Placet is a fictional planet that appeared in the Science Fiction story Placet is a Crazy Place by Fredric Brown.
Placet is composed of both ordinary and heavy matter. The core is made of the collapsed, heavy matter, giving the planet a gravity three-fourths that of Earth. Bird-like animals made of heavy matter fly through the earth above the heavy matter core.
Placet revolves faster than the speed of sound in a figure eight pattern around two stars, Argyle I and Argyle II. Argyle I is normal matter and Argyle II is contraterrene or negative matter. Half way between them is a field where light slows to the speed of sound. This field allows Placet to be able to eclipse itself twice at the same time, run into itself every 40 hours and chase itself out of sight.
This field also affects the brains of human beings, resulting in extremely vivid visual hallucinations. The sense of touch is unaffected by the hallucinations.