Jupiter’s Moon Ganymede

Jupiter's moon Ganymede. Credit: NASA

In 1610, Galileo Galilei looked up at the night sky through a telescope of his own design. Spotting Jupiter, he noted the presence of several “luminous objects” surrounding it, which he initially took for stars. In time, he would notice that these “stars” were orbiting the planet, and realized that they were in fact Jupiter’s moons – which would come to be named Io, Europa, Ganymede and Callisto.

Of these, Ganymede is the largest, and boasts many fascinating characteristics. In addition to being the largest moon in the Solar System, it is also larger than even the planet Mercury. It is the only satellite in the Solar System known to possess a magnetosphere, has a thin oxygen atmosphere, and (much like its fellow-moons, Europa and Callisto) is believed to have an interior ocean.

Continue reading “Jupiter’s Moon Ganymede”

The Planet Jupiter

Jupiter and Io. Image Credit: NASA/JPL
Jupiter and Io. Image Credit: NASA/JPL

Ever since the invention of the telescope four hundred years ago, astronomers have been fascinated by the gas giant known as Jupiter. Between its constant, swirling clouds, its many, many moons, and its Giant Red Spot, there are many things about this planet that are both delightful and fascinating.

But perhaps the most impressive feature about Jupiter is its sheer size. In terms of mass, volume, and surface area, Jupiter is the biggest planet in our Solar System by a wide margin. And since people have been aware of its existence for thousands of years, it has played an active role in the cosmological systems many cultures. But just what makes Jupiter so massive, and what else do we know about it?

Size, Mass and Orbit:

Jupiter’s mass, volume, surface area and mean circumference are 1.8981 x 1027 kg, 1.43128 x 1015 km3, 6.1419 x 1010 km2, and 4.39264 x 105 km respectively. To put that in perspective, Jupiter diameter is roughly 11 times that of Earth, and 2.5 the mass of all the other planets in the Solar System combined.

But, being a gas giant, it has a relatively low density – 1.326 g/cm3 – which is less than one quarter of Earth’s. This means that while Jupiter’s volume is equivalent to about 1,321 Earths, it is only 318 times as massive. The low density is one way scientists are able to determine that it is made mostly of gases, though the debate still rages on what exists at its core (see below).

Jupiter orbits the Sun at an average distance (semi-major axis) of 778,299,000 km (5.2 AU), ranging from 740,550,000 km (4.95 AU) at perihelion and 816,040,000 km (5.455 AU) at aphelion. At this distance, Jupiter takes 11.8618 Earth years to complete a single orbit of the Sun. In other words, a single Jovian year lasts the equivalent of 4,332.59 Earth days.

However, Jupiter’s rotation is the fastest of all the Solar System’s planets, completing a rotation on its axis in slightly less than ten hours (9 hours, 55 minutes and 30 seconds to be exact. Therefore, a single Jovian year lasts 10,475.8 Jovian solar days. This orbital period is two-fifths that of Saturn, which means that the two largest planets in our Solar System form a 5:2 orbital resonance.

Structure and Composition:

Jupiter is composed primarily of gaseous and liquid matter. It is the largest of the gas giants, and like them, is divided between a gaseous outer atmosphere and an interior that is made up of denser materials. It’s upper atmosphere is composed of about 88–92% hydrogen and 8–12% helium by percent volume of gas molecules, and approx. 75% hydrogen and 24% helium by mass, with the remaining one percent consisting of other elements.

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons
This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons

The atmosphere contains trace amounts of methane, water vapor, ammonia, and silicon-based compounds as well as trace amounts of benzene and other hydrocarbons. There are also traces of carbon, ethane, hydrogen sulfide, neon, oxygen, phosphine, and sulfur. Crystals of frozen ammonia have also been observed in the outermost layer of the atmosphere.

