Jovian Planets

The solar system is an intricate and busy place. Planets, asteroids, moons and comets are all in movement around the Sun. All these bodies are so varied and unique, and nothing shows that more clearly than the planets.

There are two very broad classes of planets within the solar system: terrestrial planets and giant planets, also called Jovian planets after the planet Jupiter.

This guide is a great introduction to the Jovian planets, and is supplemented with a useful telescope for viewing giant planets.

What are the Jovian Planets?

Giant planets are any massive planets, including exo-planets. The four known giant planets in the Solar System are Jupiter, Saturn, Uranus and Neptune. They differ from terrestrial planets that are made mostly of solid rocky and metallic materials. Instead, giant planets are composed primarily of gases and/ or ‘ices’, although a few may have dense molten cores of rocky materials. The core of a massive planet may also be completely dissolved if the planet is hot enough. If this is the case, the elements of the vaporized core will be dispersed through the planet. Even though giant rocky planets do exist around other stars, the majority of massive planets consist mainly of gases and ice.

It used to be quite common for scientists and the general public alike to dub all the giant planets in the Solar System “gas giants” – a term that has stuck with many. This is technically incorrect though. It is far more common and accurate for astronomers today to class the giant planets separately into the gas giants Jupiter and Saturn, and the ice giants Uranus and Neptune. This is because of their very distinct compositions. Jupiter and Saturn’s main components are hydrogen and helium; whereas ammonia, methane and water make up the principal components of Uranus and Neptune.

Gas Giants

Jupiter and Saturn are made mostly out of thick atmospheres of hydrogen and helium, though both gas giants do contain heavier elements that make up anywhere from 3% to 13% of planets’ mass. Astronomers think that both planets probably have a rocky molten core containing heavier elements. The cores are thought to be up to 20 000 Kelvin and very pressurized. Scientists do not completely understand the conditions and properties at the cores of the gas giants.

A layer of liquid metallic hydrogen is believed to surround the molten core. This hydrogen is called metallic hydrogen because it actually becomes an electrical conductor due to the high pressures of the planets. This layer of liquid metallic hydrogen is the most abundant element in both gas giants. A thick atmosphere of molecular hydrogen makes up the outer layers of the planets, with the outermost parts containing layers upon layers of visible water and ammonia clouds.

Ice giants

Uranus and Neptune have a distinct interior compositions which classes them as ice giants rather than gas giants. Like the gas giants, Uranus and Neptune have hydrogen abundant atmospheres. This atmosphere extends from the upper clouds down to about 80% – 85% of the radius in Uranus and Neptune respectively. Below the hydrogen-rich atmosphere are layers of ‘ices’ made from ammonia, methane and water. These layers also contain some rocky and gaseous elements, though the amounts and ratios of these components is yet to be determined. There are layers of the upper atmosphere that contain trace amounts of methane which may be responsible for the light blue appearance of Uranus, and the aquamarine color of Neptune.

The two planets do have some pronounced differences between them. For one, Uranus has a higher quantity of hydrogen and helium that Neptune does, even though Neptune is the more massive of the two. Neptune’s greater mass means that it is denser and hotter than Uranus, and has a more active atmosphere than its ice giant counterpart.


The great Jupiter is the fifth planet from the Sun. It lies at an average distance of 778 million kilometers away from the Sun, and takes 11.86 years to complete one orbit around our star.

It is the largest and most massive planet in the Solar System, with a mass two and a half times greater than that of all the other planets in the Solar System combined. Jupiter is still small in comparison to the Sun though, being only a thousandth of the mass of our host star. Ancient peoples and astronomers have always known about Jupiter, with records of the planet dating back thousands of years. This is because the planet is very bright in the night sky: it can get as bright as magnitude -2.94, which makes it the third brightest object in the night sky after the Moon and the Venus. The planet gets its name after the king of the Roman gods.

