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'''Uranus''' is the seventh [[planet]] from the [[sun]], and the third largest. To superficial appearances, it is featureless and dull. Yet the circumstances of its formation, and the characteristics of the system of moons and rings that encircle it, make it one of the most remarkable planets in all the [[solar system]].

'''Uranus''' is the seventh [[planet]] from the [[Sun]], and the third largest. To superficial appearances, it is featureless and dull. Yet, the circumstances of its formation and the characteristics of the system of moons and rings that encircle it make it one of the most remarkable planets in all the [[Solar System]].

== Discovery ==

== Discovery ==

== Orbital characteristics ==

== Orbital characteristics ==

Uranus's average distance from the sun (semi-major axis) is 19.1 AU, which is close to the 19.6 AU predicted by the [[Titius-Bode law]]. Its sidereal year is 84.02 [[Earth]] years. The orbital excentricity is 0.046, which is nearly circular (though the orbit of Earth is more circular still). Uranus's orbit is very slightly inclined to the plane of the ecliptic; in fact its orbital inclination with respect to the ecliptic is the least of those of all the planets.

Uranus's average distance from the Sun (semi-major axis) is 19.1 AU, which is close to the 19.6 AU predicted by the [[Titius-Bode law]]. Its sidereal year is 84.02 [[Earth]] years. Its orbital eccentricity is 0.046, which means that its orbit is nearly circular (though the orbit of Earth is more circular still). Uranus's orbit is very slightly inclined to the plane of the ecliptic; in fact its orbital inclination with respect to the ecliptic is the least of those of all the planets.

== Rotational characteristics ==

== Rotational characteristics ==

Uranus's sidereal day is 17.24 Earth hours. Uranus's rotation is ''retrograde'' with respect to its orbit around the Sun and the orbits of most of the other planets. Moreover its axis is tilted 97.86° with respect to its orbital plane, so that in a sense the planet is rolling on its side.

Uranus's sidereal day is 17.24 Earth hours. Uranus's rotation is ''retrograde'' with respect to its orbit around the Sun and the orbits of most of the other planets. Moreover its axis is tilted 97.86° with respect to its orbital plane, so that in a sense the planet is rolling on its side.

== Physical characteristics ==

== Physical characteristics ==

=== Magnetosphere ===

=== Magnetosphere ===

Uranus has a magnetic field, whose axis is oriented 60° away from its rotational axis and, furthermore, is displaced from the center of mass by as much as one-third the planet's radius. Comparison of Uranus' present magnetic dipole moment with the expected magnetic dipole moment at creation<ref name=Humphreys/> suggests a magnetic-field half-life of 2215 Julian years, slightly more than 1.5 times the half-life of the magnetic field of [[earth]].

Uranus has a magnetic field whose axis is oriented 60° away from its rotational axis and, furthermore, is displaced from the center of mass by as much as one-third the planet's radius. Comparison of Uranus' present magnetic dipole moment with the expected magnetic dipole moment at creation<ref name=Humphreys/> suggests a magnetic-field half-life of 2215 Julian years, slightly more than 1.5 times the half-life of the magnetic field of [[earth]].

== Ring system ==

== Ring system ==

The [[nebular hypothesis]] would not even have allowed Uranus and its twin, [[Neptune]], to form at its tremendous distance from the [[Sun]] in the time generally allotted. As the journal ''Astronomy'' stated the problem:{{cquote|Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.}} <ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref>

The [[nebular hypothesis]] would not even have allowed Uranus and its twin, [[Neptune]], to form at its tremendous distance from the [[Sun]] in the time generally allotted. As the journal ''Astronomy'' stated the problem:{{cquote|Pssst … astronomers who model the formation of the solar system have kept a dirty little secret: Uranus and Neptune don’t exist. Or at least computer simulations have never explained how planets as big as the two gas giants could form so far from the sun. Bodies orbited so slowly in the outer parts of the sun’s protoplanetary disk that the slow process of gravitational accretion would need more time than the age of the solar system to form bodies with 14.5 and 17.1 times the mass of Earth.}} <ref>R.N., Birth of Uranus and Neptune, ''Astronomy'' '28'(4):30, 2000</ref>

Of course, the time allotted for the formation of the [[solar system]] is generally taken to be the alloted age of the [[Earth]]. The problem is that Uranus and Neptune would require more than twice the supposed age of the earth to accrete their present masses. In theory, the solar system could be older than the earth itself, but not much older. The solar system has other bodies in it that are much "younger." Nor does the above account for the body of evidence that the solar system is in fact quite young.

Of course, the time allotted for the formation of the [[Solar System]] is generally taken to be the alloted age of the [[Earth]]. The problem is that Uranus and Neptune would require more than twice the supposed age of the earth to accrete their present masses. In theory, the Solar System could be older than the Earth itself, but not much older. The Solar System has other bodies in it that are much "younger." Nor does the above account for the body of evidence that the solar system is in fact quite young.

Desch<ref name=Desch>Desch, S. J. "[http://eagle.la.asu.edu/desch/papers/2007/Desch2007.pdf Mass Distribution and Planet Formation in the Solar Nebula]." Arizona State University, September 21, 2007. Accessed April 29, 2008.</ref> recognizes the problem, and supports a radical alteration of the nebula hypothesis that calls for a ''decretion'' disk, not an [[Accretion Disk|accretion disk]]. But even his model requires that Uranus and Neptune ''exchange places'' early in the process, primarily because Neptune, though about 10 AU further out from the Sun than is Uranus, is yet more massive.

The astrophysicist Steven J. Desch<ref name=Desch>Desch, S. J. "[http://eagle.la.asu.edu/desch/papers/2007/Desch2007.pdf Mass Distribution and Planet Formation in the Solar Nebula]." Arizona State University, September 21, 2007. Accessed April 29, 2008.</ref> recognizes the problem, and supports a radical alteration of the nebula hypothesis that calls for a ''decretion'' disk, not an [[Accretion Disk|accretion disk]]. But even his model requires that Uranus and Neptune ''exchange places'' early in the process, primarily because Neptune, though about 10 AU further out from the Sun than is Uranus, is yet more massive.

=== Orientation of the Uranian system ===

=== Orientation of the Uranian system ===

== Satellites ==

== Satellites ==

Uranus has twenty-seven named satellites and a number of concentric rings. The orbits of its satellites are slightly inclined to Uranus's own equator. Remarkably, many of the revolutions of Uranus's moons are retrograde. Uranus's satellites include [[Miranda]], easily the most remarkable natural satellite in all the [[solar system]].

Uranus has twenty-seven named satellites and a number of concentric rings. The orbits of its satellites are slightly inclined to Uranus's own equator. Remarkably, many of the revolutions of Uranus's moons are retrograde. Uranus's satellites include [[Miranda]], easily the most remarkable natural satellite in all the [[Solar System]].

== Missions to Uranus ==

== Missions to Uranus ==

The only deep-space probe to visit Uranus thus far has been [[Voyager 2]], which flew by Uranus on January 24, 1986. Voyager 2 discovered ten of Uranus's twenty-seven satellites, and two previously unknown rings. Voyager 2 then continued onward to [[Neptune]] before heading out of the [[solar system]].

The only deep-space probe to visit Uranus thus far has been [[Voyager 2]], which flew by Uranus on January 24, 1986. Voyager 2 discovered ten of Uranus's twenty-seven satellites, and two previously unknown rings. Voyager 2 then continued onward to [[Neptune]] before heading out of the [[Solar System]].

== Gallery ==

== Gallery ==