M級行星又稱類地行星， 是行星等級劃分的。M 級行星出現在恆星的 "可居住區", 直徑通常在 10,000~15,000 公里大氣中含有氧及氮, 充滿水及生物但若地表 97% 以上皆被水掩蓋, 則歸類為 N 級行星。典型的 M 行星包括地球，Cardassia and vulcan。
英文原文：
Class M (Terrestrial):
Class M planets are found in a star's "habitable zone" and are usually
between 10,000 and 15,000 kilometers in diameter. Their atmospheres contain
oxygen and nitrogen. Water and life-forms are abundant. If over 97% of the
surface is covered by water, then the planet is designated a Class N planet.
Examples of Class M planets include Earth, Cardassia and Vulcan.
