Extrasolar planet detection. Diagram showing the principle of detecting an extrasolar planet by the planet's gravitational influence on its star. This wobble is directly detected when the orbit (blue circle) of the unseen planet is face-on to Earth. Without any planets, the proper motion (arrow) of the star across the night sky would be a straight (dotted) line. However, a star and its planet will orbit a common centre of gravity, and so the star will trace a wave-like trail with peaks that show the time for half a planetary orbit (left-right). The more massive a planet, the more easily it is detected by this measurement of a star's position in the night sky (astrometry).
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