Uranus (pronounced either /ˈjuɹənəs/ or /jəˈɹeɪnəs/) is the
seventh planet from the Sun. It is a gas giant, the third largest
by diameter and fourth largest by mass. It is named after Uranus,
the Greek god of the sky, and progenitor of the other gods. Its
symbol is either ♅ (Unicode U+2645, mostly astrological) or (mostly
astronomical).
NASA's Voyager 2 is the only spacecraft to have visited the planet
and no other visits are planned. Launched in 1977, Voyager made its
closest approach to Uranus on January 24, 1986 before continuing on
its journey to Neptune.
In the Chinese, Japanese, Korean, and Vietnamese languages, the
planet's name is literally translated as the sky king star
(V¤¯).
Uranus is composed primarily of rocks and various ices, with only
about 15% hydrogen and a little helium (in contrast to Jupiter and
Saturn which are mostly hydrogen). Uranus (like Neptune) is in many
ways similar to the cores of Jupiter and Saturn minus the massive
liquid metallic hydrogen envelope. It appears that Uranus does not
have a rocky core like Jupiter and Saturn but rather that its
material is more or less uniformly distributed. Uranus' cyan color
is due to the absorption of red light by atmospheric methane.
Surface temperature on Uranus's cloud cover is approximately 55 K
(-218 C or -360 F).
One of the most distinctive features of Uranus is its axial tilt of
ninety-eight degrees. Consequently, for part of its orbit one pole
faces the Sun continually whilst the other pole faces away. At the
other side of Uranus' orbit the orientation of the poles towards
the Sun is reversed. Between these two extremes of its orbit the
Sun rises and sets around the equator normally.
At the time of Voyager 2's passage in 1986, Uranus' south pole was
pointed almost directly at the Sun. Note that the labelling of this
pole as "south" is actually in some dispute. Uranus can either be
described as having an axial tilt of slightly more than 90, or it
can be described as having an axial tilt of slightly less than 90
and rotating in a retrograde direction; these two descriptions are
exactly equivalent as physical descriptions of the planet but
result in different definitions of which pole is the North Pole and
which is the South Pole.
One result of this odd orientation is that the polar regions of
Uranus receive a greater energy input from the Sun than its
equatorial regions. Uranus is nevertheless hotter at its equator
than at its poles, although the underlying mechanism which causes
this is unknown. The reason for Uranus' extreme axial tilt is also
not known. It is speculated that perhaps during the formation of
the planet it collided with an enormous protoplanet, resulting in
the skewed orientation.
It appears that Uranus' extreme axial tilt also results in extreme
seasonal variations in its weather. During the Voyager 2 flyby,
Uranus' banded cloud patterns were extremely bland and faint.
Recent Hubble Space Telescope observations, however, show a more
strongly banded appearance now that the Sun is approaching Uranus'
equator. By 2007 the Sun will be directly over Uranus's
equator.
Uranus' magnetic field is odd in that it is not centered on the
center of the planet and is tilted almost 60 with respect to the
axis of rotation. It is probably generated by motion at relatively
shallow depths within Uranus. Neptune has a similarly displaced
magnetic field, suggesting that this is not necessarily a result of
Uranus' axial tilt. The magnetotail is twisted by the planet's
rotation into a long corkscrew shape behind the planet. The
magnetic field's source is unknown; the electrically conductive,
super-pressurized ocean of water and ammonia once thought to lie
between the core and the atmosphere now appears to be
nonexistent.