The interior contains denser materials, such that the distribution is roughly 71% hydrogen, 24% helium and 5% other elements by mass. It is believed that Jupiter’s core is a dense mix of elements – a surrounding layer of liquid metallic hydrogen with some helium, and an outer layer predominantly of molecular hydrogen. The core has also been described as rocky, but this remains unknown as well.

In 1997, the existence of the core was suggested by gravitational measurements, indicating a mass of from 12 to 45 times the Earth’s mass, or roughly 4%–14% of the total mass of Jupiter. The presence of a core is also supported by models of planetary formation that indicate how a rocky or icy core would have been necessary at some point in the planet’s history in order to collect all of its hydrogen and helium from the protosolar nebula.

However, it is possible that this core has since shrunk due to convection currents of hot, liquid, metallic hydrogen mixing with the molten core. This core may even be absent now, but a detailed analysis is needed before this can be confirmed. The Juno mission, which launched in August 2011 (see below), is expected to provide some insight into these questions, and thereby make progress on the problem of the core.

The temperature and pressure inside Jupiter increase steadily toward the core. At the “surface”, the pressure and temperature are believed to be 10 bars and 340 K (67 °C, 152 °F). At the “phase transition” region, where hydrogen becomes metallic, it is believed the temperature is 10,000 K (9,700 °C; 17,500 °F) and the pressure is 200 GPa. The temperature at the core boundary is estimated to be 36,000 K (35,700 °C; 64,300 °F) and the interior pressure at roughly 3,000–4,500 GPa.

Jupiter’s Moons:

The Jovian system currently includes 67 known moons. The four largest are known as the Galilean Moons, which are named after their discoverer, Galileo Galilei. They include: Io, the most volcanically active body in our Solar System; Europa, which is suspected of having a massive subsurface ocean; Ganymede, the largest moon in our Solar System; and Callisto, which is also thought to have a subsurface ocean and features some of the oldest surface material in the Solar System.

Then there’s the Inner Group (or Amalthea group), which is made up of four small moons that have diameters of less than 200 km, orbit at radii less than 200,000 km, and have orbital inclinations of less than half a degree. This groups includes the moons of Metis, Adrastea, Amalthea, and Thebe. Along with a number of as-yet-unseen inner moonlets, these moons replenish and maintain Jupiter’s faint ring system.

Jupiter also has an array of Irregular Satellites, which are substantially smaller and have more distant and eccentric orbits than the others. These moons are broken down into families that have similarities in orbit and composition, and are believed to be largely the result of collisions from large objects that were captured by Jupiter’s gravity.

Illustration of Jupiter and the Galilean satellites. Credit: NASA
Illustration of Jupiter and the Galilean satellites. Credit: NASA

Atmosphere and Storms:

Much like Earth, Jupiter experiences auroras near its northern and southern poles. But on Jupiter, the auroral activity is much more intense and rarely ever stops. The intense radiation, Jupiter’s magnetic field, and the abundance of material from Io’s volcanoes that react with Jupiter’s ionosphere create a light show that is truly spectacular.

Jupiter also experiences violent weather patterns. Wind speeds of 100 m/s (360 km/h) are common in zonal jets, and can reach as high as 620 kph (385 mph). Storms form within hours and can become thousands of km in diameter overnight. One storm, the Great Red Spot, has been raging since at least the late 1600s. The storm has been shrinking and expanding throughout its history; but in 2012, it was suggested that the Giant Red Spot might eventually disappear.

Jupiter is perpetually covered with clouds composed of ammonia crystals and possibly ammonium hydrosulfide. These clouds are located in the tropopause and are arranged into bands of different latitudes, known as “tropical regions”. The cloud layer is only about 50 km (31 mi) deep, and consists of at least two decks of clouds: a thick lower deck and a thin clearer region.

There may also be a thin layer of water clouds underlying the ammonia layer, as evidenced by flashes of lightning detected in the atmosphere of Jupiter, which would be caused by the water’s polarity creating the charge separation needed for lightning. Observations of these electrical discharges indicate that they can be up to a thousand times as powerful as those observed here on the Earth.

