Uranus is Getting Colder and Now We Know Why

Uranus is an oddball among the Solar System’s planets. While most planets’ axis of rotation is perpendicular to their orbital plane, Uranus has an extreme tilt angle of 98 degrees. It’s flopped over on its side, likely from an ancient collision. It also has a retrograde rotation, opposite of the other planets.

The ice giant also has an unusual relationship with the Sun that sets it apart from other planets.

Uranus’ uniqueness extends to its upper atmosphere, called the thermosphere-corona. That region’s 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.

When Voyager 2 flew past Uranus in 1986, it measured the thermosphere’s temperature. In the intervening decades, telescopes have continuously measured Uranus’s temperature. All these measurements show that the planet’s upper atmosphere is cooling and that the temperature has halved. None of the other planets experienced the same changes.

Scientists know that Uranus’ thermosphere is a tenuous layer. It has an embedded ionosphere, and it helps astronomers measure the thermosphere’s temperature. It’s a layer of ions that separates the lower atmosphere from the planet’s magnetosphere. H₃⁺ ions 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’s temperature with ground-based telescopes since some NIR wavelengths get through Earth’s atmosphere. That’s how they know that the upper atmosphere is cooling, while observations of the lower atmosphere show no cooling.

The cooling is puzzling, and seasonal effects were ruled out as the cause of the temperature drop. So was the Sun’s 11-year solar cycle, which sees the energy level from the Sun change.

New research published in Geophysical Review Letters has an explanation for the temperature shift. It’s titled “Solar wind power likely governs Uranus’ thermosphere temperature.” The lead author is Dr. Adam Masters from the Department of Physics at Imperial College.

According to Masters and his colleagues, the solar wind is responsible for Uranus’ cooling. The solar wind is a stream of charged particles that comes from the Sun’s outermost layer, the corona. It’s a plasma composed of mostly electrons and protons and also contains atomic nuclei and heavy ions.

“This apparently 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,” Adams said.

While the Solar wind is unceasing, its properties gradually change over timescales that match the changes in Uranus’ upper atmosphere. Since about 1990, the solar wind’s average outward pressure has been dropping slowly but significantly. The drop doesn’t correlate with the Sun’s well-known 11-year cycle, but it does closely correlate with Uranus’ changing temperature.

This suggested to the researchers that, unlike Earth, Uranus’ temperature isn’t controlled by photons.

It’s a well-known fact that photons from the Sun heat the Earth. It’s the basis for life. While our planet’s magnetosphere largely protects Earth from the solar wind, photons aren’t stopped.

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’ magnetosphere to expand.

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.

“Declining solar wind kinetic power, or near-identically total solar wind power, should mean weakening heating of Uranus’ thermosphere, leading to the observed long-term temperature decline,” the authors explain in their paper.

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.

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’s atmosphere.

The mission would address the mystery of Uranus’ 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’ unusual magnetosphere.

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.

“Beyond the solar system, this explanation for Uranus’ 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,” 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.

“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,” Adams concluded.