star and the observer, it blocks out a tiny portion of the star's light. This so-called transit method looks for repeated dips in a star’s light to confirm the presence of an orbiting planet.
A fourth detection method, called gravitational microlensing, comes from one of Einstein's insights in his theory of general relativity: Gravity bends space. When a planet passes in front of a more-distant star, the planet's gravity will behave like a lens to temporarily focus light from the star. This should cause a sharp increase in brightness and a change in the apparent position of the star.
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