THE SOLAR SYSTEM

On first glance, our solar system seems to be well understood. It includes a single star, planets, their moons, dwarf planets like Pluto and Ceres, and smaller bodies like asteroids, comets, and the outer solar system Kuiper Belt objects. Yet, scientists continue to discover fascinating new findings about our solar system, and Hubble has contributed to these discoveries. For example, researchers used Hubble to study the trajectory of a mysterious object called ‘Oumuamua as it passed through the inner solar system. They are confident that this body is from another star system and has traveled into our solar system from interstellar space.

By providing a detailed look at the planets, moons, rings, asteroids, comets, and other objects in our celestial backyard, Hubble is helping to answer age-old questions about how the solar system began, how planets formed, and how the Earth evolved.

What has Hubble taught us about storms in the solar system?

Many astronomical phenomena occur over millions of years. But since its launch in 1990, the Hubble Space Telescope has kept a watchful eye on events within our own solar system, which happen on the timescale of days, weeks, and years. The short-term phenomena Hubble has witnessed on other planets includes the weather — watching storms arise and dissipate across the faces of other worlds. Hubble’s ability to see ultraviolet, infrared, and visible light makes it the ideal meteorologist for the solar system, allowing it to probe below the cloud tops and investigate the massive storms on distant planets. 

Among these constantly shifting weather patterns are dust storms on Mars. In 2018, a spring dust storm erupted in the southern hemisphere and ballooned into a global event enshrouding the entire planet. Hubble’s Earth-orbit perspective allowed it to view the entirety of the global storm, while its long-term presence in space continues to allow it to monitor changes in Mars’s seasons over months and years.

In the outer solar system, turbulent storms dot the atmospheres of the giant planets — Jupiter, Saturn, Uranus, and Neptune — allowing Hubble to become an expert storm tracker. For instance, Hubble has observed the downsizing of Jupiter’s most famous feature, the spinning, cyclone-like storm known as the Great Red Spot.

On Neptune, Hubble has captured the most insightful images to date of a planet whose blustery weather bewilders scientists. Neptunian winds blow at an average of 900 miles per hour (1,450 km/h), and huge storms — some the size of Earth itself — come and go with regularity. Hubble’s observations captured springtime on Neptune for the first time, tracking waves of massive storms — each one larger than the distance from Kansas to New York – with temperatures colder than -350ºF (210ºC).

Hubble revealed Uranus, once considered one of the blander-looking planets, as a dynamic world with the brightest clouds in the outer solar system. The clouds are probably made of crystals of methane, which condense as warm bubbles of gas well up from deep in the planet's atmosphere.

As Hubble continues its mission, we will surely learn more about the wild weather of the other planets in our solar system, reminding us that these aren’t just placid chunks of rock or balls of gas orbiting the Sun, but changing, evolving, dynamic places with unique seasons and climates that we’re just beginning to understand.

How does Hubble support NASA's planetary missions?

The Hubble Space Telescope has worked hand-in-hand with NASA’s planetary missions. For example, in tandem with the Mars Global Surveyor (MGS) spacecraft in orbit around Mars in 2001, Hubble observed Mars’ biggest dust storm in decades. 

The storm enshrouded the entire planet in dust for several months, changing the temperature of the Martian surface and atmosphere. Scientists watched the warmed atmosphere closely as NASA’s Mars Odyssey spacecraft made its approach to orbit the planet.

Hubble has also been monitoring activity in Jupiter's atmosphere for many years. It occasionally witnesses giant storms that suddenly burst forth from below the clouds. These storms appear to be triggered when the heat locked up inside Jupiter warms the water clouds that lie just below the cloud tops. Following up on these observations, the Juno mission probes deep inside those cloud layers to learn what makes those storms churn.

More than two decades passed between the time NASA's Voyager-2 space probe flew by Saturn in 1981 and the arrival of the Cassini mission in 2004. But beginning in 1990, Hubble filled in the gap of high-resolution Saturn imaging by tracking storms and auroral activity, and providing crisp views of Saturn’s rings from various perspectives as the planet’s seasons change. Hubble also extensively studied the features of Saturn’s hazy moon, Titan, to prepare for the Cassini spacecraft visit. As Cassini approached Saturn at an angle from underneath the planet, the probe had a very different perspective of Saturn than Hubble's view from near Earth. For the first time, astronomers were able to compare equal-sharpness views of Saturn from two separate perspectives.

In July 2007, NASA launched its Dawn spacecraft on a four-year journey to the asteroid belt. To prepare for Dawn's first stop at Vesta, astronomers used Hubble to map the asteroid's southern hemisphere. This map prepared scientists for Dawn’s arrival at Vesta in July 2011, helping them select the most scientifically productive regions to observe. For Dawn’s second stop at Ceres, Hubble revealed bright and dark regions on the asteroid's surface that could be craters, and/or areas containing different surface material. Hubble's images gave rise to the questions that guided planetary scientists as Dawn approached Ceres, including: Does Ceres have a rocky inner core, an icy mantle, and a thin, dusty outer crust? Does it have water locked beneath its surface?

