Solar System -


Uranus revolves around the Sun once every 84 Earth years at an average distance of about 1,780 million miles, which is a little over 19 times the distance of the Earth to the sun.  While it is the third largest planet, after Jupiter and Saturn, it is actually lighter than slightly smaller Neptune.  Discovered by Sir William Herschel in 1781, it was the first ever planet discovered with a telescope.  It has a ring system and a strong magnetic field.  It is somewhat oblate, with an Equatorial  diameter of approximately 31,950 miles and a polar diameter of around 31,216 miles, giving it a volume ~63 times that of Earth, but its mass is only ~14.5 times as much. 

Uniquely among the planets, Uranus' axis of rotation is tilted sideways so that its poles are where Earth, for example, has its equator.  Thus, at times, like in 2007 and 2008, the rings are seen from Earth edge-on, virtually disappearing, while at other times, they are seen face on.  Uranus has a system of 13 known rings, and 27 known moons. 

A central, rocky core probably about half the mass of the Earth taking up about 20% of the radius.  The pressure towards the center is enough to produce temperatures of around 5,000K. 
An fluid mantle about 13.5 Earth masses.  This makes up the bulk of the planet, and is a hot and dense fluid comprising mainly water and ammonia, with a little methane.  
The gaseous upper atmosphere weighing about 0.5 an Earth mass and, again, occupying about 20 percent of Uranus' radius, consists mainly of hydrogen (about 83%) and helium (about 15%), with a little methane (about 2%) and traces of hydrogen.  Here is a detailed discussion of Uranus' atmosphere
While Jupiter and Saturn are predominantly gas giants, based on their different composition, Uranus and Neptune are really ice giants.  The other giant planets all radiate considerably more heat than they receive from the Sun.  Uniquely, Uranus radiates almost exactly the same amount as it receives, meaning it has very low internal temperature.  Hypotheses to explain the disparity include loosing much of its heat during the impact that caused its axis to tilt so severely.  Another possibility is that there are layers in the atmosphere that inhibit the release of heat.  Whatever the cause, Uranus is the coldest planet in our Solar System. 


Uranus internal structure may be summarised as: