The Hubble Space Telescope has spent more than thirty years providing some of the most incredible images of the known universe, but it’s only for the last ten that scientists have pointed its gaze at our solar system’s outer planets to observe them like never before.
For the past decade, NASA’s Outer Planet Atmospheres Legacy program (OPAL) has been obtaining detailed views of the long-term changes in the skies of the four closest giants to Earth: Jupiter, Saturn, Uranus and Neptune, each home to a unique set of atmospheric variables. OPAL data has allowed astronomers to observe weather patterns and seasons of these outer planets to better understand their dynamics and changes over time.
Hubble is capable of viewing wavelengths from ultraviolet to near-infrared light, and routinely provides high-resolution images of the gas giants once a year, as each of their orbit’s brings them closest to Earth. Now, with ten years of operation behind them, Hubble’s OPAL team at NASA will present a decadal overview of the program’s findings at the December meeting of the American Geophysical Union in Washington, D.C.
“Because OPAL now spans 10 years and counting, our database of planetary observations is ever growing. That longevity allows for serendipitous discoveries, but also for tracking long-term atmospheric changes as the planets orbit the sun. The scientific value of these data is underscored by the more than 60 publications to date that include OPAL data,” said Amy Simon of NASA’s Goddard Space Flight Center in a statement.
Jupiter
Jupiter is the solar system’s largest planet. The gases of the planet’s atmosphere churn all the way down to its core, tens of thousands of miles beneath the cloud tops. Besides its enormity, Jupiter is also known for its Great Red Spot. The unmistakable swirling red vortex on the gas giant’s face is the largest storm in the solar system; a raging typhoon nearly three times the size of Earth.
Jupiter’s orbit around the sun takes 12 years, allowing OPAL to have observed nearly a full Jovian year’s worth of seasons. Over that time, notable changes were observed in size and shape of the Great Red Spot, as well as other atmospheric phenomena in the bands wrapping the gas giant. Seasonal changes on Jupiter are minimal, because its axial tilt of only three degrees results in only about a five percent atmospheric variability through the course of its orbit, according to NASA. (Earth, on the other hand, has an axial tilt of around 23.5 degrees, which creates the different seasons our planet experiences.)
Saturn
Saturn takes more than twice as long to circle the sun, with an orbital period lasting 29 years. The ringed giant is tilted at a much steeper 26.7 degrees, leading to much higher seasonal shifts, compared to Jupiter. Over its ten-year observation, OPAL has tracked color variation and cloud depth of Saturn’s atmosphere as it cycles through the planet’s changing seasons.
Hubble has also been able to observe Saturn’s elusive dark ring spokes. First discovered by NASA’s Voyager mission in the 1980s, these dark rings only last for two or three rotations around Saturn. Thanks to Hubble, astronomers now know the rings to be a seasonally-driven phenomenon.
Uranus
Uranus is tilted almost completely on its side, placing its rotation on nearly the same plane as the planet’s orbit around the sun, which lasts a staggering 84 years.
Over the last ten years, OPAL has observed Uranus’ northern hemisphere, which has faced the inner solar system for the entirety of Hubble’s time in space, as it slowly tips toward the sun. Throughout its slow orbit around the sun, Uranus’ north polar cap has increased in brightness as the hemisphere nears a summer solstice in 2028.
Neptune
A storm like a giant dark splotch the size of Earth’s Atlantic Ocean was spotted in the atmosphere of Neptune in 2018, which Hubble followed until the storm’s demise toward the planet’s equator.
Another, in 2021, was observed through its formation and similar equatorial dissipation. Using OPAL data from Hubble, scientists were able to determine that the behavior of Neptune’s clouds is somehow linked to the 11-year solar cycle, responsible for recent occurrences of lower-latitude northern lights on Earth.