Uranus emits more heat than previously thought

Uranus emits more energy than it gets from the sun, two new studies report — a discovery that contradicts findings from the venerable Voyager spacecraft.

When Voyager 2 sped past Uranus on January 24, 1986, the spacecraft detected no significant excess heat from the planet, making it seemingly unique among the sun’s giant worlds. However, new observations from space- and ground-based telescopes reveal that Uranus does in fact radiate more energy than sunlight provides, two research teams report in work submitted to arXiv.org in late February.

“Uranus is not as odd as we thought it was,” says planetary scientist Patrick Irwin of the University of Oxford, a coauthor on one of the studies.

Both teams say that Uranus, which takes 84 years to orbit the sun, reflects a bit more sunlight into space than Voyager had found. This means that the sun heats the planet less than previously thought, suggesting that Uranus must generate some heat to explain its temperature.

The upshot: “Uranus does indeed have internal heat,” says Liming Li, a planetary scientist at the University of Houston and coauthor on the other study. This heat is presumably left over from the planet’s birth.

Li’s team estimates that Uranus emits 12.5 percent more energy than it receives from the sun. Irwin’s team pegs the excess at 15 percent, consistent with the other team’s result.

“Uranus is still an outlier,” Irwin says, because the other giant planets — Jupiter, Saturn and Neptune — radiate more than twice as much energy as they get from the sun.

No one knows why Uranus is so subdued. But the planet has another oddity: It rotates on its side, with an axial tilt of 98 degrees — compared with 3 degrees for Jupiter, 27 degrees for Saturn and 28 degrees for Neptune.

Planetary scientists have long suspected that a giant object knocked Uranus over. If so, the impact may have dredged up hot material from the interior, Irwin says, causing Uranus to lose much of its heat during its youth.