NASA researchers have discovered that Uranus emits more energy than it absorbs from the Sun, indicating an internal heat source – and undermining previous theories about the formation of giant planets.
After four decades of mystery, the planet Uranus, once dubbed an "inexplicable cold giant," is showing signs that it is emitting internal heat – a discovery that could change our understanding of the evolution of planets in the solar system and beyond.
When NASA's Voyager 2 spacecraft flew by Uranus in 1986, it discovered an extremely cold planet, much colder than expected. This is in contrast to other gas giants like Jupiter, Saturn and Neptune, which emit more heat than they absorb from the sun – evidence that their interiors are still hot from their formation 4.5 billion years ago.
Now, using advanced computational models and a reanalysis of Voyager data and ground-based and space-based telescopes (including the Hubble Telescope), scientists have found that Uranus emits about 15% more energy than it absorbs – a hint that it has an internal heat source, even if it is weak compared to its neighbor Neptune.
Uranus has a unique rotational inclination – it “lies on its side,” so that each pole faces the sun for 42 years straight. It also rotates in the opposite direction to most planets. Some researchers believe that a massive collision in its past tilted it – and may have also caused the loss of internal heat.
The energy calculation of Uranus
To estimate the amount of energy Uranus receives—that is, how much light it absorbs and how much it reflects or emits—it is necessary to measure the amount of light it reflects from all possible angles. To do this, the researchers developed an integrated model that took into account cloud layers, fog, and seasonal changes.
They found that Uranus reflects more light than previously thought – and therefore still emits internal heat, albeit at a low level. This result is confirmed in another study published in July 2025. Further research is still needed to understand the source of its remaining heat – and perhaps reexamine hypotheses about the collision it underwent.
Beyond understanding the history of our solar system, the study also provides insights into planets outside the solar system – many of which are the size of Uranus, and therefore understanding it is essential for understanding "extrasolar Uranuses."
More of the topic in Hayadan:
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I always knew that Uranus had the ability to generate internal heat.
It is built in
The Earth also has an internal heat source: slightly radioactive atoms, which are found deep in the mantle, and slowly decay.
In the Gemara (Tractate Shabbat, page 30, page 2), the verse "There is nothing new under the sun" (Ecclesiastes 1:9) is cited, and it is explained that the meaning of the verse is that for everything that will happen in the future, there is already something similar to it.