Studying the Outer Planets and Moons

In 1994, just four years into its long tenure of collecting data, Hubble watched as 21 fragments of Comet Shoemaker-Levy 9 bombarded the giant planet Jupiter with a sequential train of impacts. Each fragment left a temporary black scar that revealed deeper layers of the planet’s atmosphere. Hubble was able to document how Jupiter's atmosphere changed these impact scars over time, providing valuable insight into the planet's dynamic atmosphere. It was the first time astronomers witnessed such an event. The most recent collision Hubble observed occurred in 2009, when a suspected asteroid plunged into Jupiter’s atmosphere and left a temporary dark feature the size of the Pacific Ocean.

Four images of Jupiter, from bottom to top: 1) Jupiter with a puff of bright light near its limb. 2) The deep maroon scar of the impact is visible against the stripes of Jupiter's clouds. 3) The impact scar is larger with two distinct regions. 4) The impact scar is becoming more diffuse.

In 1994, Comet Shoemaker-Levy 9 broke into 21 large pieces that rained down upon the thick clouds of Jupiter. Shown here, beginning at the lower right and ending in the upper left, is a series of four images documenting the spectacular blackened impact sites as they rotated into view from behind the planet.

NASA, R. Evans, J. Trauger, H. Hammel, and the HST Comet Science Team

The Outer Planet Atmospheres Legacy (OPAL) project is a Hubble observing program that makes yearly observations of Jupiter, Saturn, Uranus, and Neptune to understand their atmospheric dynamics and evolution over time. Using Hubble, OPAL astronomers regularly measure and monitor features on each of these giant gas planets. One of those features is Jupiter’s Great Red Spot, a giant storm roughly the size of Earth that has been shrinking for at least 90 years. OPAL astronomers chart its changing size and cloud structure to determine why it is slowly disappearing. They also found that its winds are speeding up, exceeding speeds of 400 miles per hour (more than 640 kilometers per hour).

From its vantage point high above Earth’s atmosphere, NASA’s Hubble Space Telescope has completed its annual grand tour of the outer solar system – returning crisp images that are almost as good as earlier snapshots from interplanetary spacecraft. This is the realm of the giant planets – Jupiter, Saturn, Uranus, and Neptune – extending as far as 30 times the distance between Earth and the Sun. Credit: NASA's Goddard Space Flight Center; Lead Producer: Paul Morris

Hubble also monitors the changing storms on Neptune. When NASA’s Voyager 2 flew by Neptune in 1989, it captured a Great Dark Spot in Neptune’s far southern hemisphere. In 1994, when Hubble looked at Neptune, the spot had vanished. The following year, Hubble observations spotted another dark storm in Neptune’s northern hemisphere. Hubble’s regular observations of the planet for more than three decades revealed that these storms live for a few years before vanishing or fading away.

Like the speed of an advancing race car driver, the winds in the outermost “lane” of Jupiter’s Great Red Spot are accelerating – a discovery only made possible by NASA’s Hubble Space Telescope, which has monitored the planet for more than a decade. Researchers analyzing Hubble’s regular “storm reports” found that the average wind speed just within the boundaries of the storm, known as a high-speed ring, has increased by up to 8 percent from 2009 to 2020. In contrast, the winds near the red spot’s innermost region are moving significantly more slowly, like someone cruising lazily on a sunny Sunday afternoon. Credit: NASA's Goddard Space Flight Center; Lead Producer: Paul Morris

Hubble also captured images of bright auroras on Jupiter, Saturn, and Uranus. Auroras are brilliant curtains of light that appear in the upper atmosphere of a planet with a magnetic field. They develop when electrically charged particles trapped in the magnetic field spiral inward at high speeds toward the north and south magnetic poles. When these particles hit the upper atmosphere, they excite atoms and molecules there, causing them to glow in a similar process to that of a neon light.