Hubble also observed the dwarf planet Pluto’s system in anticipation of the New Horizon mission, which launched in 2006 and flew by Pluto in July 2015. Hubble’s discovery of four additional Plutonian moons – including two discovered after New Horizons launched – was critical to the mission’s planning by identifying potential hazards, verifying the optimal spacecraft trajectory, and establishing the need to include observations of the moons as part of the flyby observing sequence. Without Hubble, New Horizons would have discovered the tiny moons only a few months before its visit to Pluto.

In June 2014, Hubble discovered the next target for the New Horizons spacecraft — 2014 MU69, now known as Arrokoth. New Horizons flew by Arrokoth — the farthest and most primitive object solar system object ever explored by humankind — in the early hours of New Year's Day 2019. Thanks to Hubble, New Horizons was afforded the rare opportunity to visit an object discovered after the spacecraft launched.

What has Hubble learned about Pluto’s neighborhood?

We think we know Pluto. It's the smallest ball in our old solar system models. It’s a chunk of ice, rock, and hydrocarbons that drifts 4.67 billion miles (7.5 billion km) from Earth at its orbit’s farthest point. It's the tiny former planet that stirred up controversy when it was reclassified as both a dwarf planet and a member of the collection of icy cosmic objects we call the Kuiper Belt.

But Pluto and the Kuiper Belt still hold most of their secrets. Before the New Horizons mission flew through the dwarf planet's system in July 2015, astronomers had to rely on other ground- and space-based observatories, including the Hubble Space Telescope, to investigate those distant reaches of our solar system.

Hubble observations of Pluto in 2005 revealed two never-before-seen moons: Nix and Hydra. Six years later, in 2011, Hubble's keen vision found another moon, designated Kerberos, while searching Pluto for rings. In 2012, Hubble discovered tiny Styx while looking for potential hazards for the New Horizons spacecraft. This discovery expanded the size of Pluto's known satellite system to five moons, including its largest, Charon, which was discovered in 1978 and first imaged by Hubble shortly after launch in 1990. With five moons now known in the Pluto system, scientists are intrigued that such a small planet can have such a complex collection of satellites.

These discoveries provide additional clues for unraveling how the Pluto system formed and evolved. The dwarf planet's entire moon system is believed to have formed by a collision between Pluto and another planet-sized body early in the history of the solar system. The smashup flung material into orbit around Pluto, which then coalesced into the family of satellites now seen.

Astronomers can use the infrared vision of NASA's James Webb Space Telescope for follow-up observations. The Webb telescope is able to measure the surface chemistry of Pluto, its moons, and many other bodies that lie in the distant Kuiper Belt along with Pluto.

How do comets shape our solar system?

Since its launch in 1990, the Hubble Space Telescope has tracked many comets on their journeys through the inner solar system, and traced their orbits to their farthest reaches. These icy wanderers, remnants of the debris cloud that once encircled our newborn Sun, give astronomers clues to the formation and evolution of our solar system.

Most comets spend their lives beyond the orbit of Neptune, where they were pushed by gravitational interactions with the newly formed giant planets during the early development of the solar system. Occasionally, gravitational interactions with one another result in the orbit of one of these objects being perturbed until it swings into the inner solar system. When a comet gets within the orbit of Mars, the Sun's light warms the comet's ices. The ices begin to evaporate directly into a gas, and the comet brightens. At that point, the Hubble Space Telescope can observe the comet, detecting changes in brightness, noting expulsion of gases, and analyzing its composition.

Scientists can learn much about the building blocks of our newborn solar system by studying the composition of comets, but they can also examine interactions between comets and other celestial bodies to glean clues about planet formation and composition. For example, Hubble observed Comet Shoemaker-Levy 9 impact Jupiter in 1994. A series of Hubble observations spanning the year after impact revealed some surprising results, including unexpectedly low amounts of water in Jupiter's atmosphere.

Hubble observed Comet ISON as it made its first voyage to the inner solar system, contributing its study of the comet's activity to a wealth of worldwide observations. Although the breakup of Comet ISON was too close to the Sun for Hubble to observe, the telescope has observed the disintegration of other comets, including Comet LINEAR (C/1999 S4), Comet 73P/Schwassmann-Wachmann 3, and Comet ATLAS (C/2019 Y4). Hubble also created a striking image of Comet Siding Spring's flyby of Mars. Hubble continues to observe comets as they travel through our solar system, bearing witness to the eventual destruction of those that edge too close to the Sun.