Period 6 element
Platinum is a chemical element with the chemical symbol Pt and an atomic number of 78.
Planet symbols
Earth is not one of the classical planets (the word "planet" by definition describing "wandering stars" as seen from Earth's surface). 
Its status as planet is a consequence of the development of heliocentrism.
Nevertheless, there are ancient symbols for Earth, notably a cross representing the four cardinal directions, as a cross in a circle also interpreted as a globe with equator and a meridian;
The "Earth" symbol is encoded by Unicode at U+1F728 (; alternative characters with similar shape are: U+2295 ⊕ CIRCLED PLUS; U+2A01 ⨁ N-ARY CIRCLED PLUS OPERATOR).
Alternatively, there is the globus cruciger (U+2641 ), now most commonly used as planetary symbol.
The "globus cruciger" symbol is also used as an alchemical symbol of antimony.
Uranus (U+26E2 ), also U+2645 , a globe surmounted by the letter "H" for Herschel):
The symbols for Uranus were created shortly after its discovery in 1781. One symbol, , invented by J. G. Köhler and refined by Bode, was intended to represent the newly discovered metal platinum; since platinum, commonly called white gold, was found by chemists mixed with iron, the symbol for platinum combines the alchemical symbols for iron, ♂, and gold, ☉. This symbol also combines the symbols of Mars (♂) and the Sun (☉) because in Greek Mythology, Uranus represented heaven, and represents the combined power of Mars' spear and the Sun. 
Another symbol, , was suggested by Lalande in 1784. In a letter to Herschel, Lalande described it as "un globe surmonté par la première lettre de votre nom" ("a globe surmounted by the first letter of your name").