The Outer Region of the Solar System

From the Lecture Series: A Field Guide to the Planets

By Sabine Stanley, Ph.D., John Hopkins University

Does the solar system end at Neptune? The answer is a strict no. There are four gas giants located in the outer region of the solar system, and the last planet is Neptune. However, the solar system is not just made up of planets, and the Sun’s gravity force reaches far beyond Neptune, pulling comets inwards.

Infinite space background with silhouette of telescope.
The solar system reaches far beyond the eight planets. (Image: Vadim Sadovski/Shutterstock)

The Solar System is divided into two main regions, each consisting of four primary planets. Mercury, Venus, Earth, and Mars create the inner region, while Jupiter, Saturn, Uranus, and Neptune make up the outer region. However, there are also moons, comets, dust, and energetic particles in the system. The Kuiper belt and Oort cloud are among the furthest solar system regions man has been able to reach until now.

Learn more about How Our Sun Defines Our Solar System.

Jupiter, the Giant with many Moons

Jupiter is the biggest planet in the solar system, with four large moons and at least 75 other smaller ones. Even though it is a gas giant, its diameter is less than 10% of the Sun’s. Jupiter is made up of hydrogen and helium, the same materials that build the Sun, from the surface to the core. Originally, the planet had a huge rocky-icy core that attracted vast amounts of hydrogen and helium, so vast that now the whole planet is made of gas.

There is no solid surface on Jupiter, but the gas gets denser, and temperatures rise the closer we get to the core, so hydrogen transforms into new phases. For example, at about 15,000 kilometers depth in Jupiter, the pressure reaches 2 million bars, and hydrogen turns into ‘metal.’ The pressure becomes so high that the hydrogen atoms are pressed close enough together for the electrons to move amongst the hydrogen nuclei freely, just like electrons do in metals on Earth.

This is a transcript from the video series A Field Guide to the Planets. Watch it now, on The Great Courses Plus.

View of Jupiter from space.
The Great Red Spot on Jupiter is a hurricane that has lasted since 1830. (Image: SquareMotion/Shutterstock)

Gas planets also experience immense storms. Jupiter has had a giant oval-shaped hurricane since 1830, called the Great Red Spot. The Great Red Spot is bigger than the Earth in horizontal extent and about 200 miles deep. Wind speed in this hurricane can reach 250 miles per hour. Another significant storm on Jupiter is a giant polar vortex surrounded by eight cyclones. Each cyclone is about the width of the United States.

Learn more about Mighty Jupiter, The Ruling Gas Giant.

Saturn, the Planet with Rings

Saturn is one of the gas giants in the outer region of the solar system. Like Jupiter, it is made of helium and hydrogen, but the rings around are unique to Saturn and its characteristics.

The rings around Saturn are made of particles ranging from dust to boulder size, from almost pure water ice. The particles orbit the planet in disks much thinner than paper, with a speed of 24 kilometers per second. There are waves formed among the particles as a result of gravitational forces from vibrations in Saturn and nearby moons. The same moons can cause large gaps in the rings. Pan is one of these moons, creating the Encke Gap in Saturn’s A ring with its 35-kilometer flat surface.

Model of Saturn like planet with asteroid rings for a space background, with clipping path included in the file
Saturn’s rings are made up of different particles. (Image: 3000ad/Shutterstock)

As a gas giant, Saturn also has great storms. However, these storms are considerably different from those in Jupiter. For instance, the Great White Spot on Saturn formed in 2010, broadened out in longitude until it encircled the whole planet, ate its tail, and then died away within a year. Similar huge storms occur on Saturn almost every 30 Earth years, i.e., one Saturn year. Thus, the planet-wide storms on Saturn seem to be seasonal, like hurricanes on Earth. Saturn also has polar storms, but they act strangely. For example, the winds surrounding Saturn’s north pole form a giant hexagon. Each side of the hexagon is bigger than the diameter of the Earth.

Learn more about Saturn and the Rings: Gravity’s Masterpiece.

Does the Solar System End at Neptune?

Perhaps, this would be the most accurate answer: Neptune is the last main planet of the solar system, but its orbital distance is less than 1/10 of 1% of the distance to the farthest solar system inhabitants. The farthest objects are small icy bodies in the Oort cloud. They are gravitationally bound to the Sun but orbit the Sun 50,000 times farther than Earth’s orbit. Some of the long-period comets originate from the Oort cloud.

The eight main planets of the solar system almost occupy no space in the whole picture! Almost 99.9% of the system is occupied by the Sun. No matter how big the planets in the outer region are, the Sun is by far the biggest object at the core of our solar system.

Common Questions about Ice in the Solar System

Q: What is the outer solar system?

The inner solar system consists of the Sun and the first four planets. The outer solar system comprises the next four planets and everything beyond the planet Neptune but within the gravitational pull of the Sun. The Kuiper Belt and the Oort Cloud are also parts of the outer region.

Q: What are the regions of the solar system?

There are two main regions: the inner region and the outer region of the solar system. The inner region reaches up to Mars, and everything beyond Mars, even the Kuiper Belt and the Oort Cloud are in the outer region.

Q: What are the four outer planets?

The gas giants of the solar system are Jupiter, Saturn, Uranus, and Neptune, all located in the outer region of the solar system. These four planets are also called Jovian planets after Jupiter.

Q: Is Jupiter a failed star?

Jupiter is the first planet in the outer region of the solar system. It is called a failed star because it is made of the same elements as the Sun, i.e., hydrogen and helium. However, it is not massive enough to have the internal pressure and temperature necessary to cause hydrogen to fuse to helium. This energy source powers the Sun and most other stars.

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