A color composite image of the June 3rd Jupiter impact flash. Credit: Anthony Wesley of Broken Hill, Australia.
A color composite image of the June 3rd Jupiter impact flash. Credit: Anthony Wesley

Historical Observations of the Planet:

As a planet that can be observed with the naked eye, humans have known about the existence of Jupiter for thousands of years. It has therefore played a vital role in the mythological and astrological systems of many cultures. The first recorded mentions of it date back to the Babylon Empire of the 7th and 8th centuries BCE.

In the 2nd century, the Greco-Egyptian astronomer Ptolemy constructed his famous geocentric planetary model that contained deferents and epicycles to explain the orbit of Jupiter relative to the Earth (i.e. retrograde motion). In his work, the Almagest, he ascribed an orbital period of 4332.38 days to Jupiter (11.86 years).

In 499, Aryabhata – a mathematician-astronomer from the classical age of India – also used a geocentric model to estimate Jupiter’s period as 4332.2722 days, or 11.86 years. It has also been ventured that the Chinese astronomer Gan De discovered Jupiter’s moons in 362 BCE without the use of instruments. If true, it would mean that Galileo was not the first to discovery the Jovian moons two millennia later.

In 1610, Galileo Galilei was the first astronomer to use a telescope to observe the planets. In the course of his examinations of the outer Solar System, he discovered the four largest moons of Jupiter (now known as the Galilean Moons). The discovery of moons other than Earth’s was a major point in favor of Copernicus’ heliocentric theory of the motions of the planets.

The first star party? Galileo shows of the sky in Saint Mark's square in Venice. Note the lack of adaptive optics. (Illustration in the Public Domain).
Galileo shows of the sky in Saint Mark’s square in Venice. Note the lack of adaptive optics. Credit: Public Domain

During the 1660s, Cassini used a new telescope to discover Jupiter’s spots and colorful bands and observed that the planet appeared to be an oblate spheroid. By 1690, he was also able to estimate the rotation period of the planet and noticed that the atmosphere undergoes differential rotation. In 1831, German astronomer Heinrich Schwabe produced the earliest known drawing to show details of the Great Red Spot.

In 1892, E. E. Barnard observed a fifth satellite of Jupiter using the refractor telescope at the Lick Observatory in California. This relatively small object was later named Amalthea, and would be the last planetary moon to be discovered directly by visual observation.

In 1932, Rupert Wildt identified absorption bands of ammonia and methane in the spectra of Jupiter; and by 1938, three long-lived anticyclonic features termed “white ovals” were observed. For several decades, they remained as separate features in the atmosphere, sometimes approaching each other but never merging. Finally, two of the ovals merged in 1998, then absorbed the third in 2000, becoming Oval BA.

Beginning in the 1950s, radiotelescopic research of Jupiter began. This was due to astronomers Bernard Burke and Kenneth Franklin’s detection of radio signals coming from Jupiter in 1955. These bursts of radio waves, which corresponded to the rotation of the planet, allowed Burke and Franklin to refine estimates of the planet’s rotation rate.

Infrared image of Jupiter from SOFIA’s First Light flight composed of individual images at wavelengths of 5.4 (blue), 24 (green) and 37 microns (red) made by Cornell University’s FORCAST camera. A recent visual-wavelength picture of approximately the same side of Jupiter is shown for comparison. The white stripe in the infrared image is a region of relatively transparent clouds through which the warm interior of Jupiter can be seen. (Visual image credit: Anthony Wesley)
Infrared image of Jupiter from SOFIA’s First Light flight composed of individual images at wavelengths made by Cornell University’s FORCAST camera. Credit: Anthony Wesley/Cornell University

Over time, scientists discovered that there were three forms of radio signals transmitted from Jupiter – decametric radio bursts, decimetric radio emissions, and thermal radiation. Decametric bursts vary with the rotation of Jupiter, and are influenced by the interaction of Io with Jupiter’s magnetic field.

