Voyager missed it, but now we know Uranus has a fiery secret

The discovery resolves a long-standing scientific mystery about the giant planet, because observational analyses from Voyager 2 in 1986 didn't suggest the presence of significant internal heat -- contradicting scientists' understanding of how giant planets form and evolve.

However, using decades of spacecraft observations and computer models, a team of scientists found Uranus releases more heat than it receives from sunlight, said Xinyue Wang, the first author on the paper and a former doctoral student of UH's Department of Earth and Atmospheric Sciences within the College of Natural Sciences and Mathematics.

"This means it's still slowly losing leftover heat from its early history, a key piece of the puzzle that helps us understand its origins and how it has changed over time," she explained.

The paper published July 14 in Geophysical Research Letters, a top journal publication by the American Geophysical Union. The discovery is consistent with an independent study conducted by a research group led by Professor Patrick Irwin at the University of Oxford.

However, Uranus's internal heat is weaker than its other giant counterparts in the solar system, emitting about 12.5% more heat than it absorbs via sunlight. This is lower compared to fluxes of upwards of 100% measured for Jupiter, Saturn and Neptune, said Xun Jiang, co-author and professor in the Department of Earth and Atmospheric Sciences.

"From a scientific perspective, this study helps us better understand Uranus and other giant planets. For future space exploration, I think it strengthens the case for a mission to Uranus."

Xinyue Wang, UH's Department of Earth and Atmospheric Sciences

It's uncertain why this happened, but researchers said Uranus may have a different interior structure or evolutionary history compared to the other giant planets.

Another notable takeaway from the study was that Uranus's energy levels also change with its long seasons, which last about 20 years. These seasonal changes are likely caused by the planet's off-center orbit and tilted spin, Wang said.

Liming Li, co-author and professor in UH's Department of Physics, said this study could improve planning for NASA's flagship mission to orbit and probe Uranus, an initiative the National Academies of Sciences, Engineering and Medicine classified as its highest priority for the 2023-32 decade.

"From a scientific perspective, this study helps us better understand Uranus and other giant planets," Wang said. "For future space exploration, I think it strengthens the case for a mission to Uranus."

Additionally, the team's methodology provides testable theories and models that could also be applied to explore radiant energy of other planets within and beyond our solar system, Li said. It could even impact technology innovation and climate understanding on Earth.

"By uncovering how Uranus stores and loses heat, we gain valuable insights into the fundamental processes that shape planetary atmospheres, weather systems and climate systems," Li said. "These findings help broaden our perspective on Earth's atmospheric system and the challenges of climate change."