What Is The Largest Planet In The Solar System?

The eight planets in our solar system come in a wide variety of sizes. Some are true behemoths, while others are relatively small. Which planets are the biggest and which are the smallest?

Jupiter - Diameter Of 142,800 Km

Jupiter as Captured by the Hubble Telescope, NASA
Jupiter as Captured by the Hubble Telescope, NASA

Jupiter is the largest planet in our solar system by size, mass, and volume. By size, Jupiter is gigantic, having a diameter of 142,800 kilometers or about 11 Earths across. In terms of volume, you could fit every other planet inside Jupiter, and there would still be space left over. Jupiter is more than 300 times the mass of the Earth. 

Jupiter was able to grow to its current size, likely due to its location in the solar system. Shortly after the Sun formed, its energy pushed lighter elements towards the outer solar system. Hydrogen and helium, the two most lightweight elements in the universe, could not stay in the inner solar system and were instead pushed away by the Sun's energy. Since there is a far higher amount of hydrogen and helium than any other element, they existed in large amounts in the outer solar system, allowing the outer gas giants to grow to immense sizes.

Like the Sun, Jupiter is composed mainly of hydrogen and helium. These two elements make up over 90% of Jupiter's composition. In addition to being the largest planet, Jupiter also has the strongest magnetic field of all the planets. If humans could see Jupiter's magnetic field, it would be one of the brightest objects in the night sky. Jupiter also has the fastest rotation in the solar system, spinning once about its axis every 10-hours. It's this fast rotation that causes Jupiter to have the long bands of gas that are scattered throughout its atmosphere. Jupiter's rotation is so fast that it stretches the gas in Jupiter's atmosphere, creating the long bands that make Jupiter so recognizable.

Saturn - Diameter Of 120,660 Km

Saturn as Captured by the Hubble Telescope, NASA
Saturn as Captured by the Hubble Telescope, NASA

Saturn is the second-largest planet in our solar system, with a diameter of 120,660 kilometers, or about 9.5 Earths across. By volume, you could fit 764 Earths inside Saturn. By mass, Saturn is 95 times the mass of Earth.

The reasons behind Saturn becoming such a giant planet are the same as those for Jupiter. The outer solar system contained vast amounts of hydrogen and helium, allowing planets like Jupiter and Saturn to become the largest planets in the solar system. Interestingly, Jupiter and Saturn are probably the two most similar planets in the solar system. Both are composed chiefly of hydrogen and helium and are covered in large bands of gas. However, Saturn's bands are not nearly as prominent as Jupiter's. Like Jupiter, Saturn also rotates fast, completing one rotation every 11-hours. With such a quick rotation, the process by which the large bands of gas form are similar to Jupiter. However, due to Saturn's further distance from the Sun, it does not receive as much sunlight, which may contribute to how these large bands form. Furthermore, the distribution of some chemicals, such as water, ammonia, and sulfur, may also contribute to Saturn's watered-down appearance. 

Uranus - Diameter Of 51,118 Km

Uranus as Captured by the Hubble Telescope, NASA
Uranus as Captured by the Hubble Telescope, NASA

Uranus is the third-largest planet in the solar system, with a diameter of 51,118 kilometers, or about four Earths across. Despite being only four times larger than Earth, Uranus is still a massive planet at 14.5 times the mass of Earth. 

Like Jupiter and Saturn, Uranus is composed mostly of hydrogen and helium. However, although Uranus is also a gas giant, it contains a much higher abundance of water, methane, and ammonia when compared to Jupiter and Saturn. As a result, Uranus belongs to another classification of planets called ice giant. Uranus was the first planet to be discovered using a telescope. In 1781, the astronomer William Herschel discovered Uranus. Uranus has only ever been visited by a spacecraft once, the Voyager 2 flyby, in the 1980s. Most of what scientists know about Uranus was discovered during Voyager 2's flyby. Unlike Jupiter and Saturn, Uranus was virtually featureless. It had no large bands spread across its atmosphere and no visible storm systems. For the most part, Uranus appeared rather dull. As it later turned out, Voyager 2 simply had bad timing. Uranus experiences seasonal changes as it orbits the Sun, and future observations from Hubble have revealed atmospheric features and storms. One of the most interesting properties of Uranus is that the entire planet is tipped on its side. Unlike the other planets, Uranus does not rotate from side to side. Instead, its axis is flipped a full 90 degrees. Visually, Uranus rolls around the Sun like a wheel. 

