Jupiter: The solar system giant


Most Jupiter-sized planets orbit the mother star in a highly elliptical orbit. This means they will often cross the orbit of any Earth-like planet and fling it into outer space, making life impossible. But our Jupiter travels in a near-perfect circular orbit, preventing a collision with any Earth-like planet, making life possible

-Michio Kaku

The solar systems very first, a planet so huge, we call it the Giant! And why not, it’s this Giant that saves us from the raging comets and asteroids. Jupiter.



The planet was known and was being observed long before the telescopes were invented. This dates to at least the Babylonian astronomers of the 7th or 8th century BC.

It is one of the five planets visible to the naked eye if you know where to look. You can use tools like Jupsat Pro to know the exact position of the planet in the sky.

After the discovery of telescope by Hans Janssen, Galileo Galilei made some improvements to the design and used it for astronomical observations.

1610, that’s when he discovered the four large moons of Jupiter. These moons are also called the Galilean Moons.

This observation of the moons by Galileo is thought to be the first telescopic observation of moons other than that of Earth’s.

Simon Marius, independently discovered the moons just one day after Galileo. He also named these moons – Io, Europa, Ganymede, and Callisto. But it was until 1614 when he published his discovery.

The discovery was a major proof for the heliocentric theory of Copernicus.

heliocentric model
heliocentric model of the solar system

But there was one big problem

In that time the Catholic Church believed that the earth was centered in the universe with all the planets, sun and all other celestial bodies revolving around it.

Anyone that went against this would be punished.

And when Galileo supported the Copernican theory, he was under the threat of the Church.

Galileo before the holy office
Galileo before the Holy office

In the year 1955, two astronomers, Bernard Burke, and Kenneth Franklin discovered bursts of radio signals coming from Jupiter. These signals were coming at a frequency of 22.2 MHz (no, there weren’t any aliens playing radio!).   

Modern exploration includes all the automated space crafts that have visited the Giant since 1973.

These were,

Pioneer 10, Pioneer 11, Voyager 1, Voyager 2, Ulysses, Cassini, New Horizon, and Juno

Among these, Pioneer 10 was the first one to fly past the planet, revealing many things about the planet.

The Galileo Probe is the first to orbit the planet.

Galileo spacecraft
the Galileo space craft


Formed about 4.5 billion years ago. Born from the primordial dust, Jupiter is the first planet to be born in the solar system and so, the eldest of all.

The name Jupiter means the king of Ancient Roman God.

It is the 5th planet from the sun, and of course, the largest planet in the solar system.

size comparison of planets
size comparison of the planets

But how huge is this planet actually? Well, let’s take a look at the numbers.

The planet is 318 times more massive than the earth. Now that’s humongous!

That’s just one of the shocking things. The planet has 2.5 times more mass than all the planets combined!

And also, it is the only planet whose barycenter lies outside the volume of the sun, i.e. above the sun’s surface, at 1.068 solar radii from the suns center.

Its radius is 1/10 the radius of the sun. That means, if we could place 10 Jupiter in line with each other then we will get the radius of the sun.


Although the planet is so huge, it has no true surface. Jupiter joins the group of the gas giants.

In the year 1997, the planet was suggested to have a core. It is thought to have a dense core with a mixture of elements. The core takes a mass of about 12 to 45 times that of Earth. And that makes up approximately 4% to 14% of the total mass of Jupiter.

The next layer is an ocean of liquid, metallic hydrogen with a small amount of Helium. This is the largest layer of the planet covering about 78 % of the radius of the planet. Through this layer, helium and neon particles precipitate downwards in the form of droplets, like rain.

Followed by this, comes a layer of liquid hydrogen. The temperature inside the planet shows a steady increment as we move towards the core, this is mainly because of the Kelvin – Helmholtz Mechanism.

The outer most layer makes up the cloudy atmosphere. Jupiter, as expected, has the largest planetary atmosphere in our solar system. How large you ask? It stretches 5,000 km of altitude. Compare it to the atmosphere of Earth which is just 100 km.

The atmosphere of the Giant comprises of clouds of ammonia crystals and possibly ammonium hydrosulfide.

The beautiful color of the planet, shades of white, brown, yellow and red are caused by the diffusion of chemical components in those areas.

colors of Jupiter


Despite the size of the planet, its density is much lower than that of the Earth. This is because the planet has a volume that is equal to 1,321 times that of our Earth but it is just 318 times more massive than Earth.

Density of Jupiter – 1.33 g/cm³. Earth’s density – 5.51 g/cm³.

