Uranus releases about 15% more energy than it receives from the Sun, according to two new papers published in the journal Monthly Notices of the Royal Astronomical Society and the journal Geophysical Research Letters.
Composite image of Uranus. Image credit: Marcos van Dam / W. M. Keck Observatory.
Uranus is unlike any other planet in our Solar System. It spins on its side, which means each pole directly faces the Sun for a continuous 42-year ‘summer.’
This planet also rotates in the opposite direction of all planets except Venus.
The data from NASA’s Voyager 2 Uranus flyby in 1986 also suggested the planet is unusually cold inside, challenging scientists to reconsider fundamental theories of how planets formed and evolved throughout our Solar System.
“Since Voyager 2’s flyby, everybody has said Uranus has no internal heat,” said Dr. Amy Simon, a planetary scientist at NASA’s Goddard Space Flight Center and co-author of the first paper.
“But it’s been really hard to explain why that is, especially when compared with the other giant planets.”
“These Uranus projections came from only one up-close measurement of the planet’s emitted heat made by Voyager 2.”
“Everything hinges on that one data point. That is part of the problem.”
Using an advanced computer modeling technique and revisiting decades of data, Dr. Simon and colleagues found that Uranus does in fact generate some heat.
A planet’s internal heat can be calculated by comparing the amount of energy it receives from the Sun to the amount it of energy it releases into space in the form of reflected light and emitted heat.
The Solar System’s other giant planets — Saturn, Jupiter, and Neptune — emit more heat than they receive, which means the extra heat is coming from inside, much of it left over from the high-energy processes that formed the planets 4.5 billion years ago.
The amount of heat a planet exudes could be an indication of its age: the less heat released relative to the heat absorbed from the Sun, the older the planet is.
Uranus stood out from the other planets because it appeared to give off as much heat as it received, implying it had none of its own.
This puzzled scientists. Some hypothesized that perhaps the planet is much older than all the others and has cooled off completely.
Others proposed that a giant collision — the same one that may have knocked the planet on its side — blasted out all of Uranus’ heat.
But none of these hypotheses satisfied scientists, motivating them to solve Uranus’ cold case.
“We thought, Could it really be that there is no internal heat at Uranus?” said University of Oxford’s Professor Patrick Irwin, lead author of the first paper.
“We did many calculations to see how much sunshine is reflected by Uranus and we realized that it is actually more reflective than people had estimated.”
The researchers set out to determine Uranus’ full energy budget: how much energy it receives from the Sun compared to how much it reflects as sunlight and how much it emits as heat.
To do this, they needed to estimate the total amount of light reflected from the planet at all angles.
“You need to see the light that’s scattered off to the sides, not just coming straight back at you,” Dr. Simon said.
To get the most accurate estimate of Uranus’ energy budget yet, the scientists developed a computer model that brought together everything known about Uranus’ atmosphere from decades of observations from ground- and space-based telescopes, including the NASA/ESA Hubble Space Telescope and NASA’s Infrared Telescope Facility in Hawai’i.
The model included information about the planet’s hazes, clouds, and seasonal changes, all of which affect how sunlight is reflected and how heat escapes.
The authors found that Uranus releases about 15% more energy than it receives from the Sun, a figure that is similar to an estimate from the second study.
These studies suggest Uranus it has its own heat, though still far less than its neighbor Neptune, which emits more than twice the energy it receives.
“Now we have to understand what that remnant amount of heat at Uranus means, as well as get better measurements of it,” Dr. Simon said.
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Patrick G.J. Irwin et al. 2025. The bolometric Bond albedo and energy balance of Uranus. MNRAS 540 (2): 1719-1729; doi: 10.1093/mnras/staf800
Xinyue Wang et al. 2025. Internal Heat Flux and Energy Imbalance of Uranus. Geophysical Research Letters 52 (14): e2025GL115660; doi: 10.1029/2025GL115660