Decimetric radio emissions – which originate from a torus-shaped belt around Jupiter’s equator – are caused by cyclotronic radiation from electrons that are accelerated in Jupiter’s magnetic field. Meanwhile, thermal radiation is produced by heat in the atmosphere of Jupiter. Visualizations of Jupiter using radiotelescopes have allowed astronomers to learn much about its atmosphere, thermal properties and behavior.


Since 1973, a number of automated spacecraft have been sent to the Jovian system and performed planetary flybys that brought them within range of the planet. The most notable of these was Pioneer 10, the first spacecraft to get close enough to send back photographs of Jupiter and its moons. Between this mission and Pioneer 11, astronomers learned a great deal about the properties and phenomena of this gas giant.

Artist impression of Pioneer 10 at Jupiter. Image credit: NASA/JPL
Artist impression of Pioneer 10 at Jupiter. Image credit: NASA/JPL

For example, they discovered that the radiation fields near the planet were much stronger than expected. The trajectories of these spacecraft were also used to refine the mass estimates of the Jovian system, and radio occultations by the planet resulted in better measurements of Jupiter’s diameter and the amount of polar flattening.

Six years later, the Voyager missions began, which vastly improved the understanding of the Galilean moons and discovered Jupiter’s rings. They also confirmed that the Great Red Spot was anticyclonic, that its hue had changed sine the Pioneer missions – turning from orange to dark brown – and spotted lightning on its dark side. Observations were also made of Io, which showed a torus of ionized atoms along its orbital path and volcanoes on its surface.

On December 7th, 1995, the Galileo orbiter became the first probe to establish orbit around Jupiter, where it would remain for seven years. During its mission, it conducted multiple flybys of all the Galilean moons and Amalthea and deployed an probe into the atmosphere. It was also in the perfect position to witness the impact of Comet Shoemaker–Levy 9 as it approached Jupiter in 1994.

On September 21st, 2003, Galileo was deliberately steered into the planet and crashed in its atmosphere at a speed of 50 km/s, mainly to avoid crashing and causing any possible contamination to Europa – a moon which is believed to harbor life.

Artist impression of New Horizons with Jupiter. Image credit: NASA/JPL/JHUAPL
Artist impression of New Horizons with Jupiter. Image credit: NASA/JPL/JHUAPL

Data gathered by both the probe and orbiter revealed that hydrogen composes up to 90% of Jupiter’s atmosphere. The temperatures data recorded was more than 300 °C (570 °F) and the wind speed measured more than 644 kmph (400 mph) before the probe vaporized.

In 2000, the Cassini probe (while en route to Saturn) flew by Jupiter and provided some of the highest-resolution images ever taken of the planet. While en route to Pluto, the New Horizons space probe flew by Jupiter and measured the plasma output from Io’s volcanoes, studied all four Galileo moons in detail, and also conducting long-distance observations of Himalia and Elara.

NASA’s Juno mission, which launched in August 2011, achieved orbit around the Jovian planet on July 4th, 2016. The purpose of this mission to study Jupiter’s interior, its atmosphere, its magnetosphere and gravitational field, ultimately for the purpose of determining the history of the planet’s formation (which will shed light on the formation of the Solar System).

As the probe entered its polar elliptical orbit on July 4th after completing a 35-minute-long firing of the main engine, known as Jupiter Orbital Insertion (or JOI). As the probe approached Jupiter from above its north pole, it was afforded a view of the Jovian system, which it took a final picture of before commencing JOI.

Illustration of NASA's Juno spacecraft firing its main engine to slow down and go into orbit around Jupiter. Lockheed Martin built the Juno spacecraft for NASA's Jet Propulsion Laboratory. Credit: NASA/Lockheed Martin
Illustration of NASA’s Juno spacecraft firing its main engine to slow down and go into orbit around Jupiter. Credit: NASA/Lockheed Martin

On July 10th, the Juno probe transmitted its first imagery from orbit after powering back up its suite of scientific instruments. The images were taken when the spacecraft was 4.3 million km (2.7 million mi) from Jupiter and on the outbound leg of its initial 53.5-day capture orbit. The color image shows atmospheric features on Jupiter, including the famous Great Red Spot, and three of the massive planet’s four largest moons – Io, Europa and Ganymede, from left to right in the image.