The Great red spot

Great Red Spot
the great red spot

The most striking characteristic of the planet. A storm larger than the size of earth, located at about 22 degrees south of the equator of the planet.

The wind speed of the storm can go up to 400 Mph. That’s two and half times faster than a category 5 hurricane.

The storm is known to be in existence since 1831. And is assumed to be there since 1665. Though the models suggest that the storm is stable and is a permanent feature of the planet, there is a decrement of about 930 km every year in the length of the storm.

When it was first discovered in the late 1800s, the size of the storm was noted to be around 41,000 km. After that, in 1979, when Voyager Flew past the planet, it showed the length of the storm to be 23,300 km and width – approx. 13,000 km.

Not long after that, Hubble observations in 1995 showed a decrease in size to 20,950 km. observations in 2009 showed the size to be 17,910 km.

The latest observations in 2015 showed the storm size approximately 16,500 km by 10,940 km.

The magic of gravity

With a gravity of 24.79 m/s², Jupiter has the strongest gravity amongst all the planets.

Since Jupiter was the first planet to be formed in the solar system, it attracted and destroyed space debris that would have helped the other planets grow in size thereby limiting the sizes of the other 7 planets.

The planet, even today serves as a shield against the comets and meteors by attracting them towards itself which otherwise would have entered and crashed on inner planets, like Earth. Thank you, Jupiter.

speaking of which, have you ever wondered what would happen if you crashed into the Jupiter atmosphere. If you are curious, then check out this video.


Jupiter has a total of 79 moons! That’s the highest number of moons a planet has in the solar system (if we don’t consider the moonlets of Saturn).

but why does the planet harbor so many moons? find out here.

To understand them better, the moons are divided into two groups, regular satellites, and the irregular satellites.

Let’s take a look at them.

Regular Satellites

These are the well behaved satellites of their parent, Jupiter.

Well behaved as in?

These satellites follow the prograde motion and their orbit is pretty much circular. Also, the satellites have a low inclination.

The regular satellites can be further divided into two groups:

Inner Satellites, also called the Amalthea group

These moons orbit very close to Jupiter. Strangely, these moons replenish and maintain the faint ring system of Jupiter.

Some of the moons that fall in this group are –

Metis, Adrastea, Amalthea and Thebe.

Amalthea is the largest among the inner satellites and is thought to be a captured body by the Solar System.

Main group, also called the Galilean Moons

This group is a box full of surprises.

Galilean Moons
Image credit: NASA/JPL/DLR

After the Sun and all the planets, these four main moons of Jupiter, are the largest celestial objects in the Solar System in terms of mass.

These moons are larger than all the known dwarf planets. In fact, Ganymede is larger than the planet Mercury.

To truly understand how massive these moons are, let me tell you this… 

Just these four moons, makeup 99.997% of the total mass in orbit around Jupiter.

Now since these are so special, let’s take a look at each one of the Galilean Moons.


Discovered in 1610, the moon is named after the mythological character, Io, the priestess of Hera.

Io is the 4th largest moon in the solar system. It is geologically the most active object in the solar system with about 400 active volcanoes.

These unusually high number of volcanoes are caused by the tidal heating that takes place from the friction generated within Io’s interior. This friction is generated as it gets pulled between Jupiter and the other Galilean satellites.

Io also has the record of having the least amount of water molecules than on any other celestial object in the Solar System.  


Again, discovered in 1610 by Galileo, Europa in Greek Mythology is the mother of King Minos of Crete.

It is the smallest among the 4 major moons and the 6th closest.

Along with that, Europa is also the 6th largest satellite in the solar system.

It is mostly comprised of silicate rock and a crust of water ice. The core of this moon is mostly made up of iron-nickel.

Size wise, the satellite is slightly smaller than earth’s moon.

The moon is said to have the smoothest surface of any other astronomical body in the solar system. And that’s not it, it also has a very thin layer of oxygen in its atmosphere.

Sounds like life right?

The moon is believed to have an ocean of water beneath its smooth surface that harbors life!


It’s no doubt you have heard about this moon. Even if you didn’t, worry not.

Ganymede is the largest moon of Jupiter, and clearly the largest in the solar system.

Its diameter is 5,268 km, which exceeds the diameter of the planet Mercury.

Its orbits the home planet at position 7.

It is comprised of an approximately equal amount of silicate rock and water ice with a liquid core, rich in iron.

Internally, the moon may contain the Liquid Ocean of water. This ocean may contain more water than all of the Earths water combined.

With that, it’s the only moon to have a magnetic field. And it has the lowest Moment of inertia factor of any other body in the Solar System.


