Planetary Properties
The image on the right
shows the planet
Uranus, the
first planet discovered in modern times.
It was
found
accidentally by William Herschel while he was
searching
the sky with a telescope in 1781.
It had actually been seen many times before but dismissed as a star.
Mass and Density
The density of Uranus
is about 1.2 grams per cubic centimeter, implying that it is mostly hydrogen, helium, and ices.
This is similar to Jupiter and Saturn, but
Uranus (like Neptune)
contains higher proportions of heavy
elements than Jupiter or Saturn.
The mass is about 15 times that of the Earth, which makes it the fourth most massive
planet. But its radius of about four times that of the Earth makes it the third largest
planet, since Neptune has a smaller radius but larger mass (because Neptune's
density is higher).
The physical properties of Uranus are summarized in this
table
and the orbital properties in
this
table.
Rotation, Rings, and Moons
The average
rotational period is a little over 17 hours, and its orbital period is 84 years,
at a mean separation of 19.2 AU from the Sun.
The rings are less extensive than those of Saturn, and may be rock rather
than ice. There are five large moons and ten small ones.
The rotation axis is unusual in that it lies only 8 degrees
out of the plane of the orbit around the Sun. Thus, at times the rings and the orbits of
the moons appear like a "bulls-eye" when viewed from the Earth.
Animation:
orbit and orientation of Uranus' spin axis