The next planned mission to the Jovian system will be performed by the European Space Agency’s Jupiter Icy Moon Explorer (JUICE), due to launch in 2022, followed by NASA’s Europa Clipper mission in 2025.


The discovery of exoplanets has revealed that planets can get even bigger than Jupiter. In fact, the number of “Super Jupiters” observed by the Kepler space probe (as well as ground-based telescopes) in the past few years has been staggering. In fact, as of 2015, more than 300 such planets have been identified.

Notable examples include PSR B1620-26 b (Methuselah), which was the first super-Jupiter to be observed (in 2003). At 12.7 billion years of age, it is also the third oldest known planet in the universe. There’s also HD 80606 b (Niobe), which has the most eccentric orbit of any known planet, and 2M1207b (Lerna), which orbits the brown dwarf Fomalhaut b (Illion).

Here’s an interesting fact. Scientist theorize that a gas gain could get 15 times the size of Jupiter before it began deuterium fusion, making it a brown dwarf star. Good thing too, since the last thing the Solar System needs is for Jupiter to go nova!

Jupiter was appropriately named by the ancient Romans, who chose to name after the king of the Gods (also known as Jove). The more we have come to know and understand about this most-massive of Solar planets, the more deserving of this name it appears.

We have many interesting articles on Jupiter here at Universe Today. Here are some articles on the color and gravity of Jupiter, how it got its name, and how it shaped our Solar System.

Got questions about Jupiter’s greater mysteries? Then here’s Does Jupiter Have a Solid Core?, Could Jupiter Become a Star?, Could We Live on Jupiter?, and Could We Terraform Jupiter?

We have recorded a whole series of podcasts about the Solar System at Astronomy Cast.

What are the Galilean Moons?

Illustration of Jupiter and the Galilean satellites. Credit: NASA

It’s no accident that Jupiter shares its name with the king of the gods. In addition to being the largest planet in our Solar System – with two and a half times the mass of all the other planets combined – it is also home to some of the largest moons of any Solar planet. Jupiter’s largest moons are known as the Galileans, all of which were discovered by Galileo Galilei and named in his honor.

They include Io, Europa, Ganymede, and Callisto, and are the Solar System’s fourth, sixth, first and third largest satellites, respectively. Together, they contain almost 99.999% of the total mass in orbit around Jupiter, and range from being 400,000 and 2,000,000 km from the planet. Outside of the Sun and eight planets, they are also among the most massive objects in the Solar System, with radii larger than any of the dwarf planets.

Continue reading “What are the Galilean Moons?”

NASA’s “Remastered” View of Europa is the Best Yet

Europa. CThe cracked, icy surface of Europa. The smoothness of the surface has led many scientists to conclude that oceans exist beneath it. Credit: NASA/JPLredit: NASA

Europa, Jupiter’s sixth-closest moon, has long been a source of fascination and wonder for astronomers. Not only is it unique amongst its Jovian peers for having a smooth, ice-covered surface, but it is believed that warm, ocean waters exist beneath that crust – which also makes it a strong candidate for extra-terrestrial life.

And now, combining a mosaic of color images with modern image processing techniques, NASA has produced a new version of what is perhaps the best view of Europa yet. And it is quite simply the closest approximation to what the human eye would see, and the next best thing to seeing it up close.

The high-resolution color image, which shows the largest portion of the moon’s surface, was made from images taken by NASA’s Galileo probe. Using the Solid-State Imaging (SSI) experiment, the craft captured these images during it’s first and fourteenth orbit through the Jupiter system, in 1995 and 1998 respectively.

The view was previously released as a mosaic with lower resolution and strongly enhanced color (as seen on the JPL’s website). To create this new version, the images were assembled into a realistic color view of the surface that approximates how Europa would appear to the human eye.