Neptune - Diameter Of 49,528 Km

NASA's Constructed Photo of Neptune from Voyager 2 Images
NASA's Constructed Photo of Neptune from Voyager 2 Images

Neptune is the fourth-largest planet in the solar system, with a diameter of 49,528 kilometers, or about 3.8 Earths across. Interestingly, despite being smaller than Uranus, Neptune is more massive. Neptune has a mass seventeen times that of the Earth, while Uranus is just over 14 times as massive as Earth. Given Neptune's location in the solar system, it makes no sense why it's more massive than Uranus. Being so far from the Sun makes it unlikely that there would have been enough material for Neptune to become as huge as it is. It is generally believed that Neptune formed closer to the Sun than Uranus, and over time, the two planets swapped places due to the gravitational forces between them and the other gas giants. 

Like the other three gas giants, Neptune is composed mostly of hydrogen and helium. Like Uranus, Neptune is classified as an ice giant due to its higher abundance of things like water, methane, and ammonia. Interestingly, the higher abundance of methane in Neptune's atmosphere is why the planet is so blue. The methane absorbs red light from the Sun and scatters blue light, giving Neptune its signature color. 

Earth - Diameter Of 12,756 Km

An Image of Earth Tweeted by Astronaut Wiseman from the International Space Station, NASA
An Image of Earth Tweeted by Astronaut Wiseman from the International Space Station, NASA

Earth is the fifth-largest planet in our solar system, with a diameter of 12,756 kilometers. The Earth is the largest and heaviest of the four inner rocky worlds. The Earth is also the densest planet in the solar system. Earth's high density comes from the fact that Earth contains an abundance of heavy metals, with the core being composed mostly of iron. Earth is one of the only planets in the solar system that is geologically active, causing the surface of the Earth to change over time as the continents drift across the surface.

The Earth is the only planet in the solar system that orbits within the Sun's habitable zone. The habitable zone is the area around a star where, given the right conditions, water can exist in liquid form on a planet's surface. The Earth is nearly covered in water, with 75% of the surface being water. The existence of so much liquid water has allowed for the presence and evolution of life, and the Earth is the only world known so far to harbor life. 

Venus - Diameter Of 12,104 Km

Venus as Captured by NASA's Mariner 10 Spacecraft
Venus as Captured by NASA's Mariner 10 Spacecraft

Venus is the sixth largest planet in the solar system, with a diameter of 12,104 kilometers, or about 95% the size of Earth. In terms of size and composition, Venus and Earth are very similar. Both are rocky worlds that contain an iron core. However, despite their similarities, Venus has sometimes been called Earth's "evil twin." That's because Venus does not possess the life-giving environment of the Earth. Instead, Venus is the warmest planet in the solar system, having an average surface temperature of 900 degrees Fahrenheit (475 degrees Celsius). Venus is even warmer than Mercury, despite Mercury being closer to the Sun than Venus. The high surface temperature of Venus is a result of its atmospheric composition and density. Not only is the atmosphere 90 times heavier than Earth's, but it is also 96% carbon dioxide. As a greenhouse gas, carbon dioxide traps incoming heat from the Sun and keeps it near the surface, causing temperatures on Venus to skyrocket. 

Mars - Diameter Of 6,779 Km

An Image of Mars Taken by NASA's Perseverance Land Rover
An Image of Mars Taken by NASA's Perseverance Land Rover

Mars is the seventh-largest planet in the solar system, with a diameter of 6,779 kilometers, or about half the size of Earth. The Earth and Mars are very similar worlds. Both are composed primarily of rock and metal, both have a 24-hour day, and Mars likely had large amounts of water on its surface in the distant past. Mars has undergone numerous changes throughout its history. It once had a thick atmosphere with large amounts of oxygen, which allowed Mars to have rivers, lakes, and even oceans of liquid water. Billions of years ago, Mars would not have looked that different from Earth. Life may have arisen on Mars, but scientists have not found any evidence of past or current forms of life on Mars. 

Mercury - Diameter Of 4,849 Km

A Colorful View of Mercury, Produced from Images Taken by the MESSENGER's primary mission, NASA
A Colorful View of Mercury, Produced from Images Taken by the MESSENGER's primary mission, NASA

Mercury is the smallest planet in the solar system, with a diameter of 4,849 kilometers, or about 38% the size of Earth. Mercury is also the closest planet to the Sun at an average of 48-million kilometers. Being so close to the Sun, Mercury can't hold onto an atmosphere, making it difficult for Mercury to disperse the heat it receives from the Sun. The result is that Mercury experiences some of the most dramatic temperatures in the solar system. During the day, temperatures reach as high as 800 degrees Fahrenheit (450 degrees Celsius), while at night, temperatures drop as low as minus 290 degrees Fahrenheit (minus 180 degrees Celsius).

The Planets From Biggest To Smallest

Rank Planet Diameter (kilometres)

1

Jupiter 

142,800

2

Saturn

120,660

3

Uranus 

51,118

4

Neptune

49,528

5

Earth

12,756

6

Venus

12,104

7

Mars

6,779

8

Mercury

4,849

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