Caption: An animation of an astronomical transit of a planet across its host star.

This planet seems to be about 2.5 stellar radii from its host star.

In this animation, the planet does NOT seem to be tidally locked to the host star: i.e., it does not always turn the same face to the host star

In reality, a planet in such a close orbit would likely become tidally locked on the time scale of a gigayear at most.????

Unresolved astronomical transits of planets across their host stars allow one to detect the planets and determine radii by observations of the light curves of the stars. Obvious dips of the right nature in the light curves reveal the planets and their radii.

This planet detection method is called the transit method.

Credit/Permission: © User:Silver Spoon, 2012 / Creative Commons CC BY-SA 3.0.
Image link: Wikipedia: File:Astronomical Transit.gif.
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File: Planetary systems file: transit_method.html.