The puzzling, fascinating surface of Jupiter's icy moon Europa looms large in this newly-reprocessed color view, made from images taken by NASA's Galileo spacecraft in the late 1990s. Image credit: NASA/JPL-Caltech/SETI Institute
This newly-reprocessed color view of Europa was made from images taken by NASA’s Galileo spacecraft in the late 1990s. Image credit: NASA/JPL-Caltech/SETI Institute

As shown above, the new image shows the stunning diversity of Europa’s surface geology. Long, linear cracks and ridges crisscross the surface, interrupted by regions of disrupted terrain where the surface ice crust has been broken up and re-frozen into new patterns.

Images taken through near-infrared, green, and violet filters have been combined to produce this view. The images have been corrected for light scattered outside of the image to provide a color correction that is calibrated by wavelength. Gaps in the images have been filled with simulated color based on the color of nearby surface areas with similar terrain types.

These color variations across the surface are associated with differences in geologic feature type and location. For example, areas that appear blue or white contain relatively pure water ice, while reddish and brownish areas include non-ice components in higher concentrations.

The polar regions, visible at the left and right of this view, are noticeably bluer than the more equatorial latitudes, which look more white. This color variation is thought to be due to differences in ice grain size in the two locations.

Artist's concept of the Galileo space probe passing through the Jupiter system. Credit: NASA
Artist’s concept of the Galileo space probe passing through the Jupiter system.
Credit: NASA

This view of Europa stands out as the color view that shows the largest portion of the moon’s surface at the highest resolution. An earlier, lower-resolution version of the view, published in 2001, featured colors that had been strongly enhanced. Space imaging enthusiasts have produced their own versions of the view using the publicly available data, but NASA has not previously issued its own rendition using near-natural color.

The image also features many long, curving, and linear fractures in the moon’s bright ice shell. Scientists are eager to learn if the reddish-brown fractures, and other markings spattered across the surface, contain clues about the geological history of Europa and the chemistry of the global ocean that is thought to exist beneath the ice.

This is of particular interest to scientists since this supposed ocean is the most promising place in our Solar System, beyond Earth, to look for  present-day environments that are suitable for life. The Galileo mission found strong evidence that a subsurface ocean of salty water is in contact with a rocky seafloor. The cycling of material between the ocean and ice shell could potentially provide sources of chemical energy that could sustain simple life forms.

Future missions to Europa, which could involve anything from landers to space penetrators, may finally answer the question of whether or not life exists beyond our small, blue planet. Picturing this world in all of its icy glory is another small step along that path.

In addition to the newly processed image, JPL has released a new video that explains why this likely ocean world is a high priority for future exploration:

Further Reading: NASA

What are Temperatures Like on Jupiter?

A true-color image of Jupiter taken by the Cassini spacecraft. The Galilean moon Europa casts a shadow on the planet's cloud tops. Credit: NASA/JPL/University of Arizona

Jupiter, which takes its name from the father of the gods in ancient Roman mythology, is the largest planet in our Solar System. It also has the most moon’s of any solar planet – with 50 accounted for and another 17 awaiting confirmation. It has the most intense surface activity, with storms up to 600 km/h occurring in certain areas, and a persistent anticyclonic storm that is even larger than planet Earth.

And when it comes to temperature, Jupiter maintains this reputation for extremity, ranging from extreme cold to extreme hot. But since the planet has no surface to speak of, being a gas giant, it’s temperature cannot be accurately measured in one place – and varies greatly between its upper atmosphere and core.

Currently, scientists do not have exact numbers for the what temperatures are like within the planet, and measuring closer to the interior is difficult, given the extreme pressure of the planet’s atmosphere. However, scientists have obtained readings on what the temperature is at the upper edge of the cloud cover: approximately -145 degrees C.

Because of this extremely cold temperature, the atmosphere at this level is composed primarily of ammonia crystals and possibly ammonium hydrosulfide – another crystallized solid that can only exist where conditions are cold enough.

