Neptune orbits the Sun at an average distance of about 4.5 billion kilometres. Uranus is closer, at roughly 2.9 billion kilometres. The gap between them is therefore about 1.6 billion kilometres, yet NASA lists a minimum temperature in Uranus’s atmosphere of 49 kelvin, or minus 224.2 degrees Celsius, colder than parts of Neptune’s atmosphere.
Distance from the Sun clearly matters, but it is not the only source of a planet’s atmospheric energy. Neptune releases far more heat from inside itself. Uranus releases remarkably little by comparison, which helps explain why the planet nearer the Sun can reach the lower atmospheric minimum.
There is an important update to the familiar version of this story. For decades, Uranus was often described as emitting no internal heat at all. Two independent analyses published in 2025 found that it does release some. The reversal in the title remains real; the claim of a completely heatless interior does not.
The billion-kilometre inversion
NASA’s current planet pages put Uranus at about 19 astronomical units from the Sun and Neptune at about 30. Because sunlight weakens with the square of distance, Neptune receives only around 40 per cent as much solar energy per square metre as Uranus does.
If sunlight alone determined atmospheric temperature, Neptune would seem the obvious candidate for the colder minimum. It does not work that simply. An atmosphere redistributes energy through radiation, winds, clouds, chemistry and vertical motion. A giant planet can also supply heat from below as energy left from its formation and slow contraction escapes to space.
I have written before about how similar Uranus and Neptune look in carefully calibrated colour. That visual resemblance makes the thermal difference more interesting. They are close in size and broad composition, but their energy budgets are not twins.
Minus 224 degrees is not the temperature of the whole planet
The number in the title is a minimum within Uranus’s atmosphere, not a solid-surface reading or a uniform global temperature. Uranus has no ordinary surface on which to set a thermometer. The quoted low occurs near the tropopause, the cold boundary between the lower troposphere and the warmer stratosphere above.
Voyager 2 helped establish that vertical profile. A 1987 analysis of its radio-occultation measurements found a tropopause temperature of 53 plus or minus 1 kelvin in the narrow equatorial latitudes it sampled. Other atmospheric profiles and latitudes produce the lower 49-kelvin figure in NASA’s summary.
So “Uranus is colder than Neptune” is useful shorthand, but it can be misread. Temperatures change with altitude, latitude, season and pressure. The precise claim is that parts of Uranus’s atmosphere reach a lower measured minimum than corresponding regions on the more distant planet.
Why Neptune has more warmth to work with
A planet’s internal contribution is estimated through its energy budget. Researchers compare the sunlight absorbed by the planet with the thermal radiation it emits to space. If the outgoing total is larger, the excess must be supplied from within.
The canonical Voyager-era figures made the contrast stark. Neptune was estimated to emit about 2.61 times the energy it absorbed from sunlight. Uranus was put at 1.06 times, close enough to equilibrium within the old uncertainty that its internal heat could be treated as negligible.
That contrast gave planetary models a difficult problem. Why would two neighbouring ice giants of similar scale cool so differently? Proposed answers included a giant impact that altered Uranus early in its history, or layers inside the planet that suppress convection and keep deep heat from reaching the atmosphere efficiently. Neither explanation has been demonstrated.
The 2025 correction matters
In May 2025, Patrick Irwin and colleagues published a new estimate of Uranus’s Bond albedo and energy balance. They combined observations from Hubble, Gemini and NASA’s Infrared Telescope Facility with a seasonal model of the planet’s changing reflectivity. Their orbital-average result put the ratio of emitted to absorbed energy at 1.15 plus or minus 0.06.
A separate team led by Xinyue Wang and Liming Li reached a compatible conclusion in Geophysical Research Letters. Modelling a complete Uranian orbit from 1946 to 2030, they estimated an internal heat flux of 0.078 plus or minus 0.018 watts per square metre. Uranus is cooling and releasing internal energy, just less vigorously than the other giant planets.
The revision is not merely a technical footnote. It removes the old absolute claim that Uranus has no internal heat. It does not erase the comparison with Neptune. A 2026 commentary in the same journal noted that the updated Uranian heat flow is still about five times smaller than Neptune’s roughly 0.4 watts per square metre.
One flyby still carries too much of the answer
Voyager 2 remains the only spacecraft to have visited either ice giant closely, passing Uranus in 1986 and Neptune in 1989. Modern telescopes can track clouds, reflected sunlight and thermal emission from afar, but many basic interior measurements still rest heavily on those brief encounters.
This leaves the mechanism open. Uranus could have a deep interior that is hotter than its faint outward glow suggests, with composition gradients or a thermal boundary slowing the escape of energy. Its unusual 98-degree axial tilt and extreme seasons complicate the accounting further. The two 2025 papers improved the balance sheet, but they did not look inside the planet.
What I keep coming back to is that “colder” is not a simple address in the Solar System. Neptune is farther from the Sun and receives much less sunlight, yet it has a stronger flow of energy rising from within. Uranus sits closer and still manages the lower atmospheric minimum. The difference is written not only in where the two planets orbit, but in how reluctantly one of them gives up its own heat.
