Universe Today
It's called a stellar occultation, which occurs when a planet's orbit causes it to pass between Earth and a distant star. Like a planetary transit, an occultation allows astronomers to observe light passing through the planet's atmosphere. On April 7th, astronomers got a rare chance to study Uranus' atmosphere and rings as it passed in front of a star 400 light-years away. The observations were part of an international campaign led by NASA's Langley Research Center involving 30 astronomers from 18 observatories.
The event lasted for about an hour and was only visible to observatories in western North America. During the observation campaign, astronomers measured the temperature and composition of the middle layer of Uranus' atmosphere (the stratosphere), which helped them determine how it has changed over the last 30 years (the last time an occultation occurred). This data will help scientists understand how Uranus' atmosphere has evolved and could inform future efforts to explore the mysterious planet.
William Saunders, a planetary scientist at NASA's Langley Research Center and science principal investigator and analysis lead, described the campaign in a recent NASA press release:
"Uranus passed in front of a star that is about 400 light years from Earth. As Uranus began to occult the star, the planet’s atmosphere refracted the starlight, causing the star to appear to gradually dim before being blocked completely. The reverse happened at the end of the occultation, making what we call a light curve. By observing the occultation from many large telescopes, we are able to measure the light curve and determine Uranus’ atmospheric properties at many altitude layers."
Like Jupiter and Saturn, Uranus is primarily composed of hydrogen and helium. However, it and Neptune contain abundant volatile elements like water, ammonia, and methane. These elements are largely found beneath Uranus' swirling atmosphere, where the pressure condenses them until they become solid. Hence why Uranus and Neptune are referred to as "ice giants." In addition, Uranus has 28 natural satellites and 13 rings composed of ice and dust.
This planned observation represented a scientific opportunity to learn more about Uranus' atmosphere and composition. It also represented a significant challenge. To date, the only mission that studied this ice giant was NASA's Voyager 2 spacecraft, which conducted a flyby in 1986 (ten years before the last occultation). As a result, astronomers can only calculate its orbit within an accuracy of about 160 km (100 mi). Astronomers were naturally eager to exploit this occultation to learn more about the mysterious ice giant.
On November 12th, 2024, researchers at NASA Langley and the collaborating observatories (including two in Japan and one in Thailand) conducted a test run to calibrate their instruments in preparation for the campaign. A similar coordinated effort was conducted by the Paris Observatory and Space Science Institute (SSI), in conjunction with NASA, to observe the occultation from two telescopes in India. These efforts confirmed that the participating observatories, when working together, could capture the occultation event in April.
They also allowed the researchers to refine their models of Uranus' expected location by 240 km (125 mi) and the timing down to the second. These investigations were made possible by the number of partner institutions that provided views of the occultation from many different instruments. This included NASA's Infrared Telescope Facility (IRTF) on the summit of Mauna Kea in Hawaii, which was first built to support NASA's Voyager missions.
As Emma Dahl, a postdoctoral scholar at Caltech in Pasadena who assisted in gathering the IRTF observations, said:
“As scientists, we do our best work when we collaborate. This was a team effort between NASA scientists, academic researchers, and amateur astronomers. The atmospheres of the gas and ice giant planets [Jupiter, Saturn, Uranus, and Neptune] are exceptional atmospheric laboratories because they don’t have solid surfaces. This allows us to study cloud formation, storms, and wind patterns without the extra variables and effects a surface produces, which can complicate simulations very quickly.”
Several proposals have been made for a possible robotic mission to Uranus, including NASA's Uranus Orbiter and Probe. Due to a shortfall in plutonium production, this mission may not be ready until the late 2030s, arriving some time in the 2050s. In the meantime, NASA hopes to make similar observations over the next six years, when Uranus will occult several dimmer stars. In 2031, it will pass before a brighter star, which could lead to even greater insights into the planet's atmosphere and ring structures.
Credit: NASA