However, if one were to descend a little deeper into the atmosphere, the pressure would increases to a point where it is ten times what it is here on Earth. At this altitude, the temperature is thought to increase to a comfortable 21 °C, the equivalent to what we call “room temperature” here on Earth.

Descend further and the hydrogen in the atmosphere becomes hot enough to turn into a liquid and the temperature is thought to be over 9,700 C. Meanwhile, at the core of the planet, which is believed to be composed of rock and even metallic hydrogen, the temperature may reach as high as 35,700°C – hotter than even the surface of the Sun.

Interestingly enough, it may be this very temperature differential that leads to the intense storms that have been observed on Jupiter. Here on Earth, storms are generated by cool air mixing with warm air. Scientists believe the same holds true on Jupiter.

A close-up of Jupiter's great red spot. Credit: NASA/JPL-Caltech/ Space Science Institute
A close-up of Jupiter’s great red spot, an anticyclonic storm that is larger than Earth. Credit: NASA/JPL-Caltech/ Space Science Institute

One difference is that the jet streams that drive storms and winds on Earth are caused by the Sun heating the atmosphere. On Jupiter it seems that the jet streams are driven by the planets’ own heat, which are the result of its intense atmospheric pressure and gravity.

During its orbit around the planet, the Galileo spacecraft observed winds in excess of 600 kph using a probe it deployed into the upper atmosphere. However, even at a distance, Jupiter’s massive storms can be seen to be humungous in nature, with some having been observed to grow to more than 2000 km in diameter in a single day.

And by far, the greatest of Jupiter’s storms is known as the Great Red Spot, a persistent anticyclonic storm that has been raging for hundreds of years. At 24–40,000 km in diameter and 12–14,000 km in height, it is the largest storm in our Solar System. In fact, it is so big that Earth could fit inside it four to seven times over.

Given its size, internal heat, pressure, and the prevalence of hydrogen in its composition, there are some who wonder if Jupiter could collapse under its own mass and trigger a fusion reaction, becoming a second star in our Solar System. There are a few reasons why this has not happened, much to the chagrin of science fiction fans everywhere!

This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons
This cut-away illustrates a model of the interior of Jupiter, with a rocky core overlaid by a deep layer of liquid metallic hydrogen. Credit: Kelvinsong/Wikimedia Commons

For starters, despite its mass, gravity and the intense heat it is believed to generate near its core, Jupiter is not nearly massive or hot enough to trigger a nuclear reaction. In terms of the former, Jupiter would have to multiply its current mass by a factor of 80 in order to become massive enough to ignite a fusion reaction.

With that amount of mass, Jupiter would experience what is known as gravitational compression (i.e. it would collapse in on itself) and become hot enough to fuse hydrogen into helium. That is not going to happen any time soon since, outside of the Sun, there isn’t even that much available mass in our Solar System.

Of course, others have expressed concern about the planet being “ignited” by a meteorite or a probe crashing into it – as the Galileo probe was back in 2003. Here too, the right conditions simply don’t exist (mercifully) for Jupiter to become a massive fireball.

While hydrogen is combustible, Jupiter’s atmosphere could not be set aflame without sufficient oxygen for it to burn in. Since no oxygen exists in the atmosphere, there is no chance of igniting the hydrogen, accidentally or otherwise, and turning the planet into a tiny star.

Scientists are striving to better understand the temperature of Jupiter in hopes that they will eventually be able to understand the planet itself. The Galileo probe helped and data from New Horizons went even further. NASA and other space agencies are planning future missions that should bring new data to light.

To learn more about Jupiter, check out this article on how weather storms on Jupiter form quickly. Here’s Hubblesite’s News Releases about Jupiter, and NASA’s Solar System Explorer.

We’ve also recorded an entire show just on Jupiter for Astronomy Cast. Listen to it here, Episode 56: Jupiter, and Episode 57: Jupiter’s Moons.