On April 7, Uranus passed in front of a distant star located 400 light-years away, giving NASA scientists a rare cosmic alignment in which to investigate the distant ice giant.
During the hour-long event, called a stellar occultation, Uranus’ atmosphere refracted the star’s light, dimming it before blocking it out completely. Though it’s counterintuitive, the dimming light provided an opportunity to illuminate aspects of Uranus that are usually difficult to spot.
30 astronomers at 18 observatories across North America observed the occultation. Researchers then used the data from the event to create a light curve demonstrating how the starlight changed over time. From that light curve, scientists can extract key info about Uranus’ stratosphere, including temperature, density, and pressure, all at different altitudes.
“NASA will use the observations of Uranus to determine how energy moves around the atmosphere and what causes the upper layers to be inexplicably hot,” said NASA planetary scientist William Saunders, in an agency release. “Others will use the data to measure Uranus’ rings, its atmospheric turbulence, and its precise orbit around the Sun.”
Despite being nearly 2 billion miles from the Sun, Uranus’ upper atmosphere is hotter than physics says it should be. The light curve may provide some explanations.
This is the first time NASA has organized such a large, collaborative observation of a Uranus occultation, the release stated. The team was able to make a dry run of the observations in November 2024, when they captured a dimmer Uranus occultation using telescopes in Asia.
Those observations helped fine-tune the timing of the occultation and update Uranus’ position by about 125 miles (202 kilometers), which might not sound like much—unless you’re trying to aim multiple telescopes at the same moving target 2 billion miles (3.22 billion km) away from the opposite side of the world.
Beyond Uranus’ strange and turbulent atmosphere, this event also provided data on the planet’s ring system and its exact orbit around the Sun. That’s more important than it sounds: The first and only time a spacecraft flew by Uranus was Voyager 2 in 1986, and we still don’t know the planet’s position in space to better than about 100 miles.
Uranus has 13 known rings, 27 moons, and a whole lot of mystery. It’s made mostly of hydrogen, helium, water, ammonia, and methane. The planet is classified as an ice giant because of the weird chemical cocktail that makes up its interior. Think cold, gassy slushy with gusty winds and storms.
NASA is already looking ahead to the next big occultation of Uranus in 2031, which will involve an even brighter background star. That could mean airborne or even space-based observations next time around—which could help shore up scientists’ understanding of our local oddball, a tilted planet with mysterious rings, Shakespearian moons, and plenty more secrets yet to be revealed.
