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One of the largest planets in the Solar System is also one of the most enigmatic. Uranus is the coldest planet in the Solar System at -224° Celsius. The refrigerated dwarf Pluto is colder, but by consensus it has been relegated to the planetesimal category and no longer qualifies as a fully fledged planet.
No one knows why Uranus is so cold—colder than its more remote neighbor Neptune at -214° Celsius. Neptune lies at a mean distance from the Sun of 4,504,300,000 kilometers and Uranus at 2,870,990,000 kilometers, so it receives four times less solar radiation. However, astronomers think that the low temperature anomaly on Uranus could be due to its rotational vector.
Uranus is tipped on its side by 97° past vertical, prompting scientists to speculate that the gas giant was involved in a massive collision sometime in its early history. It is also thought that colliding with that other large object might have released much of its heat, although the precise mechanism for how a collision can cool down a planet remains a mystery.
Uranus does not radiate a great deal of energy in the infrared wavelengths, so its interior is much colder than that of Jupiter, for instance. Based on observations by the Keck telescope, Jupiter's infrared signature corresponds to an internal temperature of approximately 29727° Celsius, whereas the internal temperature of Uranus is about 4727° Celsius. If the Sun were to disappear tomorrow, Jupiter would still be visible as a faint, reddish glow, but Uranus would be almost invisible against the black background of space.
Like its other gas giant siblings, Uranus has a complex system of rings in orbit with a bevy of satellites. Currently, there are 27 known moons circling the planet, ranging in size from Miranda (470 kilometers in diameter) to Titania (1578 kilometers in diameter). The most unusual thing about the rings and moons is their placement.
Uranus is revolving around the Sun while lying on its side, and its moons and rings are also orbiting along with it in the same orientation. Instead of circling the planet near the plane of the ecliptic, they are inclined from the vertical by 97°, perpendicular to the rest of the Solar System. However, there are several small moons that orbit in the "correct" angle of inclination, crossing from axial pole to axial pole, as well as moving in a retrograde fashion. Why this is so is not known, although it has been suggested that these moons were "captured" by Uranus and did not evolve with it out of a hypothetical nebular cloud.
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