Jupiter is often called a ‘failed star’ due to its great mass, but mainly because of its composition which is similar to stars: composed of mainly hydrogen and helium. Helium accounts for approximately a quarter of Jupiter’s mass. The outer atmosphere is clearly divided into a number of layers at distinct latitudes. Storms and turbulence are common at points where the separate layers meet and interact. The most powerful and well known result of this is a huge storm called the Great Red Spot, which is bigger than the Earth that has been raging on Jupiter for hundreds of years. Jupiter does not have a well-defined solid surface, but theories suggest that the giant planet may have a molten rocky core of heavier elements regardless.

Jupiter rotates the most rapidly of all the major planets, taking a little less than 10 hours to complete a revolution about its axis. The planet has a bulge at its equator due to its rapid rotation. It also has a faint planetary ring system and a strong magnetosphere, fourteen times stronger than Earth’s magnetic field, and which causes aurora to form near the planet’s poles.

The planet has an impressive number of natural satellites – 79 known moons. This includes the four large moons discovered by Galileo Galilei in 1610, known as the Galilean moons after his discovery.

We have been sending spacecraft to Jupiter since 1973. The first close-ups of Jupiter’s atmosphere and several moons were caught by the Pioneer missions in a number of flybys. The Voyager flyby missions, which followed six years after the Pioneer missions, revealed a greater understanding of the Great Red Spot as while as the Jovian moon system. These flybys also discovered Jupiter’s rings. In 1995, the Galileo probe entered orbit around Jupiter – a first for the planet – and remained in orbit for seven years. Some of the highest resolution images we have of Jupiter where taken by the Cassini probe in the year 2000, as the spacecraft made its way toward Saturn. The most recent probe to enter Jupiter’s orbit is the Juno spacecraft in 2016.


The other gas giant in the Solar System is Saturn, the sixth planet from the Sun. It orbits at a distance of over 1.4 billion kilometers from the Sun, taking 29½ years to complete a full revolution around our star. There are different ways of determining the planet’s rotation rate which yield different answers for how long a saturnine day is, with the latest estimate at 10 hours 32 minutes and 35 seconds. It is the second largest of the planets after Jupiter, and is named for the Roman of agriculture. Despite the planet’s large size in comparison to Earth, and despite being over 95 times more massive than our home planet, Saturn only has an eighth of the average density of Earth.

Along with its giant neighbor Jupiter, Saturn is a firm favorite amongst amateur astronomers when it comes to observing Solar System targets. The planet’s beautiful, prominent ring system is the main reason why it is so captivating to view even through small telescopes. The rings are made mostly of water ice particles, and small amounts of dust and rock debris. Each ring is an average of 20 meters thick, and the furthest rings stretch out 120 700 kilometers away from the planet’s equator.

Saturn is composed of mainly hydrogen and helium. Scientists suggest that the planet’s core is rocky, probably composed of a core of iron–nickel, silicon and oxygen compounds, and about 9 to 22 times the mass of Earth. Models further suggest that the core is surrounded by a layer of liquid metallic hydrogen, followed by a layer of liquid hydrogen and helium, all surrounded by an outer layer of gases.

Ammonia crystals in its upper atmosphere give the planet its pale yellow coloring. The planet appears banded due to its layers of clouds, though the bands are relatively faint when compared to those of Jupiter. Occasionally, long lasting features like storms appear on the surface, though it is mainly dull without much contrast. Saturn’s wind speeds can reach 1,800 km/h.

The ringed planet has a magnetic field smaller than Jupiter’s and also slightly weaker than Earth’s magnetosphere. Saturn has 53 officially named moons, and at least 62 known moons in total. The planet also has hundreds of little moonlets in its rings in addition to the 62 moons.

The first flyby passed Saturn was made by Pioneer 11 in 1979, which captured low resolution images of the planet and a few of its moons. The Voyager 1 flyby in 1980 captured the first high resolution images of Saturn. In 2004, the Cassini-Huygens space probe entered Saturn’s orbit. The probe recovered information on Saturn’s atmosphere, moons, and rings. It remained in orbit until 2017.


Uranus is the seventh planet from the Sun and one of the two ice giants. It is the third largest planet measured by radius, and the fourth largest measured by mass (the smallest jovian planet). It orbits the Sun at an average distance of 3 billion kilometers, taking 84 years to complete an orbit. The planet is the only one named for a god of Greek mythology: the god of the sky Ouranos. Unlike the other major planets, which were already known to be planets as a result of their prominence and brightness, Uranus was mistaken for a star. It was only in the late 1700s that astronomer William Herschel took note and proposed it was a comet, and astronomer Johann Elert Bode proposed it was a planet.