It is the 3rd largest moon in the solar system and the 2nd largest moon of Jupiter.

Its diameter is 4821 km, that’s 99 % of the diameter of planet Mercury but only 1/3rd the mass.

The surface of Callisto is the most heavily cratered in the solar system.

With a density of just 1.83 g/cm³, it has the lowest density and surface gravity amongst the other major moons of Jupiter.

Also this moon is composed of approximately equal amounts of rock and ice.


rings of Jupiter
KelvinsongJupiter diagramCC BY-SA 3.0

The ring system of Jupiter is not as alluring as that of Saturn. This ring system of Jupiter is called the Jovian Ring System.

It was discovered by the Voyager 1 space probe in 1979. Overall, it was the third time that humanity had seen a planet with rings, after Saturn and Uranus.

It was not until 1990 that the rings were closely investigated by the Galileo Orbiter. After that, the Hubble telescope also made its observation. Observation of the rings from the surface of Earth takes the most powerful telescopes that are there.

The Jovian Rings can be divided into 4 parts:

Halo ring

This is the innermost ring of Jupiter. The ring starts at a distance of 100,000 km radius and ends at approximately 122,500 km radius.

It’s vertically the thickest of all the rings. The outer edge of the ring coincides with the next ring, i.e. the main ring.

The shape of this ring is like a torus.

And though the inner radius is 100,000 km, there are still some materials present till around 92,000 km.

Main ring

This ring follows just after the Halo ring and is the brightest of all the four rings of Jupiter.

The ring extends to 6,500 km with its inner radius at 122,500 km and its outer radius at 129,000 km

New Horizon spacecraft conducted a deep search to see if there were any small moons in the main ring of the planet in February – March 2007, but no satellites were found that were greater than 500 m in length.

Since most of the dust in the ring is made up of ice which is, well, Volatile, it gets evaporated quickly.

Also, the dust gets removed by a combination of Poynting – Robertson drag and electromagnetic forces from the Jovian Magnetosphere.

Gossamer Rings

These are further divided into two rings:

The Amalthea Gossamer Ring

As the name suggests, this ring makes contact with the moon Amalthea.

The ring extends from the Amalthea at 182,000 km to 129,000 km.

It’s the next after the main ring and hence its inner region appears to be in contact with the main ring. Since the main ring is very bright, the inner boundary of the ring is not clearly defined.

Overall the ring is a very faint structure.

Thebe Gossamer Ring

Again as the name suggests, the ring extends from Jupiter’s 4th Moon from the surface, Thebe at around 226,000 km to about 129,000 km.

Just like its sibling (Amalthea Gossamer Ring), the Thebe Gossamer’s inner boundary is not well-defined. This is because of the brightness of the main ring.

Overall the ring is very faint. In fact, it is the faintest ring of Jupiter. It gets brighter only at the top and bottom, but not as much as the other rings.

This ring extends beyond the outer edge to about 280,000 km, but is barely visible and is called the Thebe Extension.


Jupiter has the strongest magnetic field in the solar system (except for the sunspots). This magnetic field is 14 times stronger than that of Earth.

The magnetic field is generated in the liquid metallic core of the planet and is thought to be generated by eddy currents.

When this strongest magnetic field comes in contact with the solar wind at about 75 Jupiter radii from the planet, it results in a bow shock.

The four major moons of Jupiter fall in the vicinity of its magnetosphere, hence are saved from the solar winds.


Yes, how can this giant miss out the auroras!

Auroras on Jupiter
auroras on Jupiter

Hubble telescope has observed Jupiter daily for several months. Scientists used the far-ultraviolet images from the Hubble space telescope imaging spectrograph and created videos that demonstrate the movements of these vivid Auroras.

The Auroras on Jupiter are so large that they outgrow earth in size


Jupiter is the largest planet in the solar system.

it is 318 times more massive than the earth.

due to the huge gravity of the planet, it attracts a large number of comets and meteors towards itself hence protecting other planets from. dangerous impacts.

the planet has no true surface.

the density of Jupiter is less than that of the earth. 1.33 g/cm³.

it has a permanent storm larger than the earth called the Great Red Spot.

has a total of 79 known moons.

The four moons – Io, Europa, Ganymede, and Callisto are the main moons of Jupiter and are the largest objects in the solar system after the sun and all the planets.

This ring system of Jupiter is called the Jovian Ring System.

A question for you – what do you think is the reason behind the enormous size of the planet?


Sahil Asolkar

writer and co-founder

Sahil Asolkar is a writer, poet, and shows a good interest in astronomy. His work can be seen in the articles he writes for Astronomiac.

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