This composite video illustrates the auroras on Jupiter relative to their position on the giant planet. The Jupiter auroras observed by Hubble are some of the most active and brightest ever caught by the telescope, reaching intensities over a thousand times brighter than those seen on Earth. Hubble's sensitivity to ultraviolet light captures the glow of the auroras above Jupiter's cloud top. The auroras were photographed on May 19, 2016, during a series of far-ultraviolet-light observations taking place as NASA's Juno spacecraft approaches and enters into orbit around Jupiter. The aim of the program is to determine how Jupiter's auroras respond to changing conditions in the solar wind, a stream of charged particles emitted from the sun. The full-color disk of Jupiter in this video was separately photographed at a different time by Hubble's Outer Planet Atmospheres Legacy (OPAL) program, a long-term Hubble project that annually captures global maps of the outer planets. Auroras are formed when charged particles in the space surrounding the planet are accelerated to high energies along the planet's magnetic field. When the particles hit the atmosphere near the magnetic poles, they cause it to glow like gases in a fluorescent light fixture. Jupiter's magnetosphere is 20,000 times stronger than Earth's. These observations will reveal how the solar system's largest and most powerful magnetosphere behaves. Credit: NASA, ESA, J. Nichols (University of Leicester), and G. Bacon (STScI); Acknowledgment: A. Simon (NASA/GSFC) and the OPAL team

NASA's Hubble Space Telescope Imaging Spectrograph took a series of snapshots of Saturn's aurora dancing in the sky. The ultraviolet images were taken on Jan. 24, 26, 28 and 30, 2004. This dissolve sequence shows the aurora appearing as a ring of light circling the planet's polar region. Collisions with atoms and molecules make the gases in the planet's atmosphere glow in visible, ultraviolet, and infrared light. Credit: NASA, ESA, J. Clarke (Boston Univ.) and G. Bacon (STScI)

Studying and Finding Moons

Jupiter’s moons are also interesting targets for the telescope. Their compositions and environments can provide important clues in the search for life beyond Earth. Hubble found the best evidence yet for an underground saltwater ocean on Jupiter’s moon Ganymede, the largest moon in the solar system, by detecting related activity in Ganymede’s own auroras. This subterranean ocean may have more water than all the water on Earth’s surface. Hubble also recorded evidence of short-lived changes in the atmosphere above the surface of Jupiter’s moon Europa – the result of gas plumes expelled from a subsurface ocean.

Diagram showing Uranus in the center and the paths of its rings and moons around the planet.

Hubble found two new moons and two new outer rings around Uranus.

NASA, ESA, and A. Feild (STScI)

Hubble also found new moons around Uranus and Neptune. In 2003, astronomers using Hubble discovered two small moons around Uranus. These moons, called Cupid and Mab, are so small and faint that NASA’s Voyager 2 spacecraft was unable to detect them during its 1986 flyby of the planet. The moons are about 8 to 10 miles across (12-16 km), or roughly the size of San Francisco. Hubble also detected two, large, outer rings around Uranus.

In 2013, a small, faint moon called Hippocamp was discovered in images Hubble took of the planet Neptune between 2004 and 2009. Astronomer Mark Showalter discovered the moon while studying faint ring-arcs around the planet. He then combed through 150 archived Hubble images of Neptune looking for the moon. Using those Hubble images, Showalter was able to plot Hippocamp’s 23-hour orbit around Neptune.

Learn More

Comet Fragment Slams Into Jupiter
In July 1994, 21 chunks of comet Shoemaker-Levy 9, which had broken apart a year earlier, slammed into Jupiter, and Hubble recorded this spectacular event.
Hubble Images Suggest Rogue Asteroid Smacked Jupiter
Without warning, a mystery object struck Jupiter on July 19, 2009, leaving a dark bruise the size of the Pacific Ocean.
Hubble Captures Vivid Auroras in Jupiter's Atmosphere
Hubble photographed Jupiter's auroras during a series far-ultraviolet-light observations taking place as NASA's Juno spacecraft approached and entered its orbit around Jupiter.
Saturn's Auroras Defy Scientists' Expectations
Hubble observations of Saturn helped researchers overturn theories about how Saturn's magnetic field behaves and how its auroras are generated.
Hubble Spies Tiny Moons Circling Uranus
Two new moons, uncovered by Hubble, are so faint they eluded the Voyager 2 spacecraft when it flew by Uranus in 1986.
Hubble Finds New Moon of Neptune
The moon is so small and dim that it is roughly one hundred million times fainter than the faintest star we can see with our unaided eyes.