Uranus is an oddball among the Solar System\u2019s planets. While most planets\u2019 axis of rotation is perpendicular to their orbital plane, Uranus has an extreme tilt angle of 98 degrees. It\u2019s flopped over on its side, likely from an ancient collision. It also has a retrograde orbit, opposite of the other planets.<\/p>\n
The ice giant also has an unusual relationship with the Sun that sets it apart from other planets. <\/p>\n
<\/p>\n Uranus\u2019 uniqueness extends to its upper atmosphere, called the thermosphere-corona. That region\u2019s temperature is above 500 Celsius, and the heat sources responsible have puzzled astronomers. The corona extends as far as 50,000 km above the surface, which also sets it apart from other planets. Even weirder, its temperature is dropping.<\/p>\n When Voyager 2 flew past Uranus in 1986, it measured the thermosphere\u2019s temperature. In the intervening decades, telescopes have continuously measured Uranus\u2019s temperature. All these measurements show that the planet\u2019s upper atmosphere is cooling and that the temperature has halved. None of the other planets experienced the same changes.<\/p>\n Scientists know that Uranus\u2019 thermosphere is a tenuous layer. It has an embedded ionosphere, and it helps astronomers measure the thermosphere\u2019s temperature. It\u2019s a layer of ions that separates the lower atmosphere from the planet\u2019s magnetosphere. H3<\/sub>+<\/sup>\u00a0ions in the ionosphere quickly reach thermal equilibrium with the surrounding neutrals. The ions emit photons in the near-infrared (NIR) that allow astronomers to monitor the thermosphere\u2019s temperature with ground-based telescopes since some NIR wavelengths get through Earth\u2019s atmosphere. That\u2019s how they know that the upper atmosphere is cooling, while observations of the lower atmosphere show no cooling.<\/p>\n The cooling is puzzling, and seasonal effects were ruled out as the cause of the temperature drop. So was the Sun\u2019s 11-year solar cycle, which sees the energy level from the Sun change.<\/p>\n New research published in Geophysical Review Letters has an explanation for the temperature shift. It\u2019s titled \u201cSolar wind power likely governs Uranus\u2019 thermosphere temperature.\u201d The lead author is Dr. Adam Masters from the Department of Physics at Imperial College. <\/p>\n According to Masters and his colleagues, the solar wind is responsible for Uranus\u2019 cooling. The solar wind is a stream of charged particles that comes from the Sun\u2019s outermost layer, the corona. It\u2019s a plasma composed of mostly electrons and protons and also contains atomic nuclei and heavy ions. <\/p>\n \u201cThis apparently very strong control of Uranus\u2019 upper atmosphere by the solar wind is unlike what we have seen at any other planet in our Solar System,\u201d Adams said. <\/p>\n While the Solar wind is unceasing, its properties gradually change over timescales that match the changes in Uranus\u2019 upper atmosphere. Since about 1990, the solar wind\u2019s average outward pressure has been dropping slowly but significantly. The drop doesn\u2019t correlate with the Sun\u2019s well-known 11-year cycle, but it does closely correlate with Uranus\u2019 changing temperature. <\/p>\n This suggested to the researchers that, unlike Earth, Uranus\u2019 temperature isn\u2019t controlled by photons.<\/p>\n It\u2019s a well-known fact that photons from the Sun heat the Earth. It\u2019s the basis for life. While our planet\u2019s magnetosphere largely protects Earth from the solar wind, photons aren\u2019t stopped. <\/p>\n Uranus is much further away from the Sun than Earth is, almost 3 billion km, while Earth is only about 228 million km from the Sun. The number of photons that reach Uranus is not enough to heat the planet. Instead, the decreasing solar wind is allowing Uranus\u2019 magnetosphere to expand. <\/p>\n Since the magnetosphere shields Uranus from the solar wind, its expansion makes it more difficult for the solar wind to reach the planet. Energy flows through the space around the planet, eventually reaching the thermosphere and controlling its temperature. <\/p>\n \u201cDeclining solar wind kinetic power, or near-identically total solar wind power, should mean weakening heating of Uranus\u2019 thermosphere, leading to the observed long-term temperature decline,\u201d the authors explain in their paper. <\/p>\n This means that for close-in planets like Earth, starlight controls the temperature of the thermosphere, while for planets further away, the solar wind takes over. <\/p>\n This discovery could affect a proposed future mission to Uranus. The Planetary Science and Astrobiology Decadal Survey 2023-2032 identified a mission to Uranus as a top priority, though so far, none have been approved. The mission concept is called Uranus Orbiter and Probe (UOP), and one of its main goals is to study the ice giant\u2019s atmosphere.<\/p>\n The mission would address the mystery of Uranus\u2019 cooling, but scientists struggled to understand it. These findings mean the mission goals can be updated, and the question becomes how the energy from the solar wind gets into Uranus\u2019 unusual magnetosphere.<\/p>\n This study not only answers a puzzling question about Uranus but also extends to exoplanets. If this solar-wind cooling can happen here, it can happen elsewhere. <\/p>\n \u201cBeyond the solar system, this explanation for Uranus\u2019 thermosphere cooling implies that exoplanet companions to host stars without strong local driving (like at Jupiter) and with sufficiently large magnetospheres will undergo a predominantly electrodynamic interaction with their parent star,\u201d the authors write. For these exoplanets, the stellar wind will strongly govern the thermal evolution of the upper atmosphere, not stellar radiation. The stellar wind may also drive certain types of aurorae. <\/p>\n \u201cThis strong star-planet interaction at Uranus could have implications for establishing if different exoplanets generate strong magnetic fields in their interiors \u2013 an important factor in the search for habitable worlds outside our Solar System,\u201d Adams concluded. <\/p>\n![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/11\/grl68299-fig-0001-m-1024x559.jpg\")
![\"This](\"https:\/\/www.universetoday.com\/wp-content\/uploads\/2024\/11\/Uranus-Orbiter-and-Probe.jpg\")
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