New images yield clues to seasons of Uranus
Near-infrared images from the Keck II telescope show the planet Uranus in 2005 (left), with the rings at an angle of 8 degrees, and at equinox in 2007 (right pair), with the planets ring system edge-on. In all images, the south pole is at the left and the equator is directly below the rings. Uranus, which has an 84-year orbit around the sun, has seasons that last twenty-one years. With the aid of new imaging technologies and telescopes, scientists had their best chance to observe the change of seasons on the distant planet and to look for seasonal effects on some of the solar systems most mysterious weather features.

With an 84-year orbit around the sun, it isn't often that planetary scientists have an opportunity to observe the change of seasons on Uranus, a planet some 19 times farther from the sun than the Earth.
But in 2007, the planet reached equinox, the point in time where the sun is directly over the planet's equator and what little sunlight the planet gets is distributed evenly over its northern and southern hemispheres, giving scientists their best opportunity to probe the seasonal dynamics of the ringed planet.
Speaking in Ithaca, N.Y., today (Oct. 13) at a meeting of the American Astronomical Society's Division for Planetary Sciences, a team led by University of Wisconsin-Madison researcher Lawrence Sromovsky shared crisp new Keck II telescope images of the planet as it changed seasons.

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Title: Constraints to Uranus' Great Collision. IV. The Origin of Prospero
Authors: Gabriela Parisi (IAR-LaPlata), Giovanni Carraro (ESO-Santiago), Michele Maris (OATS), Adrian Brunini (LaPlata)

It is widely accepted that the large obliquity of Uranus is the result of a great tangential collision (GC) with an Earth size proto-planet at the end of the accretion. We attempt to constraint the GC scenario as the cause of Uranus' obliquity as well as on the mechanisms able to give origin to the Uranian irregulars. Different capture mechanisms for irregulars operate at different stages on the giant planets formation process. The mechanisms able to capture the Uranian irregulars before and after the GC are analysed. Assuming that they were captured before the GC, we calculate the orbital transfer of the nine irregulars by the impulse imparted by the GC. If their orbital transfer results dynamically implausible, they should have originated after the GC. We investigate and discuss the dissipative mechanisms able to operate later. In particular Prospero could not exist at the time of the GC. Different capture mechanisms for Prospero after the GC are investigated. Gas drag by Uranus' envelope and pull-down capture are not plausible mechanisms. Capture of Prospero through a collisionless interaction seems to be difficult. The GC itself provides a mechanism of permanent capture. However, the capture of Prospero by the GC is a low probable event. Catastrophic collisions could be a possible mechanism for the birth of Prospero and the other irregulars after the GC. Orbital and physical clustering should then be expected. Either Prospero had to originate after the GC or the GC did not occur. In the former case, the mechanism for the origin of Prospero after the GC remains an open question. In the latter case, another theory to account for Uranus' obliquity and the formation of the Uranian regular satellites on the equatorial plane of the planet would be needed.

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Astronomers have captured remarkable new images of the rings of Uranus.
The rings are currently edge-on to Earth, in an event that only happens every 42 years.
A team, led by Imke de Pater from University of California, Berkeley, US, has analysed the rings' structure, with some surprising results.

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This series of images from NASA's Hubble Space Telescope shows how the ring system around the distant planet Uranus appears at ever more oblique (shallower) tilts as viewed from Earth - culminating in the rings being seen edge-on in three observing opportunities in 2007. The best of these events appears in the far right image taken with Hubble's Wide Field Planetary Camera 2 on August 14, 2007.
The edge-on rings appear as two spikes above and below the planet. The rings cannot be seen running fully across the face of the planet because the bright glare of the planet has been blocked out in the Hubble photo (a small amount of residual glare appears as a fan-shaped image artefact). A much shorter colour exposure of the planet has been photo-composited to show its size and position relative to the ring plane.
Earthbound astronomers only see the rings' edge every 42 years as the planet follows a leisurely 84-year orbit about the Sun. However, the last time the rings were tilted edge-on to Earth astronomers didn't even know they existed.

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Credit: NASA, ESA, and M. Showalter (SETI Institute)