Uranus has an atmosphere much like that of Jupiter and Saturn, consisting primarily of hydrogen and helium. However, the planet contains a fair bit of “ices” including ammonia, methane and water. The model of the planet’s structure suggests that Uranus’ core is small and rocky, made of mainly silicate and iron-nickel. Surrounding the rocky core is a mantle of water and ammonia fluid which makes up most of the mass of Uranus, and finally a gaseous outer envelope of complex layers of clouds.

A unique feature setting the planet apart from every other in the Solar System is that Uranus has an axis of rotation which is tilted sideways, putting its north and south poles where the equator of other planets would lie. It takes 17 hours 14 minutes to complete a revolution about its axis.

Uranus does not have the distinct bands and storms common to the other giant planets, appearing nearly featureless. It does however have a ring system, magnetosphere, and the many moons associated with the other Jovian planets (with 27 known natural moons).

Voyager 2 encountered Uranus is a flyby in 1986; the only probe to ever have investigated the planet. The flyby gathered information on the planet’s atmosphere, rings and moons, as well as discovering 10 new moons.


Neptune is the eighth planet form the Sun; the furthest from our star and the last major planet   in the Solar System. Measuring by diameter, it is the fourth largest planet in the Solar System, and measuring by mass it is the third largest. It is also the densest of the giant planets. Neptune makes a full revolution around the Sun once every 164.8 years, and lies at an average distance of 4.5 billion kilometers away from the Sun. A day on Neptune is equal to 16 hours and 6 minutes. Its blue hue makes Neptune appear to be an ocean covered planet, and because of this it is named for the Roman god of the sea.

Neptune is the only major planet not visible to the naked eye.  It is also the only planet which was not discovered by observations, but instead by mathematical prediction by Alexis Bouvard. It was later observed for the first time by Johann Galle.

Neptune’s planetary model is very close to that of its ice giant neighbor Uranus. It’s likely that the planet has a small rocky core of silicates, iron and nickel. The surrounding mantle is abundant with ammonia, methane and water “ices”. As with Uranus, this mixture is actually a hot, dense fluid which is referred to as “icy” in planetary science. The outer layers are mainly gaseous.

Similar to the other giant planets, Neptune is made mainly of hydrogen and helium. Its atmosphere consists of bands of clouds which have different compositions depending on the altitude. Uranus and Neptune are similar in many ways, but whereas Uranus has a generally featureless surface, Neptune has active weather patterns which add visible detail to the surface. This includes having the strongest winds of all the Solar System’s planets, with speeds reaching a rapid 2100 km/h. The planet even had a Great Dark Spot, likened to Jupiter’s Great Red Spot. The planet also has a magnetosphere, a planetary ring system (though it is faint and fragmented), and 14 known moons.

Voyager 2 is the only probe to have visited Neptune, making its closest approach to the planet in 1989, and gathering information on its moons and weather system. The spacecraft confirmed Neptune’s magnetic field, and discovered six new moons as well as an extra ring.

The Perfect Telescope for Viewing Jovian Planets – Celestron Astro Master 102AZ

With everything you know now about the Jupiter, Saturn, Uranus and Neptune, it is no wonder that you want the perfect telescope for viewing these fascinating Jovian planets. Celestron’s Astro Master 102AZ is the ideal choice.

The telescope has become a firm favorite among beginner hobbyists thanks to its quick and easy setup which requires no tools. It is also a top choice for a novice as refractors do not need to have their optics aligned, and can be ready to go straight out of the box.

Celestron brings you high quality, erect image optics which show pristine views of a variety of celestial objects. Jupiter’s bands and Galilean moons are clearly visible, as well as Saturn’s rings. Images of the moon are highly detailed, and star clusters are a must to behold through this telescope.

The Astro Master comes with a host of excellent extras including 20mm and 10mm eyepieces, a manual alt-azimuth mount which is wonderfully straightforward to use, and Celestron’s Starry Night Software.