Should we call our planet “the earth,” “earth,” or “Earth”? Generally our planet should be called “Earth.” Like the other planets, it should be capitalized and generally not be preceded by “the.” When earth refers to soil, it should not be capitalized.
We don’t say, the Mars, and certainly not the mars, or the Venus or the Pluto. It is in keeping with the Gaia theory that we think of Earth as having a kind of organic unity and therefore receive a personal name. Earth is a subject.
Technically there was no Greek God name Earth. In Greece, the personification of Earth was Gaia, one of the Greek primordial deities. According to Wikipedia: These deities are a group of gods from which all others descend. They most notably include Uranus (Father Sky) and Gaia (Mother Earth), who preceded the Titans, who themselves preceded the Olympians. In Roman mythology, Tellus or Terra Mater was a goddess of Earth.
Wikipedia gives this etymology of Earth:
The modern English word Earth developed from a wide variety of Middle English forms, which derived from an Old English noun most often spelled eorðe. It has cognates in every Germanic language, and their proto-Germanic root has been reconstructed as erþō. In its earliest appearances, eorðe was already being used to translate the many senses of Latin terra and Greek γῆ (gē): the ground, its soil, dry land, the human world, the surface of the world (including the sea), and the globe itself. As with Terra and Gaia, Earth was a personified goddess in Germanic paganism: the Angles were listed by Tacitus as among the devotees of Nerthus and later Norse mythology included Jörð, a giantess often given as the mother of Thor.
Originally, earth was written in lowercase and, from early Middle English, its definite sense as “the globe” was expressed as the earth. By early Modern English, many nouns were capitalized and the earth became (and often remained) the Earth, particularly when referenced along with other heavenly bodies. More recently, the name is sometimes simply given as Earth, by analogy with the names of the other planets. House styles now vary: Oxford spelling recognizes the lowercase form as the most common, with the capitalized form an acceptable variant. Another convention capitalizes Earth when appearing as a name (e.g. “Earth’s atmosphere”) but writes it in lowercase when preceded by the (e.g. “the atmosphere of the earth”). It almost always appears in lowercase in colloquial expressions such as “what on earth are you doing?”
The Online Etymology Dictionary, gives this derivation:
Old English eorþe ”ground, soil, dirt, dry land; country, district,” also used (along with middangeard) for “the (material) world, the abode of man” (as opposed to the heavens or the underworld), from Proto-Germanic ertho (cognates: Old Frisian erthe ”earth,” Old Saxon ertha, Old Norse jörð, Middle Dutch eerde, Dutch aarde, Old High German erda, German Erde, Gothic airþa), from extended form of PIE root er- (2) “earth, ground” (cognates: Middle Irish -ert ”earth”). The earth considered as a planet was so called from c.1400.