Observations of Uranus in the near-infrared from 1992 to 2018 reveal that the planet’s upper atmosphere (thermosphere) has steadily cooled, dropping from about 700 K to approximately 450 K.
Now, a team led by Imperial College London scientists has finally understood why Uranus’s upper atmosphere has been cooling for decades. They determined that unpredictable long-term changes in the solar wind are behind the drop.
The team predicts that Uranus’ upper atmosphere will either continue to cool or potentially warm up again, depending on how the solar wind changes in the coming years.
Lead researcher Dr Adam Masters, from the Department of Physics at Imperial, said: “This very strong control of Uranus’ upper atmosphere by the solar wind is unlike what we have seen at any other planet in our Solar System.
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“It does mean that planets outside the Solar System could be in the same situation. These insights could, therefore, help researchers investigating exoplanets by shedding light on the kinds of signals that might be detected coming from similar planets around distant stars.”
In 1986, the Voyager 2 spacecraft made a flyby to Uranus. It took the temperature of the upper part of Uranus’ atmosphere, called the thermosphere.
Since the 1990s, Earth-based telescopes have regularly measured the temperature of Uranus’ thermosphere, revealing that its temperature has approximately halved over time. Unlike Uranus, Earth’s thermosphere hasn’t experienced such a dramatic temperature change, nor have other Solar System planets with monitored thermospheres.
Scientists initially considered whether the 11-year solar cycle, linked to sunspot activity, might explain the cooling trend. However, after 30 years of data collection, no consistent pattern tied to the solar cycle was found—only a steady temperature decline. A seasonal effect was also ruled out, as Uranus’ equinox occurred in 2007, eliminating it as a cause.
The mystery was resolved when the study’s authors, who were working in different fields at the time, came together at a conference. They realized that the cooling of Uranus’ thermosphere might be linked to gradual changes in the properties of the solar wind over the same period.
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In Earth’s thermosphere, temperature is primarily influenced by sunlight. Photons from the Sun cause reactions that warm the atmosphere. The intensity of these photons fluctuates with the 11-year solar cycle.
However, in Uranus’ case, the temperature decline appears linked to changes in the solar wind rather than sunlight. Since around 1990, the outward pressure of the solar wind has been steadily decreasing, with little correlation to the solar cycle. This decline mirrors the cooling of Uranus’ thermosphere.
As with Earth, the researchers proposed that photons do not control Uranus’ thermosphere temperature. Instead, the reduced solar wind pressure has caused the planet’s magnetic “bubble,” or magnetosphere, to expand. This larger magnetosphere is a barrier to the solar wind, reducing its direct impact on the planet’s surface. As the bubble grows, it redirects energy flow through space around Uranus, ultimately affecting the thermosphere’s temperature.
The cooling of Uranus’ thermosphere had long been a mystery, with no apparent cause, making it difficult to develop a testable theory for future missions. However, the discovery linking the thermosphere’s cooling to changes in the solar wind has now provided a clearer understanding.
This insight not only predicts how Uranus’ thermosphere will continue to evolve but has also reshaped the science goals for upcoming missions. Future missions will focus on how solar wind energy interacts with Uranus’ unique magnetosphere.
The team is also interested in exploring whether a similar phenomenon could occur at Neptune, which, like Uranus, has yet to be visited since the Voyager missions in the 1980s.
Beyond the Solar System, this discovery could be valuable for characterizing exoplanets. In systems where the stellar wind behaves similarly to the solar wind around Uranus, emissions from an exoplanet’s upper atmosphere, such as auroras, could be highly sensitive to changes in the stellar wind.
The team suggests that observers should pay closer attention to exoplanets farther from their parent stars or in systems with strong stellar winds, as emissions from these planets may have been underestimated in previous studies.
Dr Masters explained: “This strong star-planet interaction at Uranus could have implications for establishing if different exoplanets generate strong magnetic fields in their interiors – an important factor in the search for habitable worlds outside our Solar System.”
Journal Reference:
- A. Masters, J. R. Szalay et al. Solar Wind Power Likely Governs Uranus’ Thermosphere Temperature. Geophysical Research Letters. DOI: 10.1029/2024GL111623
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