Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. Uranus is similar in composition to Neptune, and both have bulk chemical compositions which differ from that of the larger gas giants Jupiter and Saturn. For this reason, astronomers sometimes place them in a separate category called “ice giant”. Uranus’s atmosphere is similar to Jupiter’s and Saturn’s in that it is composed mostly of hydrogen and helium, with small amounts of methane, water vapor, ammonia, and trace quantities of other hydrocarbons. It has the coldest atmospheric temperature of any planet in the Solar System at −216 °C (−357 °F), and has a complex, layered cloud structure with water thought to make up the lowest clouds and methane thought to make up the uppermost layer of clouds. The interior of Uranus is mainly composed of ices such as water ice, ammonia ice, carbon monoxide ice, methane ice as well as rocks. Trace amounts of hydrogen sulfide are present in Uranus’ lower atmosphere. Its core heat comes from radioactive decay like Jupiter’s and Saturn’s, but it radiates very little heat into space because it has no solid surface on which an internal ocean could form to transfer its heat outwards.
Uranus was discovered on 13 March 1781 by William Herschel. Although it was considerably smaller than Jupiter or Saturn, its unusual color prompted Herschel to name it afterthe classical godofthe sky, Ursa Major (Latin for Great Bear) being one sidereal constellation visible from Europe at that time. Thirteen years later John Couch Adamsand Urbain Le Verrier used Newtonian mechanics toget independent estimatesofMASSES for Neptunethat were within 1% offrom observational data; these results were presented nearly simultaneouslyin 1846/47, although Le Verrier seems nottohave been awareofAdams’ prior work when he submitted his paper independently several months later. Johann Galleanddieterich Burchardtwere ableto locateNeptune asthe predicted positionin 1846;GalleyellowedLe Verrierfor credit during a meetingon 23 September 1846 where they viewed Neptune through d’Arrest’s telescope togetherwith Alexander von Humboldtand WilhelmBeer;BurchardtdiscoveredTriton just over amonthlater on 16 October 1846 while searching for possible satellites near Neptune using d’Arrest’s method ratherthan followinguponthe predictionsmadeby eitherAdamsor Le Verrier .:156–7 As all three men had workedindependentlythey are often jointlycreditedwith findingNeptune .:290–1 Modern historians generallyagreethat Adamsdeservesprimarycredit having initiatedworkwhich led tousingmechanics inthe predictionwhileLe Verrier getspredominantcredit for actuallypredictingits existence& location , with some giving primary credit t o Adams & joint creditsto all three men . :217–222 In 1978 Brian Marsden reanalyzedAdams’ papers& concludedthat he should be given top billing among those who calculationsled directlyto discovery ; since then most textbooks & scientists considerhim to be co-discoverer ratherthan merelypredictor.”) Subsequent observations revealed Triton ‘sanomalously high albedo(reflectivity)and retrograde orbit , which suggested thatit hadbeencapturedby Neptune ‘sgravitationalfield relativelyrecentlyin astronomical historyas colliding bodies do not normallyachieve such orbits unless disturbedafterformation . This hypothesiswas supportedwhen Voyager 2 found evidencefor ongoing geologic activity on Triton whichexplainsits relativelyhigh temperatures(despite being fartherfromthe Sun thansome asteroids ) & seemsto indicatethattidal heatingmay still be occurringnow due its closeorbital relationshipwith Neptune despite their great differenceinsize.” [33):281ff See also:Great Comet C/1996 B2 Hyakutake