M-155 was a planet in the Kapeshet system, in Sector Epsilon Z-3. It was discovered by Commander Spock of the USS Enterprise on stardate 1305.4, and given its designation per the Murasaki Index. It was quickly nicknamed "The Planet That Wasn't There" due to its mysterious disappearance and reappearance on ship's sensors. It was later discovered, after a landing party beamed down and also disappeared, that the planet was the subject of time-manipulation experimentation by Parneb. (TOS novel: Strangers from the Sky)